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Audio Gain Structure for Pro AV Systems

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2. microphones set the gain level for a specific brand name and E E Table Mic Boundary E E Table Mic Gooseneck Ei E Lectem Mic Gooseneck model and turn on phantom power if required for that mic A high pass filter is inserted o compensate for common microphone problems such as ow frequency noise thumping or plosives popping A bass and treble shelving filter is also inserted into the filter block with Q3 Clock Audio C3 RF Series 5 Clock Audio C800 RF Shure MX400 S Cardioid amp Shure MX400 S Supercardioid Shure MX400 S Omni directional Q3 Shure MX400 D Cardioid gS Shure MX400 D Supercardioid Shure MX400 D Omni directional L1 Beyer Dynamic Classis GM 301 Beyer Dynamic Classis GM 302 Beyer Dynamic Classis GM 303 q Beyer Dynamic Classis GM 304 amp Beyer Dynamic Classis GM 305 AKG CK 31 AKG CK 32 AKG CK 33 Q3 Sennheizer ME 34 gain level set to O dB This allows a Soest E p Ceiling Mic E Handheld Mic adjustment of the microphone Creaa DVD Player one if necessary either via the VCR DVD Conte j iPod DSP Configurator Software or by e lt r programming the gain parameters Wireless Mic Rx unbal f Q3 Wireless Mic Rx balanced nto a control system and allowing he user to make adjustments Finally a compressor is inserted into a dynamics processing block applying light compression to he channel for the purpose of both normalizing diverse
3. Figure 1b Figure 1d A sequence of valves in a plumbing system is an effective analogy for visualizing audio gain structure Audio Gain Structure for Pro AV Systems Extron How Does Plumbing Relate to Audio Gain Structure Consider the water impurities as an equivalent to circuit noise or in an audio system the noise floor which can be heard as hiss Sufficient water pressure ensures that the quantity of impurities at our faucet is inconsequential Similarly having sufficient audio levels through each AV device ensures that the noise floor will not be noticeable at the loudspeakers Consider the bursting pipes as equivalent to clipping or distortion Too much water pressure even in only one pipe can put the entire plumbing system at risk of a burst pipe With audio signal clipping or distortion in one component will affect the entire signal path For example if there is clipping at a microphone input the rest of the system will pass that distortion through with great clarity and accuracy What you hear will be objectionable and your customers won t be pleased Why is Proper Gain Structure Needed Proper gain structure makes the difference between good and bad sound Maintaining a low noise floor and preventing clipping distortion are the two most notable reasons for proper gain structure See Figure 2 0 dBFS Maximum or Clipping 21 dBu Headroom Nominal 17 dB 4 dBu Reference 17 dBFS S N Dynam
4. These are more advanced tools for an AV engineer including a professional SPL meter AV signal test generator multimeter and an acoustical sound source These tools enable precision in setting gain levels as well as accuracy in establishing listening levels When setting gain levels in an audio system it is always important to have some equipment on hand for generating reference audio test signals and measuring their levels This will allow for precise determination of signal levels at all stages of the system from the signal source through the DSP to the amplifiers and loudspeakers The test signals and measurement tools are used to properly set nominal signal levels through the system and ensure that ample headroom is available to prevent the possibility of clipping Sound Pressure Level Meter A sound pressure level SPL meter measures the acoustic level in the listening environment For gain structure setup it is used to establish a nominal volume setting for the system as well as a sonic reference for a human voice Many electronic supply companies offer an affordable SPL meter and there are many apps available for mobile devices There are also higher performance SPL meters from Br el amp Kjaer and others that conform to industry standards Advanced Kit A senior AV engineer or audio professional typically uses this type of kit for sound system commissioning which includes gain Structure setup and sound power response o
5. ausim Bit n BASS c nna TREE POMP gLOUD g DIY GAIN 2c FBS pass CMP LOUD 5 DLY GAIN SS TREB T FIT D GAIN L UC URS URS URS UR amp Muted No Input Met O Meus Que 5 ket Extron DSP Figure 12 Telephone hybrid and codec routing 1 and 2 The inputs are routed to the outputs for the ceiling speakers and ALS The hybrid input is also routed to the codec and likewise the codec input is sent to the hybrid 3 The hybrid and codec inputs are also routed with the program submix into Virtual Return E to be used as the reference for AEC processing 4 Gain blocks are selected for a control system to provide user adjustment of volume for the hybrid and codec far end 4 In defining volume control points for the videoconferencing codec and the telephone hybrid we can use either additional submixes or we can keep things simple and use the post processing input gain 4 in Figure 12 These will be used for interfacing into a control system 6 It is often the case that the volume controls within the videoconference codec will be used due to the fact that they are user friendly have on screen volume and mute indicators and allow consistency between fully integrated and standalone videoconference systems Listening Tests and Final System Adjustments Page 24 For this final step we will perform a series of system tests to finalize system gain structure and listening levels We begin by first ensuring
6. above 4 Repeat Step 3 for all lectern microphones 5 Repeat Step 3 for all wireless microphones 6 Repeat Step 3 for all ceiling microphones Make no presumptions on how loud or soft a customer s voice might be On one particular occasion in the author s experience with setting up audio systems an assumption was made that a particular CEO would talk in a normal and reserved manner It was discovered at the first event that the CEO spoke like a motivational Speaker sending his input into distortion Needless to say some quick adjustments had to be made B Microphone Automixing 1 Turn on or activate the AEC and automixer for each microphone input Open the output meter for the ALS telephone hybrid or codec We will use this meter to monitor the overall voice signals being mixed Again AEC and automixing are beyond the scope of this document and will be covered in future publications 2 With as many people as you can gather have everyone position themselves at the various microphones and conduct a mock meeting Watch the output meter to ascertain if the voice mix is adequate Is the signal hovering around 4 dBu 17 dBFS Is it too low or too high Page 25 Extron Audio Gain Structure for Pro AV Systems Page 26 3 If necessary minor gain adjustments of a couple of dB can be made at the output matrix mixing points 3 in Figure 10 If major adjustments are needed go back through the microphone signal path to make
7. push the blank configuration to the unit with all settings at their defaults This includes gain settings at unity or O dB and no mixing or DSP processing activated If a DSP configuration file has been provided for you then open the file Set all processing objects to bypass mode all gain settings to unity and mute all inputs and outputs to put the DSP into a safe status Be sure to push the updated configuration settings to the DSP Amplifier Ceiling Speakers Signal Generator Sine Wave at 1 kHz 4 dBu Amplifier Program Speakers 4 4m Codec Recorder ALS Input ADC DAC A 1 kHz 4 dBu sine wave is used as the signal reference for confirming O dB or unity gain within the DSP Page 11 Extron Audio Gain Structure for Pro AV Systems Application Reference Equivalent Signal Level Voltage Level Consumer 10 dBV 316 mV rms Professional 4 dBu 1 23 V rms Broadcast 8 dBu 1 95 V rms Table 1 Standard audio reference levels b For a DSP based on an open architecture a new configuration should have all input and output levels at unity and no connections between the inputs and outputs If you are presented with a pre programmed DSP configuration set gain levels throughout to unity or O dB bypass all processing objects and mute all inputs and outputs Push the updated configuration settings to the DSP 2 Set up the audio signal generator for a sine wave at 1
8. random approach 12 00 is easy to remember easy to see in the back of the rack and is generally 10 dB below maximum A buffer of 10 dB helps to account for headroom and provides a good margin of safety for amplifiers that are almost always larger than needed and speakers that are often tapped higher than necessary Audio Gain Structure for Pro AV Systems Extron The use of 12 00 is one of acceptable convenience It is a case of practicality outweighing precision Using 9 00 or 3 00 or any other setting is just as viable What is important is to be consistent from amp to amp and indeed from project to project so that anyone servicing the system can easily restore the system to nominal settings So when is it appropriate to set the amplifier input gain above or below 12 00 When the amplifier power rating or loudspeaker tap value is larger than needed you may find that turning the amplifier input gains lower than 12 00 keeps the noise floor down and gives you plenty of gain When the amplifier power rating or loudspeaker tap value is smaller than needed turning the amplifier input gains higher than 12 00 will get you sufficient gain but at the expense of headroom Figure 7 illustrates the procedure for setting up the amplifiers and loudspeakers If at this point you are pressed for time and the system has only one or two loudspeaker zones you could skip the following steps and proceed to section D 1 Begin with the input sensitiv
