Lexicon 20/20 AD Power Supply User Manual
Contents
1. J13 75 OHM COAXIAL Jumper settings as set at the factory 10 d T 5 OHM IASSIS CONNECTED TO RTH GROUND ROUGH POWER CABLE OR GROUND STRAP 20 20 AD User Guide Using the 20 20 AD The following figure indicates the locations and factory settings of these jumpers on the board In their closed positions the jumpers have their right two or lower two pins shunted together To open a jumper move the shunting block to the left or upper two pins Note that the jumpers in J13 have only two pins to open simply remove the shunting block O OmOm aaO On Om mO Om wi w2 w9 Wi0 Wii W12 W13 W14 Analog Jumpers Digital Jumpers Jumper locations shown with factory settings Note that the 20 20 AD s analog inputs are not transformer isolated It is therefore necessary to have one ground connection between the 20 20 AD and the audio source In addition to the ground lift jumpers each analog input has a 600Q termination jumper If the analog source expects to see a 600Q load these jumpers should be closed2. 20 20 AD 20 Bit A D Converter lexicon Precautions The 20 20 AD is a rugged device with extensive electronic protection However you should observe the same reasonable precautions that apply to any piece of audio equipment Always use the correct line voltage e Do not install the 20 20 AD in a closed unventilated rack or directly above heat producing equipment such as power amplifiers Never attach audio power amplifier outputs speaker outputs directly to any of the 20 20 AD s connectors To prevent fire or shock hazard do not expose the 20 20 AD to rain or moisture Notice This equipment generates and uses radio frequency energy and if not installed and used properly that is in strict accordance with the manufacturer s instructions may cause interference to radio and television reception It has been type tested and found to comply with the limits for a Class A computing device in accordance with the specifications in Subpart J of Part 15 of FCC Rules which are designated to provide reasonable protection against such interference in a residential installation However there is no guarantee that interference will not occur in a particular installation If this equipment does cause interference to radio or television reception which can be determined by turning the equipment OFF and ON the user is encouraged to try to correct the interference by one or more of the following measures Reorient the receivi
3. u nunssseensnnnnnnsnnnnnnnnennnnnnnnnnnnennnnnnnn Speeiflcations 2 2 2 namen Definition Of Terms nes ea Contents 20 20 AD User Guide Introduction The 20 20 AD is an analog to digital converter which can be configured for two 20 bit channels or four 18 bit channels Twin on board signal processors provide 20 bit to 16 bit compression in the digital domain as well as DC removal and a choice of four dither types The compressor allows leeway in setting levels and provides insurance against clipping in the digital domain In addition it provides 195 soft compression curves and 13 time constants to allow for more creative functions such as emulation of tape saturation or analog compression This manual provides some basic information on conversion but assumes the user will be familiar with the principles of analog to digital conversion as well as with the digital audio interface formats output by the 20 20 AD Information on specific data which may be required by any target device should be obtained from the manufacturer Lexicon has attempted to provide correct information and functional capability corresponding to digital audio standards known and available to us at the time of publication of this document and manufacture of this instrument As standards evolve Lexicon may make updates to documentation and or system software available for purchase Lexicon s warranty on this product excludes consequenti