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10. 2 Deciding whether to put videoconferencing far end content into ceiling speakers or program speakers is rather interesting Suffice it to say ceiling speakers provide a more consistent level of the far end audio throughout the room and as such makes it easier for the AEC algorithm to do its job 3 To establish an AEC echo reference route the telephone hybrid videoconference codec and program submix to the appropriate echo reference destination In some DSP models this is a bus and in other DSP products this is a direct link to each microphone input For our example using an Extron DMP 128 Virtual Return D will be used for the AEC echo reference 3 in Figure 12 AEC configuration will be covered in detail in a subsequent publication Page 23 Extron Audio Gain Structure for Pro AV Systems EE SP Configurator Conference Room 2A DMP 128 C P AT po Een LR er Outputs il TRIM 08 Mp pra Kum f vou Ceiling Spkr pr D jp mme 7 TRIM SEOUD EJ Donon m 2 25 5 a gt Doo S cmm s TRIM BOVE BLY BIE me sodec Tx EJ rai Love zomp s E 71 J Virtual Send Bus Expansion Outputs ENTRE i a E A J s emm D hh th m s em D 98 5 9 5 5 2 3 09g erum gt 53 999 5302 LE fe oem Eo GS CS ap 4 1 can race DAN sue ow Pousti HR Coc CAA e HOGG IOIR IE DE HHAH jamm EET oo Ht NEN m NE DE Ia ua o ue aa y ELI memes O EI x mem Ss so ney HRS 7
11. Gain Structure for Pro AV Systems Extron e Sound pressure level meter A device that measures the level of sound in a given location e Signal generator A device that generates audio reference signals such as an Extron VTG 400 Series test generator Acoustical sound source such as a portable loudspeaker Used to approximate the human voice for configuring microphones It should be noted that smartphones tablets and laptops can be used with specific apps to generate test signals measure sound pressure levels and function as a loudspeaker The use of these devices should be considered when in a situation without proper test gear However while such devices are handy they are not a true substitution for purpose made properly calibrated audio test equipment The sidebar on the next page provides more detailed information on the equipment essential to properly setting up gain structure Setting Gain Structure for Pro AV Systems It may seem complex and overly time consuming to properly set up an audio system However it really takes just four essential steps to successfully set up gain structure We ve developed a straightforward step by step procedure that encompasses them and can be applied to any sound system installation in the field By following this procedure step by step AV technicians can get an audio system up and running with very good performance Audio professionals will find this information somewhat more simplif
12. and set up the system to achieve desired room SPL and levels to external devices Audio Gain Structure for Pro AV Systems Extron A Initial DSP Setup Ultimately the goal in setting up gain structure for a DSP is to attain the best signal to noise ratio that the system will afford with nominal source signals coming into and exiting from the DSP at suitable levels and extreme signal levels handled without clipping It is particularly important that the input analog to digital converters be capable of handling the full dynamic range of incoming signals without overloading As an example when properly set up the Extron DMP 128 provides 17 dB of available headroom above 4 dBu nominal level which should be more than plenty for the speech and music content typically encountered in pro AV applications Begin the following setup after all necessary connections have been completed This includes the DSP outputs to the amplifiers amplifier connections to the loudspeakers and power Y Be sure that all amplifiers are powered off before continuing 1 Connect your laptop to the DSP and launch the DSP application software Load the configuration file provided to you or begin with a new blank configuration with the specific model of the DSP you are working with Establish a live connection to the DSP by setting the software to the live or online mode a If the DSP is based on a fixed processing architecture such as the Extron DMP 128
13. any deficiencies and work around them But passing important system details onto others or remembering them three months later can be rather difficult Improper automixer operation An automixer manages multiple microphone signals to ensure low background noise clear and intelligible speech and maximum system gain before feedback However if the microphone Signal level coming into an automixer is too low the gate may cut off soft voices intermittently making them sound chopped If the microphone level coming into the automixer is too high the gate may not attenuate background noises such as participants shuffling their papers Also the automixer may be less effective in managing microphone signals when participants are conversing back and forth Difficult troubleshooting When service technicians are called to site they are often given the unenviable task of finding an issue in a system they are unfamiliar with When an AV company uses a consistent approach to audio gain structure the service technician needs less time to became familiar with the system and can jump right to finding the issue at hand Unhappy customers Inconsistent audio within and between systems can quickly turn a satisfied customer into an unhappy customer What Can Cause Bad Gain Structure There are many reasons why AV systems end up improperly calibrated for audio system gain Audio system setup and optimization which includes gain structure equalizatio
14. kHz with the output level set to 4 dBu This is the standard reference or nominal level for professional audio If available you can instead use the signal generator built into the DSP and skip ahead to 4 If the signal generator does not specify the output level in dBu use the multimeter to measure the voltage at the output of the generator A 4 dBu signal level is equal to 1 23 volts rms Table 1 lists standard reference signal levels for audio Figure 6 At unity gain a 4 dBu signal reference into an Extron DMP 128 will read 17 dBFS 3 Connect the signal generator to an input of the DSP see Figure 5 Unmute the input Then through the DSP Software use the DSP input meter to measure the level of the incoming 1 KHz sine wave With the DSP input gain at unity the meter should be reading nominal see Figure 6 Some DSP models will show incoming signal levels in dBu while Extron DSP products display them in units of dBFS dB Full Scale The sidebar Audio Signal Voltage Units provides more information about audio voltage units including dBu and dBFS 4 Route the audio signal generator through the matrix mixer to an unoccupied output of the DSP ensuring that all routes are set for unity or O dB of gain Then unmute the output and check the meter which should be reading the same or equivalent signal level as the generator You can further validate proper signal level by checking for 1 23 volts rms at the unoccu
15. mV 40 dBu dBV Volts 50 dBu dBu Audio professionals frequently convert between volts dBu and dBV A convenient online calculator is available at www extron com product audiotools aspx dBFS is a measurement unit for peak audio signals in the digital domain with O dBFS being the upper threshold for digital audio signals When connecting two audio DSP units using a digital audio pathway O dBFS in one unit will equate to O dBFS in the other with few exceptions In converting from or to the analog domain O dBFS represents the maximum peak signal level that can be accepted by the analog to digital and digital to analog converters O dBFS is most often correlated to a device s maximum input or output level expressed in dBu When connecting two audio DSP units using an analog audio pathway O dBFS in one unit may not necessarily equate to O dBFS in the other Any correlation between the devices is predicated on the maximum dBu output of the first device matching the maximum dBu input of the next device This is further complicated by the fact that dBFS is a peak measurement unit while dBu is an rms measurement unit It should be noted that there is no universal calculator for converting dBu or dBV to dBFS Given the difficulty of converting between these measurements it is still useful to have a frame of reference between the analog and digital domains In the nomograph dBu dBV and voltage scales have been aligned with the dBFS scale for an Ex
16. noise was introduced into the Extron DSP and the output of the DSP presented to the RTA The RTA was set to A B subtraction mode with the total average curve subtracted from the real time pink noise The resulting response on the RTA was an electrical facsimile of the average acoustic response of the loudspeakers Parametric filters were then employed on the DSP to flatten the response curve on the RTA Those filters were then saved into the Building Block for the specific Extron ceiling speaker model The purpose of the loudspeaker Building Blocks is to normalize the inherently non linear response of a ceiling speaker array as opposed to flattening the on axis response of a single loudspeaker Depending on the source additional sweetening of the signal may be desired For microphones a high pass filter should be employed to eliminate popping and in certain cases tone shaping of the frequency range for speech may be desired For music sources a bass boost is often employed to sweeten the sound at lower levels Since the sweetening for microphones and music sources are for all intents and purposes somewhat opposite adding this to the outputs would render an output ideal for one or the other but not both As such Extron has included the sweetening curves into the input and virtual bus Building Blocks allowing voice lift and music to coexist in the same loudspeaker system while maintaining attributes needed for both Page 29 E
17. of frequencies vibrates sympathetically with a physical attribute of the loudspeakers and or the room Helping to avoid feedback Equalization procedures vary and are outside the scope of this document It is a very important topic that will be addressed in a future publication After equalization has been completed adjustments for EQ and any other DSP signal processing may have boosted or attenuated levels such that the reference SPL levels set in section C above may no longer be accurate If time permits it is recommended to repeat Steps 4 through 6 in section C above for each of the amplifier outputs or loudspeaker zones Doing so ensures even coverage between loudspeaker zones after equalization If you are using an Extron DSP and have an installation of Extron ceiling speakers you could apply Extron Building Blocks as a time saving alternative to performing an equalization procedure Building Blocks are a collection of pre designed processor settings optimized for various input and output devices with predefined levels filters equalization Settings dynamics and more For Extron ceiling speakers Building Blocks provide equalization curves determined from extensive sound power response measurements of an array of nine loudspeakers of a given Extron ceiling Speaker model within an acoustically controlled environment More information about Building Blocks can be found in the Appendix E Other Destination Devices If other destina
18. send to the videoconferencing codec turned up too high You get the same levels in the end but you also introduce a lot of noise 1 In the matrix mixer unmute and slowly turn up the voice lift gain control not the microphone submix volume control This is 4 in Figure 10 Audio Gain Structure for Pro AV Systems Extron 2 Have someone speak into a microphone at a normal talking level Gradually bring up the voice lift gain control until you have reached a desired listening level 3 Listen for intelligibility In a very live room without good acoustical treatment reflections off of hard surfaces can compromise voice intelligibility To a small degree intelligibility problems can be dealt with using a few tricks such as 1 Applying some tone shaping to the microphone voice lift submix such as a boost at 2 kHz 2 Adding an 8 kHz low pass filter to the microphone voice lift submix Always check signal levels after applying equalization filtering or other processing The only hard and fast rule when trying to fix a room with DSP processing is Don t expect much The only proper way to address poor room acoustics is to incorporate acoustic treatments G Recorder and Assistive Listening System Check incoming signal levels for speech program and far end at the meters of the recorder For the ALS listen to the audio through the headphones Adjust levels as necessary at the volume control for the ALS output channel Final Consi
19. sure all matrix mixing points are at unity or O dB If so there may be improper settings in the automixer Bypass or deactivate the automixer to determine whether the problem is corrected Microphone Voice Lift Levels No not yet Program Audio Ts At the DSP program submix for program audio 3 in Figure 11 reduce the gain controls about halfway At this point the amplifiers have been powered off since Step 2 so turn them back on Play some program content that is fairly consistent in level and bring up the program submix gain until you can hear the music a little bit Monitor the output meters to ascertain whether the program level is adequate Is the signal hovering around 4 dBu 17 dBFS Is it too low or too high Much like the microphone mix minor adjustments of a couple dB can be made at the output matrix mixing points 2 in Figure 11 If major adjustments are needed go back through the program signal path to make sure all matrix mixing points are at unity or O dB Also check whether program signals are coming into the DSP inputs at a Sufficient level Make adjustments until you have a good solid signal to the outputs Play some program content from each source and switch between the sources to ensure that there are no major differences in level aside from differences in content Bring up the program submix gain controls until the level sounds and feels good in the room If desired add s
20. system gain structure is the result of careful selection of equipment and properly setting audio levels through each gain control within each device allowing for optimal signal to noise ratio and headroom Understanding Gain Structure To help get a better sense of gain structure in an audio system let s use plumbing as an analogy The water that we drink originates from a water tower or reservoir and then travels through a maze of pipes and valves before it comes flowing out of our faucets If some or all of the valves along the way are not opened enough we get a dribble of water from our faucet as illustrated in Figure 1a We are not getting enough water and the pressure is so low that the water is picking up impurities from the pipes If all of the valves along the way are opened too much we get plenty of water pressure but we run the risk of bursting any of the pipes along the way See Figure 1b In this case we are getting too much With a mixture of valves opened too much and valves opened not enough it is possible to get sufficient water pressure at the faucet but the water will have impurities and we run the risk of bursting a pipe See Figure 1c By setting all valves to the proper pressure we get sufficient water pressure at the faucet and enough pressure in the pipes to ensure no impurities and the pipes won t burst as shown in Figure 1d This is the ideal scenario for this plumbing system Figure 1a Es Figure 1c
21. Best Practices Guide Audio Gain Structure for Pro AV Systems By John Fish Consultant Applications Engineer S Extron Electronics ERFACING SWITCHING AND CONTROL Extron Audio Gain Structure for Pro AV Systems Contents WHO SHOULD HEAD THIS 2 2c eod eoe erg 00 4 vere rr vs 4 WHAT IS GAIN STRUCTURE issskanai kat AERRREERERRRER E E RE 4 UNDERSTANDING GAIN STRUCTURE leeeee eese 4 How Does Plumbing Relate to Audio Gain Structure lille 5 WHY IS PROPER GAIN STRUCTURE NEEDED 5 What Can Be Expected With Proper Gain Structure 0005 5 What Can Happen With Improper Gain Structure 0 0 0 eee eee 6 WHAT CAN CAUSE BAD GAIN STRUCTURE lees 6 HOW DO I GET PROPER GAIN STRUCTURE leeee 7 WHAT TOOLS ARE NEEDED FOR SETTING UP GAIN STRUCTURE 7 Essential Tools for Setting System Gain Structure llle 8 SETTING GAIN STRUCTURE FOR PRO AV SYSTEMS 9 THE FOUR ESSENTIAL STEPS iizccu n rr RLERRrRRRREE EET 10 GAIN STRUCTURE PROCEDURE FOR PRO AV SYSTEMS 10 Step 1 Back End DSP Outputs to Amplifiers Loudspeakers aud Oiler Desumellolis ac dd E Kees aes 04 REQUE ERR I ECT dE 10 A Ma ES E SS DE asc arat cic at e etc ph e dot n d ecc dL d 11 Audio Signal Voltage Units 0 0 ccc ee 13 Be ATTI CablinGian goto Gt oue m e abd eed ade PO he eas de daas 14 CAMOM S 3 0 S
22. L Video Production 78 dB SPL Music Production 78 93 dB SPL Table 2 Typical reference sound pressure levels Some information in this table from Holman T 2008 Surround Sound Up and Running Second Edition Burlington MA Focal Press If available you can use the internal test signal generator of the DSP instead of the external generator Adjust the pink noise signal level within the DSP test generator module Reading pink noise level on a meter is notoriously difficult due to the fact that the reading jumps around a lot Do your best to average the result in your head and don t try to be too precise 2 Set up the SPL meter for A weighting slow response measurements Mount the SPL meter on a tripod If using a microphone for SPL measurements mount it on a microphone stand The microphone should be at ear height for the typical seated or standing listener You can also just hold a handheld meter in position Refer to the SPL meter s user manual for information on how to properly orient the SPL meter or microphone If in doubt point the microphone 45 degrees away from the loudspeaker 3 Turn on all amplifiers Select any one of the DSP outputs pull the corresponding gain level down halfway and then unmute it Slowly bring the gain level back up until the pink noise seems sufficiently loud while still sounding comfortable At this point the gain at the DSP output will be adjusted so that at nominal or unity for th
23. TOO ony Boundary Conference Table ENEO Gm Ceiling Microphone 5 ft 1 5 m Handheld 0 5 ft 15 cm Wireless Lavaliere 0 5 ft 15 cm Wireless Headset 2 in 5 cm Table 3 Typical talker to microphone distances Why 65 68 dB SPL Depending upon which textbook or resource you use standard conversational speech is defined as measuring 60 63 dB SPL at 1 meter Similarly a raised human voice is defined as measuring 65 68 dB SPL at 1 meter Humans have a tendency to raise their voices when talking on the phone as well as in meetings with a bit of heated discussion For this reason we have selected a raised human voice as our sonic reference 2 Position the test loudspeaker at a microphone using the guidelines in Table 3 for talker to microphone distances If in doubt place the loudspeaker where the talker s head would be If the microphone is a condenser mic be sure phantom power has been selected at the DSP input Check the input meter and adjust the gain so that the incoming mic signal averages around nominal with readings between 19 dBFS to 15 dBFS for an Extron DSP or between 2 dBu and 6 dBu if using a DSP from another manufacturer 3 Repeat Step 2 for all microphones Note Steps 2 and 3 will ensure that the sensitivities of all microphones are the same with respect to a human voice D Program Sources Setting up program sources including Blu ray Disc computers and portable media players is less straightforward than
24. asi rptu aie edes Geeky dette ga duit Sepia quee aded 14 D Ecualizeuobl s Sa patio de to dog corno cabo a edna a Chak oe ane ee 17 E Other Destination Devices llli 17 Step 2 Front End Microphones and Source Devices to DSP Inputs 17 A Microphone BD sor shield cakes ope deli ode d a ERRGBIE Hd qoi ER d 18 Es Mital DSP SUDO aa dr ee dtc mtt nere drca d etna deii ad in 18 C IMIGMOPNONGS ss 3 xxckicnkdobm eo px Sete w esse Adobe b RT 18 Iv Progra SOURCES apros 62 ae EQ qc dor ddr e Trident MO Rew Be d 19 E Switchers and Matrix Switchers is s asees llle 20 Step 3 DSP Routing Processing and Internal Gain Structure 21 A Microphone SubImiX 43a cse vc RA RpDEGGU wae ee ee ewes Daa ens 21 B Programi SUHM eses snas erann aro RR POR rre Un m he Grae o 22 C Telephone Hybrid and Videoconferencing Codec Inputs 22 Page 2 Audio Gain Structure for Pro AV Systems Extron Step 4 Listening Tests and Final System Adjustments 005 24 A Microphone IMPUS s sc aras ara iier stintata koeste tnet ap eu 25 B Microphone Automixing csse 25 C Microphone Voice Lift Levels llle 26 D Program ANGIO auis 20290 8938 Alone deiode qi edes a EET BOR le PUE do ad 26 E Goriterenclng AUCIO aig asta eet ER buna eR OE RR EROR AC Rog n es 26 F Microphone Voice Lith i a aura 80 ERR RE esa dea Y PRAE ee aw ae ee 26 G Recorder and Assistive Listening System 000000 0 e