4. 5 Adjust THRESHOLD to suit the program material If the expected source material is percussive the chance of clipping is relatively slim and you don t want to color the rest of the music leave THRESHOLD at OdB Otherwise it may be necessary to lower the THRESHOLD setting until the clipped signal sounds acceptable Note The compressor can only prevent clipping if the analog digital converter does not overload Any signal which overloads the converter will also overload the compressor Although the compressor works in both 4 channel and 2 channel mode the available dynamic range in 4 channel mode is only a few dB greater than the capacity of a 16 bit recorder so nothing is gained by setting your margin greater than a few dB unless of course there is a specific audio effect you are trying to achieve Aslightly unorthodox use of the compressor allows you to automatically fine tune the level so that the highest peak will be set to exactly OdBfs when transferring analog tape or other repeatable source to the digital domain To do this 1 Do a dry run of the loudest section s of the tape through the 20 20 AD to check the level Using 1dB SCALE and PK HOLD adjust the input level until the peak comes within 6dB of clipping 2 Turn on the compressor Set GAIN to 6dB THRESHOLD to OdB and RELEASE to infinity Do another dry run of the loudest section s of the tape If you turn on DISPLAY you will see the gain reduction me
5. To minimize shock hazard the instrument chassis and cabi net must be connected to an electrical ground The instru ment is equipped with a three conductor AC power cable The power cable must either be plugged into an approved three contact electrical outlet or used with a three contact to two contact adapter with the grounding wire green firmly connected to an electrical ground safety ground at the power outlet The power jack and mating plug of the power cable meet International Electrotechnical Commission IEC safety standards DO NOT OPERATE IN AN EXPLOSIVE ATMOSPHERE Do not operate the instrument in the presence of flammable gases or fumes Operation of any electrical instrument in such an environment constitutes a definite safety hazard KEEP AWAY FROM LIVE CIRCUITS Operating personnel must not remove instrument covers Component replacement and internal adjustments must be made by qualified maintenance personnel Do not replace components with power cable connected Under certain conditions dangerous voltages may exist even with the power cable removed To avoid injuries always disconnect power and discharge circuits before touching them DO NOT SERVICE OR ADJUST ALONE Do not attempt internal service or adjustment unless another person capable of rendering first aid and resuscitation is present DO NOT SUBSTITUTE PARTS OR MODIFY INSTRUMENT Because of the danger of introducing additional hazards do not install subst
6. 2 o 1 18 03 4 8 o 12 f o0 S GAIN THRESHOLD RELEASE ATTEN ON vo CD on TAN pk Hou 7 I HAN LINK Jy 2 l sc 2 3 3 The Converter Gain Controls 1 2 3 and 4 Provide continuously vari able gain control over a 30dB range Note In 2 channel mode only 1 and 2 are active THRESH When the compressor is on indicates that the input level has risen above the selected compression threshold UNBAL Lights when there is a mis match between the input sig nal and the setting of the rear panel Balance switch 4 CHAN Push on push off selection of 2 channel or 4 channel mode lit when in 4 channel mode UNBAL THRESH UNBAL THRESH co co co UNBAL THRESH cy 1dB SCALE gt ALT e owog UNBAL THRESH co co PK HOLD Cc ATTEN MTEC SSssRan EMPH Turns pre emphasis on and off OVL 0 3 6 1 1 2 x EL 4 6 PK HOLD ATTEN 1dB SCALE Press to select 1dB Scale display on level meters Also used to select alternate modes Press to cause the level meter LEDs to hold at the highest level received Press again to deactivate Press to attenuate balanced input signals by 6dB Press again to turn attenuation off Do not use this control on unbalanced signals 20 20 AD User Guide Controls and Connectors The Level Meters When the compressor is off the behavior of the level meters is determined by the settings of the 1dB SCALE and PK HOLD swit
7. 104dB The DITHER switch automatically adds just enough dither to squeeze the remaining dynamic range into the 96dB dynamic range of 16 bit media 20 20 AD User Guide Note that the 6dB GAIN setting is given only as an example Feel free to experiment with any level in this mode GAIN settings above 12dB are not available because at that point practically all of the dynamic range of the converter will fit onto 16 bit media Set THRESHOLD to 24dB Now when the signal level exceeds 24dB the gain is gradually reduced until at OdB signal level the compression gain is also at OdB unity The graph below shows a plot of gain vs input level with compressor GAIN at 6dB and THRESHOLD set to 24dB Note that unlike some compressors the gain curve is not linear but changes smoothly This reduces high frequency artifacts which can result from aggressive compression Threshold 24dB 0 Level i 5 Q 5 O 104dB 110dB 0 Level Input The settings ofthe GAIN and THRESHOLD knobs determine the compression ratio according the formula ratio threshold threshold gain In our example the compression ratio is 24 24 6 1 33 1 This is not a very intense compression by most standards but remember the point is to pack as much music onto the recording media as possible without introducing audible artifacts As the following table of compression ratios shows most of the ratios are below 2 1 In order to minimize the au
8. If the source s isolation transformer has a center tapped output ground the center tap 3 Reduce the signal level into the 20 20 AD and make it up with the front panel gain control s 4 Remove the power cord open the 20 20 AD and short out R4 R28 R54 and R95 These are 10 K resistors brown black black red located near the input relays Note that this change may degrade the 20 20 AD s CMRR 5 Equalize the load resistances of the other box with the following procedure a With the 20 20 AD and the other box connected send a high level tone through the source so that the 20 20 AD s UNBAL light goes on b Connect a 1 Meg potentiometer set at maximum resistance between the pin 2 signal and ground c Gradually reduce the resistance until the UNBAL light goes out If the UNBAL light does not go out reconnect the pot between pin 3 and ground and repeat d Once the UNBAL light goes out continue reducing the resistance until the light comes on again Set the pot so thatitis in the middle of the range where the UNBAL light is out e Remove the pot and measure the resistance Solder a resistor of that value between the correct pin and ground 20 20 AD User Guide Using the 20 20 AD Adjust the front panel Gain knobs until the peak input levels are balanced and Front panel come within a few dB of clipping For now make sure that the DISPLAY switch Adjustments inthe DIGITAL COMP LIMITER section is set to OFF In normal oper
9. When an input signal exceeds the peak level by 4dB or less the compressor gain is automatically reduced just enough to assure that the compressor output never exceeds OdB After the overload has passed the compressor gradually brings the gain back up to 4dB The RELEASE knob determines the speed of this recovery Note This automatic gain reduction can add considerable distortion to the leading edge of the overload signal With THRESHOLD set at OdB or at any value less than or equal to the GAIN setting gain reduction will not start until the signal is about to clip too late to prevent leading edge distortion If this distortion is unacceptable try lowering the THRESHOLD setting so that compression starts before the signal clips If for example THRESHOLD is lowered to 8dB compressor gain reduction will be gradual instead of being reduced all at once at AdB It is important to remember that the lower you set THRESHOLD the sooner compression will start and the more likely you are to color signals which were not in any danger of clipping 2 Using the 20 20 AD 22 Digitizing Analog Tape Lexicon To summarize 1 Set the input levels so that the peak level is a few dB below clipping 2 Turn on the compressor 3 Set compressor GAIN to the input level safety margin and THRESHOLD to OdB 4 Adjust RELEASE to suit the program material The faster the expected clipping source the faster the release time should be
10. audio Refer to Page 7 for recommended cables Using the 20 20 AD LINK DISPLAY Digital Audio Outputs 23 Using the 20 20 AD Realigning Two DAT Recordings Digital Sinewave Generator 24 Lexicon When recording in 4 channel mode to two DAT recorders it may be useful to record a clapper signal to both DATs to facilitate realignment during post production When you press ALT LINK the 20 20 AD will generate 8mS of silence followed by an 8 cycle 1 kHz square wave burst at 12dBfs This signal is time aligned on all four output channels You can align the two DAT tapes on a disk audio production system by visually lining up the first edge of the burst or you can align them audibly by subtracting channel 3 from channel 1 and adjusting the offset until the click disppears The 20 20 AD includes a digital sinewave generator which is accurate to 20 bits with a dynamic range of more than 120dB This generator is useful in testing the digital outputs To use it 1 Use the SAMPLE RATE FORMAT and 4 CHAN buttons to select the output mode These controls are locked out in sinewave generator mode 2 Press and hold down 1dB SCALE ALT and press the 4 CHAN button 1dB SCALE and 4 CHAN will flash to indicate that the unit will now generate digital sinewaves The compressor GAIN knob adjusts the frequency of the sine wave and the THRESHOLD knob adjusts the amplitude as shown below 2062 5 3000 5062 5 18 1500 l d 7 P7 20