25. at the amplifier outputs Most amplifiers have relays that disconnect the amp from the loudspeaker line when turned off If your amplifier does not have this feature it will be necessary to disconnect the loudspeaker circuit from the back of the amplifier in order to perform the impedance measurements Always measure the impedance of all loudspeaker circuits before turning on any amplifiers This is very important to ensure that there are no dead shorts to the amplifier outputs For a constant voltage system the impedance measured should not be lower than the rated minimum impedance for the amplifier For further information about testing loudspeaker impedances refer to the owner s manual for your measurement device If an impedance meter is not available a DC resistance measurement can be made with a digital multimeter While not completely accurate for determining loudspeaker impedance a multimeter will give you a reasonable approximation and will definitively indicate dead shorts or open circuits 3 At this point you have fully tested all cable connections to and from the amplifiers ensuring that once they are turned on there will be no damage Amplifiers The next step is to establish proper gain structure at the amplifier inputs The conventional approach is to adjust each of the amplifier input sensitivity controls with a signal reference introduced into the system at a pre determined level A typical signal reference is pink nois
26. cement should be in the room For the purpose of this article we will call this matrix gain point the voice lift gain control It is also viable to use a separate submix or virtual bus for the voice lift gain control A benefit of using a separate submix is the ability to apply feedback suppression specifically to the voice lift system without affecting the signal being sent to the far end of a conferencing session But remember this gain point must either exist after or be ganged together with the microphone submix gain control shown as 4 in Figure 10 Page 21 Extron Audio Gain Structure for Pro AV Systems B DSP Configurator Conference Room 2A DMP 128 C P AT Co E Tieton Vew Toor Wadon Hep E er C Outputs E gt OOS e gt oD m Th SID SB pes m ES s Dooa Dogo as 2 2 90 9C oa Dots lt 7 DOH a EE E iind Virtual Send Bus AOC lll lll THEE we 3 Eee m 5 emmm E GAIN AAE MAD GAIN E DYN E CMP DIN T AN s GAIN ome cain A BASS D AN GAIN TREB i E BASS P u ray GAIN Face DYN S CMP DLy pues SAN 715 ome Do ooo Rx GAIN FILT GATE P E UCKS AM m T Se FES FILT SCMP LOUD 5 DLY a ETE SIUS Fes L8 55 Mong Br oup E ppy TREB eR DLL T 98 3 Fes L8 55 Komp BLoup 5 Diy TREB E SFI EY S100 DIY me omm Extron DSP Figure 10 Mic submix and routing with
27. derations Test the system to determine whether feedback can be introduced when adjusting the volume control points It may be necessary to add soft limits into the DSP program to prevent a user from increasing volume to the point of feedback It is recommended that a small amount of signal compression be added in the DSP to protect against signal overload at the output digital to analog converters This can be added to the inputs the submixes or the outputs Remember when it comes to compression less is more Set the threshold as high as possible and the ratio as low as possible to allow system protection without affecting normal signal levels Extron Building Blocks Simplify DSP Setup For Extron DSP products Building Blocks can simplify setup for various source devices destinations including Extron ceiling speakers and microphone and program submixes Extron has created Building Blocks for many different makes models and types of microphones as well as program sources including Blu ray Disc players and PCs Building Blocks can be used to quickly add new devices to a system or as a starting point for further fine tuning More information is available in the Appendix Summary Properly setting gain levels is a very important factor in allowing audio systems to perform to their full potential The ultimate goal is to achieve excellent sound quality that meets the end user s expectations for performance Following a logical method for setting
28. e at 10 or 15 dB below nominal The procedure begins with all amplifier input sensitivity controls set to full attenuation The technician then turns up an individual amplifier input control until the desired sound pressure level is measured from the speakers The audio is muted and the procedure repeated for each remaining amplifier input Once setup is completed for the entire system the amplifier input control settings are marked often by stickers in the form of dots or arrows This is done so that the system can be brought back to optimum operation after an amplifier replacement or if the levels have been altered In the days before detented gain controls it was common practice to measure voltage gain for each amplifier channel and include the measurements in the system verification test reports Technically this procedure is the proper approach to setting amplifier input sensitivity However for pro AV systems that utilize audio DSP it is more practical to fix all amplifier input gains at a predetermined level and make individual adjustments within the audio DSP By setting all amplifier gain controls the same it is easy to make a quick visual inspection and detect anything out of the ordinary With the individual output levels stored in the audio DSP restoring the system is as easy as recalling a preset or configuration file A safe default setting for the amplifier input gain controls is to set them all at 12 00 While this may seem to bea
29. e certainly is much more to it but discipline is priority number one There is always pressure to get the systems running and playing music as soon as possible This pressure comes from fellow technicians other trades anxious customers and from within ourselves Succumbing to this temptation can lead to a disappointing first impression Discipline means being able to insist to everyone that you need to complete your setup because the system isn t ready yet It also means that the first things they are going to hear are test tones and pink noise not the customer s favorite music selections or an awesome movie sound clip This actually works in your favor since there is no better way to clear a room than to play pink noise for 45 minutes straight Priority number two is creativity Site conditions and project schedules may limit how much you can get done at any given point in time Sometimes you need to be creative to get results given the practical limitations What Tools Are Needed for Setting Up Gain Structure Equipment for testing measuring and evaluating audio systems can include many types of audio devices but here we will only list what is necessary for setting up audio system gain structure For that an AV technician should have the following Digital voltmeter or multimeter A device that measures rms voltage Ideally it would be rated for a 20 Hz to 20 kHz frequency response Sound Pressure Level Meter is Test Signa
30. e channel displays a meter level of 17 dBFS Given the maximum output level of 21 dBu for the device O dBFS is equal to 21 dBu Therefore a level of 17 dBFS equals 4 dBu 21 dBu 17 dB 4 dBu When the target level is achieved at the input maintaining unity gain through the System will produce 4 dBu at the output Line Input Building Blocks Extron Building Blocks for line level devices set the gain level for the specific device and its operating line level Bass and treble shelving filters are inserted into the filter block with the gain level set to O dB This allows for tone adjustment if necessary either via the DSP Configurator Software or by programming the gain parameters into a control system and allowing the user to make adjustments Light compression is added to the channel to normalize diverse signal levels with threshold set at 17 dBFS and a 2 1 compression ratio The compressor threshold is at target Audio Gain Structure for Pro AV Systems Extron level so as not to greatly affect unity gain at the target level through the system The table below shows different line level device categories and their operating line level Gain compensation is based on the operating level and the gain required to bring the signal up to the target level When the target level is achieved at the input maintaining unity gain through the system will produce 4 dBu at the output Gain compensation for line levels in dBV is ca
31. e system the loudspeakers are playing at a desired reference SPL Reference SPLs for various audio and AV applications are listed in Table 2 For our purposes here we will use a reference of 75 dB SPL a level applicable to most pro AV sound system applications 4 Moving the SPL meter or microphone around measure the SPL at several locations within the loudspeaker zone and determine the average 5 Adjust the DSP output gain and repeat Step 4 until you measure an average SPL of about 65 dB Recall from Figure 7 that the 65 dB SPL is equal to the 75 dB SPL reference minus a 10 dB offset Once you have confirmed the 65 dB average SPL mute the DSP output Note Recall from Figure 7 that we set the signal generator to 10 dB below nominal Our measurement would therefore be 10 dB less than the desired reference level A 75 dB SPL reference minus the 10 dB offset yields a 65 dB SPL target 6 Repeat Steps 3 through 5 for the remaining outputs that feed the amplifiers Audio Gain Structure for Pro AV Systems Extron D Equalization Once the reference SPL has been established for all of the loudspeaker zones you can now begin the process of equalizing the frequency response of the loudspeaker system There are a number of reasons to perform system equalization such as Ensuring consistent tonal quality from one speaker zone to the next e Eliminating potentially annoying resonances Resonance occurs when sound at a frequency or band
32. ee 2f FINAL CONSIDERATIONS 0 00 cece eee eee eee nn nnn 27 EXTRON BUILDING BLOCKS SIMPLIFY DSP SETUP 20055 27 SUMMARY 54 434226 5GRh ied Reds eee RRO HEEL ot os 27 APPENDIX 1 EXTRON BUILDING BLOCKS 00 0c eee enews 28 APPENDIX 2 REFERENCES 0600 cetstcctvotwssesiebseveeteuereaas 30 Page 3 Extron Audio Gain Structure for Pro AV Systems Page 4 Who Should Read This In short you should AV technicians need to know gain structure so that they can set up systems in a better faster and more efficient manner AV engineers need to know gain structure so that they can design systems with proper interfacing components allowing the system to perform at its best Senior AV engineers should read this as a potential resource to educate their technical staff Salespeople should read this to obtain a cursory understanding of gain structure and how it affects the bill of materials What is Gain Structure Gain structure can be thought of as the sequence of level settings applied to audio signals as they pass through various devices in an audio or AV system When proper gain structure is applied the system as a whole performs its best with speech music and other content sounding clear dynamic and free of distortion Many audio devices such as audio digital signal processors preamplifiers mixers AV switchers amplifiers and some source devices include one or more user accessible gain controls Proper audio
33. er in front of the DSP In these cases set the switcher input and output gain levels to unity using the internal metering if available or measure the signal at the switcher output or DSP input ni Mics Amplifier Ceiling Speakers Blu ray Player Amplifier Program Speakers a e Codec Recorder ALS piunt SS SS peque ee SS A A 4 l l l Mic 1 Output Output Gain l Automixer Mic Submix Matrix Mixing l Mic 2 l i l Output l l Output Gain MicN l l l l Blu ray Output i Input Gain Output Gain l Program Submix Laptop l l Input Gain t mi As a starting point for establishing gain structure within the DSP microphone and program sources should be grouped into submixes and then routed to their respective outputs Page 20 Audio Gain Structure for Pro AV Systems Extron Set up each input source as in section D above making sure to account for level differences between consumer and pro devices DSP Routing Processing and Internal Gain Structure In this step we will create submixes in the DSP for the microphones and program sources Then we will set up the routing within the DSP for these submixes as well as the telephone hybrid and videoconferencing unit With nominal levels established for incoming and outgoing signals of the DSP the next step is to bring them together into a working system by creating two submixes one for the microphones and another f
34. evice to achieve O VU on its meters If no such input level controls are available use the appropriate outputs of the audio DSP If using an ALS without metering such as a magnetic induction loop it will be necessary to monitor the level by listening through the headphones Set the generator to output pink noise at 6 dBu Remember that this is 4 dBu minus 10 dB to provide a comfortable listening level Front End Microphones and Source Devices to DSP Inputs In this step we will test all front end cable connections and then set up gains at the DSP inputs for microphones program sources and external switchers Page 17 Extron Audio Gain Structure for Pro AV Systems Page 18 A Microphone Cabling 1 Confirm that all amplifiers are turned off Set the generator for a 1 KHz sine wave at 30 dBu The 30 dBu level roughly approximates a nominal microphone signal Unmute all microphone inputs at the DSP 2 At a microphone location disconnect the microphone from the cable and connect the generator 3 Check for a 30 dBu reading at the DSP input Then reconnect the cable to the microphone Repeat this procedure for all other microphones and wireless microphone base stations Once you are finished you have now confirmed proper cabling for all microphones in the system B Initial DSP Setup It is essential that a high pass filter be added to microphone inputs for attenuating low frequency rumbles as well as aA pl
35. ferencing Codec Inputs 1 Route the telephone hybrid to the DSP outputs feeding the ceiling speakers the videoconference codec and if used the ALS and recording system see Figure 12 1 Page 22 Audio Gain Structure for Pro AV Systems Extron Bi DSP Configurator Conference Room 2A DMP 128 C P AT o a Fie E Viv Th Widen Hep omman qus Tes o T C Outputs D mjp D 7 e e aem Th SID coge ES s Soo 6 DoS a s Dogo TRIM THT Codec Tx E Dao mmm s EE E dl Inputs Virtual Send Bus n mem D ICI SS B DESI uu THERE aree Somme EIS m ESS IS HHH 5 Wu Ey FILT I AEC g DYN cK DLY pon AMA gt GAIN f D vm D 143 eee SD Bul Returns FBS 5 FILT E OUD ony GAIN fl ap RD poe mE c Sa FRES CMP Loup DEY m ee a D m HE T ae eee eei E E Ed dip oa eee ae e 5 L i Ti Sue d a i m au i a D Figure 11 Program submix and routing 1 The laptop and Blu ray Disc stereo inputs are routed to Virtual Returns C and D to create a program left right submix 2 The submix is then routed to the outputs for the program speakers ALS hybrid and codec 3 Gain blocks are selected for a control system to provide user adjustment of program volume 2 Route the videoconferencing codec to the DSP outputs feeding the ceiling speakers the telephone hybrid and if used the ALS and recording system see Figure 12
36. gain structure results in consistent performance from system to system A system with good gain structure will often seem deceptively simple That is its elegance Page 27 Extron Audio Gain Structure for Pro AV Systems Appendix 1 Extron Building Blocks Extron Building Blocks are a quick configuration tool that can significantly reduce configuration time A Building Block is a collection of processor and gain settings for a particular portion of the signal chain This allows configuration of an entire microphone input line input virtual bus return or line output channel with just two mouse clicks A comprehensive set of preconfigured Building Blocks is available within the DSP Configurator Software If you are unfamiliar with DSP processing these Building Blocks may provide all that you need for complete configuration of your Extron DSP product The preconfigured Building Blocks can also be used as a starting point providing a basic device setup while staging an installation and a basis for fine tuning the system during commissioning in the field Building blocks can be modified and then saved allowing you to create a customized set of tools unique to your needs Alternatively Building Blocks can be built from a blank screen or an existing configuration allowing you total flexibility in how you create and deploy this useful and time saving feature Microphone Building Blocks Building Blocks Extron Building Blocks for 3
37. ic Range 88 dB 105 dB 60 dBFS Outside of Meter Range Noise Floor 105 dBFS Audio signal levels should be sufficiently high to ensure a very low noise floor but also allow for ample headroom to avoid signal peaks from being clipped Specifications and meter range are shown for an Extron DMP 128 audio DSP Targeting a nominal or average signal level of around 4 dBu which correlates with 17 dBFS in the DMP 128 allows for about 17 dB of headroom more than sufficient for pro AV applications What Can Be Expected With Proper Gain Structure More benefits than one would expect There is actually a return on investment when audio systems are set up properly e Clear undistorted audio Getting the best possible sound reproduction from the system Voices are intelligible and the music sounds good No hiss How many times have you stood near a loudspeaker and heard a hiss This is a good indication of an improperly set up gain structure A consistent mix Everyone will be heard equally in a conferencing application with the far end hearing the same thing as the near end and the recording being a good representation of both In following proper gain structure a well balanced mix becomes inherent to the system Page 5 Extron Audio Gain Structure for Pro AV Systems Page 6 Optimum acoustic echo cancellation Echo cancellation has come a long way in the past 10 years but it is not foolpro
38. ied than the procedures they use for large scale sound reinforcement systems The guidelines presented here are intended to address the practical needs and challenges specific to pro AV applications Before we proceed let s take a moment to clarify a typical sound system in a pro AV application Today s systems are typically based on an audio digital signal processor DSP that functions as the central audio device for signal routing and processing See Figure 3 The system includes source devices such as microphones media players and laptop or desktop PCs which are collectively referred to as the front end of the system There are also destination devices such Front End Back End Sources Destinations l Ceiling Speakers Mics Amplifier Blu ray Player Amplifier Program Speakers a e Codec Recorder ALS 9 9 7 A a Input ADCs Processing Mixing amp Routing Processing Output DACs ee eT Recorder L A typical sound system in a pro AV application including the front end or sources a DSP as the central audio device for signal routing and processing and the back end or destinations Page 9 Extron Audio Gain Structure for Pro AV Systems Back End DSP Outputs to Amplifers Loudspeakers and Other Destinations Begin gain structure setup with an initial configuration of the DSP Then test all back end cable connections and Set up the system
39. ilding Blocks The output channel Building Block inserts an equalization curve into the output signal chain of the DSP A compressor is inserted on the output set to a nominal level for loudspeaker protection The output level is given a nominal setting from which you can make final adjustments to suit the room Line output Building Blocks are provided for program output voice lift output and select models of Extron ceiling speakers The equalization curves used for the loudspeaker Building Blocks were generated by measuring the sound power response of an array of nine 9 identical loudspeakers installed in a large acoustically dead room For testing 400 samples were taken throughout the sound field of the loudspeaker array and averaged together to determine the average sound power response of the nine ceiling speakers This was repeated with the loudspeaker array at 6 feet 1 8 m 8 feet 2 4 m and 12 feet 3 7 m spacing between loudspeakers The results were merged into a single response curve for all three configurations comprised of over 1 200 samples total average Note An array of four 4 Extron FF 120T and FF 220T loudspeakers were used at 12 feet 16 feet 4 9 m and 20 feet 6 1 m spacing since the Flat Field Technology provides more consistent sound levels across the listening area than conventional coverage patterns This total average curve was then inverted and loaded into an RTA real time analyzer Calibrated pink
40. in the DSP Configurator Software for an Extron DMP 128 in a conferencing application 1 AEC and automixer processing blocks are first added to each of the five mic inputs 2 The mic signals are routed into Virtual Return A to create a submix 8 and 5 The submix is then routed to the outputs for the ceiling speakers ALS telephone hybrid and codec 4 A gain block is selected for a control system to provide user adjustment of microphone volume B Program Submix 1 Create a submix for the program input signal paths For an Extron DSP such as the DMP 128 the submix will be created by matrix mixing the signals into a virtual bus See Figure 11 1 2 Now route the program submix to the DSP outputs feeding the program speakers the videoconference codec the telephone hybrid and if used the ALS and recording system This is shown as 2 in Figure 11 3 With the program submix signal path now created you can again define or incorporate a volume control point for interfacing into a control system This volume control point will be used to provide a user control for adjusting the program submix level It should be separate from the gain controls at the routing points to the outputs As with the microphone submix the gains at the outputs should never be used for volume control In our example we will use the submix gains in Virtual Returns C and D as the program volume control point 3 in Figure 11 C Telephone Hybrid and Videocon