11. exit the alternate mode vi ALT DISPLAY Activates remote mode front panel lockout When acti vated the 1dB SCALE and DISPLAY LEDs flash and front panel controls are locked into their current set tings To unlock hold ALT and press DISPLAY again ALT LINK Only active in 4 channel mode Activates a short tone burst See Realigning Two DAT Recordings ALT ON Reserved for future en hancements Lexicon SAMPLE RATE Selects the sample rate of the A D conversion This switch should be set to the same sample rate as your sync source When Word Clock is selected as an exter nal sync source the AES Channel Status Sample Rate bit will be set to the sample rate value you have selected here The PL DN setting of this switch is reserved for future enhancements EXT SYNC This switch allows selection of Internal mode unlit Word Clock source or AES source ALT DITHER Activates psychoacousti cally optimized noise shap ing When activated WIDE 20BIT and HF will be lighted Note that this type of dither can only be used in 2 chan nel mode activating will dis able 4 channel mode and compressor functions 20 20 AD User Guide AC Power Standard 3 pin IEC power connector Controls and Connectors 20 20 AD Rear Panel Connectors BAL UNBAL Push on push off switch Set button to correspond to input signal type A mismatch be tween the input signal and the setting of this swit
12. 20 20 AD User Guide Using the 20 20 AD 2 00k 4 00k 6 00k 8 00k 10 0k 12 0k 14 0k 16 0k 18 0k 20 0k Plot of noise distortion vs frequency for the 20 20 AD in 2 channel mode with a 60dBfs sinewave at 1kHz 2 00k 4 00k 6 00k 8 00k 10 0k 12 0k 14 0k 16 0k 18 0k 20 0k WIDE dither is added before truncation Noise floor is ele vated further but harmonic distortion is removed PONS dither Noise floor is down almost to the untruncated level in the critical band 2 4kHz but significantly increased at high frequencies 2 00k 4 00k 6 00 8 00k 10 0k 12 0k 14 0k 16 0k 18 0k 20 20 AD in 2 channel mode truncated to 16 bits without dither Note harmonic distortion spikes at multiples of 1kHz and elevated noise floor 6 00k 8 00k 10 0k 12 0k 14 0k 16 0k 18 0k 20 0k 22 0k 24 0k HF dither is added before truncation Noise floor in audio range is lowered by 3dB at the expense of increased noise above 20kHz 4 00k 6 00k B 00k 10 0k 12 0k 14 0k 18 0k 20 0k 22 0k 24 0k 20BIT dither is added before truncation This is similar to HF dither but at a lower level The dither reduces spurious tones low level spikes visible in the first plot at the expense of a slightly elevated noise floor 17 Using the 20 20 AD Automatic Offset Removal Compressor Modes Dynamic Range Compression Lexicon DC offsets are created by input amplifiers and A D con
13. 360 the period of the waveform When two identical signals are always of the same polarity they are said to be in phase or to have a phase of 0 When they are always of opposite polarity they are said to be out of phase or to have a phase of 180 polarity the positive or negative direction of an electrical force quantization the process of converting an infinitely varying waveform into a finite series of discrete levels The inaccuracy introduced by this process is called quantization error rms amplitude root mean squared amplitude a way of assigning a DC power equivalent to an AC waveform For a sine wave this is 0 707 times the peak level sample rate the periodic interval at which an analog audio signal is sampled e g 44 1 or 48 kHz signal to noise ratio the ratio between the maximum peak signal and the wideband noise in the absence of signal This number becomes more meaningful when distortion is added It is then expressed as S N D S PDIF Sony Phillips Digital Interface Format synchronize to arrange events to occur simultaneously In audio the events are both the period and the phase of a signal THD N the ratio of the rms amplitude of the distortion and noise that is added to a signal to the rms amplitude of the signal itself This can be expressed in percent or decibels S N D 1 THD N time constant the amount of time it takes for a signal to rise or fall 63 of its full scal