41. ity controls for each amplifier set at 12 00 Next set the audio signal generator to output pink noise and reduce the output level by 10 dB from 4 dBu to 6 dBu or equivalently from 1 23 volts rms to 388 millivolts rms For the Extron DMP 128 the meter at the input gain should hover around 27 dBFS which is 10 dB below 17 dBFS Attenuating the output of the generator by 10 dB will provide for a comfortable listening level once the amplifiers are turned on Amplifier input sensitivity controls set to 12 00 or midway between min and max Ceiling Speakers Amplifier DSP SPL Signal Generator Meter Pink Noise at 6 dBu Amplifier 65 dB SPL Program Speakers averegediom several locations a em Codec Recorder ALS r 1 l l l ipi Output i l Output Gain Input Gain Matrix Mixing k i l Output l Output Gain l l l l Output I l Output Gain i L J A pink noise test signal is used to establish a 75 dB SPL reference for the listening environment The signal is attenuated by 10 dB from the 4 dBu pro audio reference level so that the SPL meter reads 65 dB SPL a more comfortable listening level for pink noise Page 15 Extron Audio Gain Structure for Pro AV Systems Page 16 Reference Sound Appleen Pressure Level Sound Reinforcement 75 dB SPL Conference Rooms Sound Reinforcement 85 dB SPL Auditoriums and higher Movie Sound 85 dB SP
42. l Source m Acoustical Sound Source Proper audio gain structure requires some essential tools for making sound measurements and generating audio reference signals Page 7 Extron Audio Gain Structure for Pro AV Systems Essential Tools for Setting System Gain Structure Signal Generator Many audio test signal generators are available including the Extron VTG 400 Series video and audio test generators as well as the NTi Audio Minirator devices commonly used by audio professionals Software audio test generators are also available for iOS Android devices and PCs It should be noted that these consumer devices will not output at the 4 dBu professional reference level unless they are used with a pro audio interface Essential Kit Economical tools for an AV technician including a general purpose SPL meter smartphone or tablet multimeter and an acoustical Source such as a portable powered speaker They are ideal for quick setup of audio systems with proper gain settings Digital Voltmeter or Multimeter A multimeter such as the classic Fluke 87 is an absolutely essential tool in setting up audio systems It is useful in setting rms voltage levels at the output of a test signal generator or other source as well as in checking for proper cable connections throughout the system A multimeter can also aid in confirming that there are no shorts at the amplifier outputs connected to the loudspeakers Enhanced Kit
43. lculated by first converting the source output level to dBu Source Type eho na Pa Target Level Consumer Player 10 dBV 411 8 dB 17 dBFS 4 dBu Computer Sound Card 0 dBV 41 8 dB 17 dBFS 4 dBu Pro Level Balanced 4 dBu 0 dB 17 dBFS 4 dBu Broadcast Level 8 dBu 4 dB 17 dBFS 4 dBu Virtual Bus Building Blocks Virtual paths within the DSP are often used to create a submix which provides a single point of control for managing a submix of microphones or program sources Submixes can also be processed as one composite signal as opposed to processing individual channels The virtual bus Building Blocks provide a starting point for submix processing and control Virtual bus Building Blocks are provided for the following submix scenarios e Microphone submix e Program channel submix For submixing microphones a high pass filter is inserted as well as bass and treble shelving filters Light compression is added which will help to protect both listeners and the system from many microphone channels activating at the same time Program channel submixes include bass and treble shelving filters plus light compression A loudness filter is also inserted in the path A program channel submix allows loudness processing to be added to program material independent of the microphones With the two submixes separately managed both microphones and program material can then be routed to the same output Line Output Bu
44. m Consumer gear with a line level of 10 dBY is approximately 12 dB lower in reference level than professional gear with a line level of 4 dBu Proper gain levels are essential in mixed consumer professional systems Not adhering to unity gain throughout the system e Setting gains to counteract wiring errors rather than checking for and fixing them first For a balanced audio cable if the or wire is not connected the circuit will pass audio but with a 6 dB loss Increasing the gain of that circuit fixes the immediate problem but what happens when the wiring connection is fixed later The audio signal is now 6 dB too high which could be enough to cause clipping distortion Even with sufficient time and budget available an additional challenge remains the need to ensure all technical staff is sufficiently trained and up to date Professional AV firms must not only embrace audio theory but video control and networking as well Adequately training and educating technical staff requires considerable time and resources Many excellent resources are available that detail the theory and concepts in setting up audio systems A list of references is provided in the Appendix It is from this body of knowledge that we have assembled a subset of practical procedures that technicians can use without prerequisite training and apply them to commercial and pro AV applications How Do I Get Proper Gain Structure In a word discipline Ther
45. microphones Precisely determining output levels from them can be difficult Although the standard reference level is 10 dBV or 316 mV for consumer devices nominal output levels often vary The headphone outputs of laptops and PC sound cards typically deliver around O dBV or 1 volt nominal output Blu ray Disc and DVD players usually have nominal output levels of 10 dBV Some program sources may deliver potentially unpredictable signal levels such as a combo VCR DVD player or a guest laptop connected into an open input For these sources consider applying automatic gain control AGC at the DSP input to normalize extreme signal levels 1 Computers When setting up a computer open the sound card mixer and set all sources and output levels to 7596 This is not a calibrated setting but a practical and repeatable setting that will provide an adequate signal level and a measure of safety By limiting a user to just 25 percent additional volume on the computer we help prevent the possibility of overloading the DSP input 2 Media DVD and Blu ray Disc There are three ways program source devices can be set up for a normalized nominal playback level through the System a Use calibrated commercially available test signals recorded at the standard 20 dBFS reference level to establish a 20 dBFS level at the DSP input These signals can be especially useful in setting up portable media players with variable audio outputs In the past test refere
46. n and other procedures requires adequate time and resources for signal measurements system programming and evaluation of audio performance Ideally this would occur after all AV equipment has been installed and connected acoustic treatments hung on the walls and all furniture in place Given all the usual procedures tasks and training necessary to complete the AV project on schedule there is often very limited time available for audio system setup This can be complicated further by construction delays pushing into fixed immovable completion dates AV can often be the victim of the big squeeze at the end of new construction projects With the commoditization of small and medium AV systems aggressive AV integration budgets sometimes do not provide for adequate audio setup and optimization This is an unfortunate but very real problem in the AV industry It is often the case that insufficient hours devoted to proper audio setup during installation later cut into the service budget Common practices that lead to improper gain structure include the following Improper input gain This is probably the most prevalent error Once an input gain has been set incorrectly it usually forces more errors all the way through the system to try to compensate Amplifier input gains set wide open with DSP output gains at less than 30 Audio Gain Structure for Pro AV Systems Extron Combining consumer gear with professional gear in the same syste
47. nce CDs have been available with signals recorded at 20 aBFS but are not as easy to find today There are also test reference DVDs and Blu ray Discs Internet resources are available for downloading test signals However beware that these signals are not necessarily calibrated to a standard reference level Page 19 Extron Audio Gain Structure for Pro AV Systems b Use the intended playback material from the end user This material can be talking or music content It is the best for setting up system nominal level since the content will actually be used in the application For talking content set the input gain so that the levels average between 20 dBFS and 15 dBFS on an Extron DSP such as the DMP 128 For music set the gain so the meters read approximately 15 dBFS to 12 dBFS with peaks at about 6 dBFS to ensure a safe 6 dB of headroom from the O dBFS clipping level C Use your own program material AV technicians should always have familiar audio material on hand This Should include talking content music and a sonic excerpt of a movie with very wide dynamic range The movie Sound clip can be useful in delivering signal level extremes to the DSP to check for clipping If the system is intended to be used for surround sound playback be sure that the peaks in movie content do not exceed 6 dBFS E Switchers and Matrix Switchers In some systems there may be video sources with analog audio connected into a switcher or matrix switch
48. of By providing an adequate signal and mix to the echo cancellation algorithm it is better able to do its job Commissioning time is reduced Following a systematic procedure for gain structure minimizes troubleshooting efforts saving money for your customer Service calls are expedited When an organization employs a logical and consistent approach to gain structure they establish a common baseline for all technicians to follow This unified approach aids the service technicians in resolving an issue in a quick and efficient manner What Can Happen With Improper Gain Structure Many more problems than one would expect Unfortunately audio systems are frequently performing below their potential because of improper or arbitrary gain settings Clipping Apart from the intended effects of turning up guitar amps to eleven distortion is not pleasing to the ear Slight distortion can be perceived as a tonal quality issue Many have tried to equalize it out not realizing that it is distortion Speech intelligibility can also be affected by distortion e Excessive noise As discussed above poor gain structure can increase the noise in a system to an audible degree e Bad mix Errors in gain structure at the inputs to a DSP can make mixing and routing to various destinations much more difficult While they can be fixed in the mix technicians may find setup and programming to be more complex A technician may be able to keep track of
49. ome loudness processing to the program submix to make the audio sound more full and satisfying Conferencing Audio Ts Turn on the telephone hybrid and ensure that you can hear the dial tone in the ceiling speakers For the DMP 128 activate the built in telephone hybrid within the DSP Configurator Software by loading the telephone Building Blocks and then opening up the Phone Dialer window Using the audio tools in the videoconference codec play whatever test tones or files are available to ascertain that the signal level coming from the codec is adequate in the room Make a videoconference call to a far end location and have someone there evaluate the levels of microphone and program content you are sending Make any necessary gain adjustments at the output matrix mixing points for the microphone and program submixes 3 in Figure 10 and 2 in Figure 11 Have the far end talker speak into the system Confirm that signals are coming into the DSP at adequate levels Listen to the audio over the loudspeakers and adjust the gain controls at the input as needed Repeat the same process for the telephone hybrid Microphone Voice Lift Now we can bring up the voice lift gain control or 5 in Figure 10 Why did we wait Because using the voice lift gain control as a basis for system levels is one of the most common mistakes in setting up audio systems It usually results in the microphone submix being too low and the
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51. or the program sources See Figure 9 These mixes will be routed to the appropriate outputs If present in the system the videoconferencing far end and the telephone hybrid will also be routed to the outputs A comprehensive look at the how s and why s of configuring and programming an audio DSP is beyond the scope of this document This is a topic that deserves in depth discussion and will be addressed in a future publication A Microphone Submix 1 Add automixer processing blocks to the microphone signal paths in the DSP and configure as appropriate If this is a system equipped for conferencing and the DSP is equipped with acoustic echo cancellation AEC then add AEC processing blocks as well As an example a GUI screenshot of the DSP Configurator Software for a DMP 128 is shown in Figure 10 AEC and automixer blocks are highlighted in the areas defined by 1 and have been activated for the five microphone input paths AEC is necessary for conferencing applications and will be covered in detail in a subsequent publication Automixing is useful in automatically managing the use of several microphones in voice reinforcement and conferencing applications It is a topic that merits more discussion and we will address it in a future publication 2 At the outputs of the automixer combine the microphone signals into a submix For some DSP models the submix is automatically created within the automixer For Extron DSP products s
52. osives consonants such as p that cause a popping or thumping sound when pronounced over a microphone 1 For each microphone input set the high pass filter to 100 Hz at 6 or 12 dB per octave In rooms with significant low frequency resonances it may be necessary to set the high pass filtering at 150 Hz or higher 2 f there are wireless handheld headset or lavaliere microphones follow the manufacturer s recommendations for proper setup and then proceed with setting input gain levels in the DSP C Microphones 1 Connect the signal generator to a portable loudspeaker Set up the generator so that it outputs pink noise from the loudspeaker at 65 68 dB SPL when measured 1 meter away by the SPL meter see Figure 8 This will provide a sonic reference for a raised human voice 0 5 5 ft 15 cm 1 5 m Signal Generator Pink Noise Speaker 65 68 dB SPL at 1 meter to the DSP Blu ray Player c NE C S C 3 Amplifier Ceiling Speakers Program Speakers e Recorder ALS Output Gain Matrix Mixing Output Output Gain Output Output Gain A signal generator and loudspeaker serve as a reference for a human talker and are used to establish nominal levels for incoming mic signals Audio Gain Structure for Pro AV Systems Extron Microphone Type Distance From Talker Gooseneck Lectern 1 ft 30 cm Gooseneck Conference Table S
53. pied DSP output connector Page 12 Audio Gain Structure for Pro AV Systems Extron Audio Signal Voltage Units Measurements of audio signal levels are expressed in volts dBu dBV dBFS and a number of other units While signal levels in decibels dB express the relationship between two known levels dBu and dBV are measurement units relative to universally accepted rms voltages as zero reference points They are relevant to audio signals in the analog domain and are defined by the following equations dBu dBV 2 214 dBu 20 log d dBV 20 log 0 775 volts 1 volt Expressing voltages in decibels provides a convenient means of quantifying signal gain and attenuation signal ratios and relating changes in signal levels to changes in listening levels Nominal signal voltages are distinctly different between the professional dBu and consumer audio dBV worlds with a 2 214 dB difference in the zero references for the two scales A visual relationship between dBu dBV and volts can be observed in the nomograph below T dBFS Scale Varies by Product 21 dBu X es DMP 128 20 dBu Maximum 5V Input 10 dBV 10 dBu 10 dBFS 4 dBu 1 23 V 17 dBFS Professional 0 dBV 1 0V Reference 0 dBu 20 dBFS Level 500 mV 10 dBV 10 dBV 316 mV ES Consumer 10 dBu 30 dBFS Reference Level 20 dBV 100 mV 20 dBu 40 dBFS 50 mV 30 dBV 30 dBu 50 dBFS dB Full Scale Extron DMP 128 40 dBV 10
54. ptimization Included are one or more calibrated test microphones and a laptop running audio analysis Software together with a multimeter and an acoustical sound source Acoustical Sound Source This is essentially a compact loudspeaker for reproducing pink noise test signals It is used as a model for a human talking voice providing a consistent steady state volume level for setting up microphone gain levels at the inputs of a DSP The ATI Audio NG 1 and Whirlwind Qbox are compact battery powered loudspeakers that include internal test signal generators Another option is a portable studio monitor such as the Fostex 6301B which accepts balanced audio from a test signal generator You can also use any other small self powered loudspeaker When nothing else is available a mobile device and its own loudspeaker such as a smartphone or tablet can be used to quickly get a system up and running Loudspeaker Impedance Meter While not necessary for setting up gain structure this is a very important tool to confirm proper cable connections from the amplifiers to the loudspeakers Loudspeaker impedances should always be measured prior to powering up amplifiers for the first time to make sure that there are no dead shorts to the amplifier outputs that could result in system downtime The TOA ZM 104A is a commonly available impedance meter while the NTi Audio Minirator MR PRO signal generator includes impedance metering Page 8 Audio
55. r turn down or mute the voice lift gain control defined earlier in Step 3 Section A 5 and shown as 5 in Figure 10 2 Deactivate the AEC and automixer blocks for all microphones 3 For all table microphones perform the following 8 Have someone speak at a normal talking level into each microphone and ensure that there is sufficient level at each microphone input Observe the DSP input meter and take note of the results but make no adjustments yet b Have someone speak loudly into each microphone and ensure that there is sufficient headroom at each microphone input Does the microphone input clip or does the input meter go into the red Take note of the results but make no adjustments yet C If any of the microphones within this group behaves differently from the rest then troubleshoot that microphone to find out if there are wiring errors DSP errors or physical problems with the microphone d Once all of the microphones are behaving similarly we can make adjustments If the input meter levels observed from normal talking are well below 4 dBu equivalent value of 17 dBFS in the DMP 128 see Figure 2 then we may want to raise the input gain levels However if the levels observed from loud talking are approaching maximum then we may want to lower the input gain levels Either way we want to adjust all microphones in this group by the same amount Remember we calibrated the microphones to their sensitivities in Step 2 Section C
56. signal evels and protecting the system Compressor threshold is set at 12 dBFS with a soft knee added and a 2 1 compression ratio The compressor threshold is above target level so as not to affect unity gain at the target level through the system Gain levels for the microphone Building Blocks are based on several factors e Reference SPL level from the talker at 1 meter e Distance of talker from microphone e Microphone sensitivity provided by the manufacturer e Target channel level The table below shows different mic categories the reference Page 28 SPL level at the mic talker level typical talker to mic distance and the target channel level The reference SPL level accounts for a heightened level produced by a loud talker at 74 dB SPL based on 65 to 68 dB SPL being a raised talking level at 1 meter A loud talker level was chosen to ensure that all microphone inputs have sufficient headroom Reference Source Microphone Type Level Distance Target Level Lectern Goosenecks 74 dB SPL 1 foot 30 cm 17 dBFS 4 dBu Table Mics Boundary 74 dB SPL 2 feet 60 cm 17 dBFS 4 dBu Table Mics Goosenecks 74 dB SPL Ceiling Mics 74 dB SPL 5 feet 1 5 m 17 dBFS 4 dBu Handheld Mics 2 feet 60 cm 17 dBFS 4 dBu 74 dB SPL 0 5 feet 15 cm 17 dBFS 4 dBu The target channel level is 4 dBu which allows for sufficient headroom 4 dBu on th