14. SYNC if you intend to daisy chain several 20 20 ADs from the same Word Clock you should open the leftmost jumper on J13 on all units except the last one in the chain If an alternative ground path exists between the word clock generator and the 20 20 AD W16 can be opened to prevent a ground loop Note that unlike the analog and AES SYNC inputs the Word Clock input is not transformer isolated and requires a ground connection between the 20 20 AD and the signal source The Word Clock input requires a 75Q RG 59 U coaxial cable Use of ordinary audio quality cable may distort the edges of the Word Clock This will add jitter to the sample rate which will in turn add noise and distortion to the analog digital converter Refer to Page 7 for recommended cables The WORD CLOCK OUT signal is a logic level square wave at the sample frequency Itis capable of driving a 75Q load andis always available even when aninternal sync source is selected This output may be used to synchronize one ormore slave 20 20 ADs to a master As with the Word Clock input use only 75Q RG 59 U or similar cable 20 20 AD User Guide The 20 20 AD is shipped configured for Internal mode To operate in External mode press EXT SYNC until the desired sync source WC or AES lights If the selected sync source light flashes and the PRES light is not on it indicates that there is no signal present as a sync source If the selected sync source and PRES lights are on but t
15. Using the 20 20 AD Balanced and Unbalanced Input Signals Lexicon Make sure the BALANCE switches located on the rear panel are set to match the type of input OUT balanced IN unbalanced The red UNBAL lights on the front panel will indicate an error if there is a mismatch between the switch setting and the signal type Note that when an UNBAL light goes on there is a good chance that the input signal has been audibly clipped even if the level meter does not indicate an overload If it is necessary to mismatch the switch settings to the signals for example when switching quickly between balanced and unbalanced inputs you can avoid clipping by setting the peak input levels about 3dB below full scale Under some conditions the UNBAL light may remain on even though the source and the 20 20 AD are both set for unbalanced operation 1 The source has a pin 3 high unbalanced signal Try swapping pins 2 and 3 2 The source is floating transformer isolated Tie pin 3 to pin 1 at either end of the cable The UNBAL light may also flash intermittently if the source is balanced and transformer isolated and the 20 20 AD shares the source with another con verter or an amplifier The other box may be unequally loading the positive and negative source signals causing them to become unbalanced Some solutions to this problem are 1 Drive the 20 20 AD by itself This will ensure a balanced signal and minimize common mode noise 2
16. al damages resulting from the use of this product Local jurisdiction may extend the user additional rights AES EBU Interface Professional The AES EBU interface conforms to both the AES3 1992 ANSI S4 40 1992 specification and the EBU document Tech 3250 E Both inputs and outputs are balanced transformer coupled designs with a female XLR input and a male XLR output conforming to the standard convention of IEC 268 Part 12 Input and output levels comply with CCITT V 11 On this device the professional form of the digital audio and auxiliary data has been labeled AES in accordance with general usage S PDIF Sony Phillips Digital Interface Format Consumer The S PDIF interface conforms to the Channel Status Type II format as specified in the EIAJ CP 340 Digtial Audio Interface Standard dated September 1987 and to IEC 958 First Edition 1989 03 As of November 20 1991 amendments to IEC 958 which include specifications for the implementation of auxiliary data for the consumer format were under review Until new specifications are published this device must conform to currently approved specifications On this device the consumer form of the digital audio and auxiliary data has been labeled SPDIF in accordance with general usage 20 20 AD User Guide Controls and Connectors After unpacking the 20 20 AD save all packing materials in case you ever need Unpacking to ship the unit Thoroughly inspect the 20 20 AD and packing m
17. ance controlled impedance data communication twisted shielded pair cable Digital Output Microphone cable may introduce a significant amount of jitter into the signal causing distortion Use commercially available consumer audio optical cable assemblies S PDIF Consumer Digital Audio Output This interface is unbalanced but because it carries digital signals it requires the Word Clock I O use of 750 RG 59 coaxial cable Below are recommended cable and cable assemblies Similar cables from sources other than those listed here may be appropriate AES EBU Belden 9271 foil shielded twisted pair 1240 12 2 pF ft S PDIF Consumer Digital Audio Belden 9259 RG 59 U 22 AWG conductor 242 O D 75Q 17 3 pF ft Belden 8218 27 AWG conductor 150 O D 75Q 20 5 pF ft Maximum recommended length 32 ft 10M S PDIF Consumer Digital Audio Optical Toshiba TOCP174y Sony POC 15 Maximum recommended length 16 ft 5M Word Clock Belden 9259 RG 59 U 22 AWG conductor 242 O D 75Q 17 3 pF ft Belden 8218 27 AWG conductor 150 O D 75Q 20 5 pF ft Maximum recommended length 32 ft 10M 20 20 AD User Guide The 20 20 AD has four analog inputs CH1 CH2 CH3 and CH4 In two channel mode only CH1 and CH2 are used and the dynamic range of the unit is extended If you have two stereo pairs one of which is used more often than the other the most often used pair should be assigned to CH1 and CH2 and the other pair to CH3 a