57. that all microphones and program sources are feeding the outputs to the ALS telephone hybrid and videoconferencing codec at proper levels before we concern ourselves with the volume levels in the room This may seem counterintuitive but it ensures that we have a good solid signal level going to the outputs before we cope with the variables introduced by the loudspeaker systems the acoustics of the room and human perception Audio Gain Structure for Pro AV Systems Extron In conferencing applications it is always important to remember that there are two prime directives One is to ensure that the audio quality of what you hear locally is at its best The second is to ensure that communication to another site is also at its best Both are of equal importance When you achieve both the meeting participants can forget that they are cities or continents apart A Microphone Inputs In this section we will use the microphone inputs that we calibrated earlier and make minor adjustments to reflect real world conditions It is ideal to perform this section with at least one other person serving as the talker If you are by yourself you can perform these tasks by connecting the laptop running the DSP software to the display You can then monitor signal levels on screen as you speak into the microphones 1 Since we do not want the voice reinforcement in the room to influence our settings just yet we mute the voice lift gain In the matrix mixe
58. tion devices happen to be in place such as the videoconferencing unit audio recorder or ALS you can set them up at this time Begin by turning off the amplifiers At the DSP create audio ties from the signal generator input to the outputs connected to these devices and then unmute the outputs Set the signal generator to output a 1 kHz sine wave at 4 dBu As an example open the software application for the videoconferencing unit enter into administrator mode if necessary and then navigate to the audio input settings page Using the input meter of the codec check that the audio signal level is at the manufacturer s recommendation for nominal e f the codec input meter indicates no audio signal presence check that the input settings in the codec are correct Also confirm that the appropriate audio DSP output is unmuted If there is still no audio detected check the wiring e f the codec input meter is hovering at or near maximum verify that the codec input is set for line level and not for microphone level Also ensure that phantom power is turned off e f the codec input meter activity appears fairly normal make slight adjustments to the codec input level control to get to the desired level If necessary make adjustments at the audio DSP output as well For a recording device you may have to put it into record mode to check whether the meters are showing a nominal 4 dBu level or O VU Adjust input levels on the recording d
59. to achieve desired room Front End Microphones and Source Devices to DSP Inputs Test all front end cable connections and then set up gains at the DSP inputs for microphones program sources and external switchers DSP Routing Processing and Internal Gain Structure Create submixes in the DSP for the microphones and program sources Then set up the routing within the DSP for these submixes as well as the telephone hybrid and Listening Tests and Final System Adjustments Perform a series of System tests to finalize System gain structure and listening levels SPL and levels to external devices videoconferencing unit The four essential steps to setting up an audio system for proper gain structure as amplifiers ceiling speakers program speakers and assistive listening devices ALS which together are known as the back end of the system Other devices including videoconferencing codecs and recorders can exist as both source and destination Similar principles can be applied to systems with both analog and digital based audio signal processing and distribution The Four Essential Steps In working with pro AV and pro audio systems usually it is best to envision the audio path from start to finish or to begin with the sources then the DSP and finally the amplifiers and loudspeakers While this sequence provides a logical signal flow in reality it is often not practical to perform gain str
60. tron DMP 128 where 21 dBu is the maximum allowable input signal level at its analog to digital converters and corresponds with O dBFS Looking at the scales for dBFS and dBu a 4 dBu nominal signal level would correlate with about 17 dBFS in the DMP 128 Signal meters in the DMP 128 and other Extron DSP products always provide measurements in dBFS For a full explanation of the dB unit in audio applications please refer to the references listed at the end of this guide Page 13 Extron Audio Gain Structure for Pro AV Systems B Amplifier Cabling Page 14 Ensure that all amplifiers are still powered off 1 Testing the cable connections from the DSP to the amplifiers Using the DSP software create audio ties from the signal generator to the outputs connected to the amplifiers Double check that the audio ties you just created are all at unity or at O dB of gain With the signal generator sending out the same 1 kHz sine wave measure the rms voltages at the amplifier inputs to confirm proper cable connections from the DSP outputs If one signal measures about 6 dB or just 50 percent of the voltage of the others this is likely a case where a lead has been disconnected or shorted to ground Once you have confirmed cable connections into the amplifiers turn off the signal generator 2 Testing for proper cable connections from the amplifiers to the loudspeakers This is accomplished using a loudspeaker impedance meter
61. uch as the DMP 128 the submix will be created by matrix mixing the signals into a virtual bus See 2 in Figure 10 3 Now route the submix through the matrix mixer to the appropriate outputs of the DSP A microphone submix typically is directed to the outputs feeding the videoconferencing codec the telephone hybrid and if used the ALS and recording system See 3 in Figure 10 4 With the microphone submix signal path now created you can define or incorporate a volume control point for interfacing into a control system This volume control point will be used to provide a user control for adjusting overall microphone levels It should be separate from the gain controls at the routing points to the outputs Additionally gains at the outputs should never be used for volume control since they are already used to set amplifier input sensitivity In our example we will use the submix gain in Virtual Return A as the microphone volume control point illustrated in Figure 10 as 4 5 If local voice reinforcement is needed the microphone submix can also be routed to the output feeding the ceiling speakers illustrated as 5 in Figure 10 It is important to note that the ideal microphone submix level for the telephone codec or ALS will not be ideal for voice reinforcement As such the 5 matrix point will be treated differently in that it will be affected by the microphone submix volume control but allow for final adjustment of how much voice reinfor
62. ucture adjustments Conflicts on the job site can delay installation of furniture onto which microphones may be mounted and in which program sources may be installed Gain structure setup can be expedited however by setting up the DSP and amplifiers already in the racks or equipment closet while system installation is still in progress Therefore the four essential steps in establishing gain structure for pro AV systems may at first seem out of order see Figure 4 There is however a rationale to this approach that speeds up the process and increases the probability of a properly set up audio system It should be noted that before performing gain structure setup the audio DSP program should already be written or configured and all control system hooks defined In complex systems this is actually a necessity However as AV field technicians will attest this is not always the case Following the four steps will allow the field technician to develop the DSP configuration as a byproduct of the procedure Control system hooks are designated functions of a DSP for interfacing with a control system They can include gain controls group masters mute functions and source selection Gain Structure Procedure for Pro AV Systems Back End DSP Outputs to Amplifiers Loudspeakers and Other Destinations Page 10 We will begin the gain structure setup with an initial configuration of the DSP Then we will test all back end cable connections
63. xtron Audio Gain Structure for Pro AV Systems Appendix 2 Heferences 1 Ballou G 2008 Handbook for Sound Engineers Fourth Edition Burlington MA Focal Press 2 Bohn D 1997 Setting Sound System Level Controls Retrieved August 21 2012 from www rane com pdf ranenotes Setting Sound System Level Controls pdf 3 Bohn D 2000 Audio Specifications Retrieved September 10 2012 from www rane com paf ranenotes Audio Specifications pdf 4 Davis D and Patronis Jr E 2006 Sound System Engineering Third Edition Burlington MA Focal Press 5 Holman T 2008 Surround Sound Up and Running Second Edition Burlington MA Focal Press 6 Holman T 2010 Sound for Film and Television Third Edition Burlington MA Focal Press T Winer E 2012 The Audio Expert Waltham MA Focal Press Page 30 Extron Electronics wun INTERFACIN G SWITCHING AND CONTROL Worldwide Sales Offices Extron USA West Worldwide Headquarters Extron Electronics 1025 E Ball Road Anaheim California 92805 USA Sales Tech Support 800 633 9876 USA amp Canada only Order Support 800 633 9873 USA amp Canada only Control Systems Support 800 633 9877 USA amp Canada only 1 714 491 1500 Fax 1 714 491 1517 Extron USA East Extron Electronics 2500 N Raleigh Boulevard Raleigh North Carolina 27604 USA Sales 800 633 9876 USA amp Canada only

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