18. and C81 These are upright tubular capacitors located near the analog input connectors Solder short lengths of wire in place of each Alternatively you can leave the capacitors installed and solder short lengths of wire across the pins of each on the solder side of the board Until you become comfortable with the operation of the rest of the 20 20 AD you should keep the compressor turned off Make sure that the DISPLAY LINK and ON buttons in the compressor block are all off unlighted and the compressor gain knob is set to OdB Although the function of the digital compressor is similar to that of a standard analog compressor its intended uses are somewhatdifferent Three of the more common uses Dynamic Range Compression Non Linear Transfer Functions and Overload Protection are described below This mode is primarily useful when recording program material with a wide dynamic range onto 16 bit media in 2 channel mode Although the compressor will work in 4 channel mode the dynamic range of 4 channel mode is so close to the 16 bit level that any further compression will only raise the noise floor This use of the compressor requires a bit of artistry because all settings must be optimized to the program material With the compressor on set compressor GAIN to 6dB This will increase the gain of low level signals below the THRESHOLD setting by 6dB effectively squeezing the dynamic range of the converter in 2 channel mode from 110dB to
19. aterials for signs of damage Report any shipment damage to the carrier at once The following accessories are included with the 20 20 AD 1 Power Cable 2 User Guide 3 Quick Reference Guide The 20 20 AD is equipped with a 3 pin IEC connector and detachable line cord Power Connect the cable end of the 20 20 AD line cord to the 20 20 AD power connector Then plug the line cord into an AC wall socket providing voltage corresponding to the data plate on the unit The 20 20 AD measures 19 W x 1 75 H x 13 9 D 483 x 45 x 353 mm Make Mounting sure that the 20 20 AD is securely screwed into the rack and that support is provided for the rear of the chassis during transport to avoid possible damage from severe mechanical shock The maximum ambient operating temperature is 95 F 35 C Provide adequate ventilation if the 20 20 AD is mounted in a closed rack with heat producing equipment Controls and Connectors Lexicon 20 20 AD Front Panel Controls The 20 20 AD front panel controls are divided into three functional groups for conversion compression and synchronization output formatting Synchronization and Converter Compressor Output Formatting ala mes UNBAL THRESH UNBAL THRESH UNBAL THRESH G oO oo oo oo DIGITAL COMP LIMITER DITHER 20BIT mwr VL 100 SAMPLE RATE FORMAT ORAT 20 20 AD EXT SYNC 20 BIT AD CONVERTER AES WC Lock 1dB SCALE DISPLAY an Rut FR 189 14 oh eme 4 8 D C 10 oe
20. ation each level meter will indicate the level at the output of the converter in dB referenced to digital full scale dBfs A lighted green or yellow LED indicates that the signal has exceeded the level printed next to it The red 0dB LED lights when the signal exceeds 0 5dBfs The red OVL LED indicates audible clipping clipping which Normal Level Meter settin lasts for several samples i 1 dB SCALE and DISPLAY set to OFF I dB SCALE Turning on the 1dB SCALE switch increases the resolution of the top end of the scale shown on the level meters In this mode each LED corresponds to 1dB Meter Ballistics Each meter consists of a peak meter indicated by an isolated LED and an average meter indicated by a growing and shrinking bar The peak meter s attack time constant is instantaneous and its decay time constant is roughly 1 second The average meter s time constants approximate those of a VU meter with attack and delay time constants of 10 mS PK HOLD a 1 dB Level Meter settings When you press PK HOLD the meters will indicate the highest level received 1 dB SCALE set to ON after PK HOLD was pressed To reset the peak turn PK HOLD off and then on DISPLAY set to OFF again Remember that the level meters will display either the normal scale or the 1dB scale dependent on the setting of the1 dB SCALE switch ATTEN The ATTEN button should only be used on balanced input signals If a Gain knob is turned completely counter clo
21. ch will be indicated on the front panel by the BAL LED Analog Input 3 pin XLR connectors In 2 channel mode only inputs 1 and 2 are active WORD CLOCK acs N OUT j SYNC AAN Digital Output Two 3 pin XLR connectors professional format AES bal anced digital outputs Two consumer format optical fiber optic outputs Two consumer format unbalanced coaxial RCA outputs Note In 2 channel mode 3 amp 4 and 1 amp 2 are identical The input and output connectors conform to published stan dards for professional and consumer formats The 20 20 AD however provides the user selected format at all connectors e g the AES format if selected will be available at the consumer type outputs as well as at the XLR outputs Sync Input 3 pin XLR connector professional format AES EBU balanced digital input 75Q unbalanced BNC connectors for word clock in out Controls and Connectors Lexicon Audio Connections Connectors Signal Mating Connector Description AES EBU XLR A3M Balanced RS 422 Digital Input AES EBU XLR A3F Balanced RS 422 Digital Output CA S PDIF Unbalanced 759 Consumer Digital Output S PDIF Consumer Digital Consumer Digital Audio Optical Output BE Word Uloc Unbalanced mem i BELLE R XLR Pinout XLR Pinout pin 2 high by convention 20 20 AD User Guide Controls and Connectors Cables This interface requires balanced connections using high quality low capaci AES EBU t
22. ches When the compressor is on pressing DISPLAY will cause gain reduction in dB to be displayed on the two right hand meters 2 channel mode or on all four meters 4 channel mode The following charts show all of the possible states of the level meter and the value in decibels for each state Switches ON Meters 1 amp 2 Meters 3 amp 4 none Normal Normal Channel TaB SCALE TaB Scale TaB Soae Mode 1 dB SCALE 1 dB Scale 1 dB Scale DISPLAY Gain Reduction Gain Reduction none Normal Blank tare a8 SCALE Noms taB Scare Mode 1 dB SCALE 1dB Scale Noma Gain Reduction DISPLAY DISPLAY Press to cause gain reduc tion to be displayed on the level meters In 2 channel mode only the right two level meters are activated in 4 channel mode all four level meters display compression Press again to deactivate LINK In 2 channel mode links the left and right compressors in 4 channel mode where 1 2 and 3 4 are already linked this control links the two pairs ON Turns the compressor on and off DISPLAY RMT LINK ms ON vio Co GAIN GAIN Adjusts the gain of low level signals below the compres sion threshold between the converter and the compres sor GAIN NORMAL 1dB SCALE REDUCTION a 2 ci 2s ok ek CON OTAHRWNH OC OCOONOaO Lh WN m ao The Compressor DIGITAL COMP LIMITER 18 14 THRESHOLD RELEASE THRESHOLD S
23. ckwise and the level meter still indicates an overload you must either press the ATTEN button which reduces the input level by 6dB or reduce the signal level at its source Note that ATTEN should not be used unnecessarily as it causes a slight reduction in the Common Mode Rejection Ratio CMRR and thereby increases the potential for hum 4 CHAN When the 4 CHAN switch is on the 20 20 AD converts four channels from analog to digital with a dynamic range of better than 16 bits When the 4 CHAN switch is off the 20 20 AD only converts CH1 and CH2 buts adds 12dB 2 bits to the dynamic range Your selection of mode depends primarily on how many channels you need to digitize and on whether or not you need the extra dynamic range If you plan to record to 16 bit media without using the on board digital compressor 2 channel mode will give you duplicate digital outputs EMPH The EMPH switch adds CD standard 15 50usec pre emphasis to each input signal before digitization Using the 20 20 AD Synchronization and Sample Rate AES Sync Word Clock Lexicon The 20 20 AD can synchronize to its internally derived sample clock at 44 1kHz or 48kHz or to an external AES or Word Clock signal in the range of 41 51kHz The EXT SYNC switch allows you to select external Word Clock WC external AES AES or internal source WC and AES off Before switching between Internal and External modes be sure to reduce monitor volume as some D A co
24. common mode noise is introduced over the length of the cable by outside interference A typical example is line hum CMRR Common Mode Rejection Ratio measures the ability of a system to reject common mode noise It is expressed as the ratio of common mode noise entering the system to the common mode noise leaving it dBfs decibel full scale this is the unit used to describe the amplitude of a digital audio signal where O equals digital full scale digital headroom does not exist see dBfs distortion an unwanted change in a waveform as it passes through an electronic component or from one medium to another dither a low level signal which is added to digital audio to reduce quantization noise in effect by replacing it with noise which is perceived as less offensive dynamic range when used to describe digital to analog or analog to digital converters dynamic range is the ratio of the full scale signal to the broadband noise 0 20 kHz measured with a 60dBfs signal In other words it is the S N D measured with a 60 dBfs input signal and a 20 kHz low pass filter EBU European Broadcast Union emphasis a form of equalization which increases high frequency signals in order to maximize the signal to noise ratio De emphasis is the inverse of emphasis equalization the process of increasing or decreasing the amplitude of some frequency ranges of a signal with respect to others frequency the number of v
25. cribe digital to analog or analog to digital converters dynamic range is the ratio of the full scale signal to the broadband noise 0 20 kHz measured with a 60dBfs signal In other words it is the S N D measured with a 60 dBfs input signal and a 20 kHz low pass filter EBU European Broadcast Union emphasis a form of equalization which increases high frequency signals in order to maximize the signal to noise ratio De emphasis is the inverse of emphasis equalization the process of increasing or decreasing the amplitude of some frequency ranges of a signal with respect to others frequency the number of vibrations per unit of time ex pressed in cycles per second Hz harmonic distortion the appearance of harmonics of the input signal at the output of a device link the tying of two channels of the 20 20 AD compressor to a shared gain curve lock two or more audio signals which have the same frequency and phase mS millisecond 1 1000 of a second oversampling a method of sampling an audio signal at many times the audio bandwidth typically 64 256 times in order to reduce the complexity of the anti aliasing filter The oversampled data is then digitally filtered to reduce the sample rate period the interval of time required for a waveform to complete one cycle and begin to repeat itself Period 1 frequency phase the angular relationship between two signals meas ured in degrees where
26. dibility of compression artifacts use ratios from the unshaded portion of the table Setting the threshold at any level higher than a 4 1 ratio will force the ratio to 4 1 Using the 20 20 AD Using the 20 20 AD THRESHOLD 20 Lexicon GAIN As both the formula and the table show raising the threshold increases the ratio making the compression more audible above the THRESHOLD setting However because signals below the threshold are boosted linearly through the compressor raising the threshold may decrease the amount of time the compressor is reducing gain This may decrease the audible artifacts Conversely if you lower the threshold all the way to 36dB you will pretty much guarantee that all but the quietest signals will experience some amount of gain reduction but the ratio will be so small 1 2 1 that the total effect may be less noticeable than a higher threshold setting Start with RELEASE time set to about 180 mS then shorten the time for percussive program material and lengthen it for material such as voice RELEASE times of OmS to are available Attack time of the compression is automatically set as a function of the signal rise time Compressor Knob Values 18 16 14 56 100 180 20 0 o 32 oP 320 e 360 THRESHOLD RELEASE in dB in mS and S Note that changing the settings of GAIN and THRESHOLD while recording may produce audib
27. e This only pertains to signals which rise and fall exponentially such as the 20 20 AD meter time constants or compression gain curves word clock a square wave used as a synchronization source Its frequency equals the sample rate 26 Definition of Terms AES Audio Engineering Society aliasing a form of distortion which occurs when an A D converter s sample rate is less than twice the highest fre quency component of a signal amplitude the maximum departure of the value of a wave form from its average value attenuation a reduction in the amplitude of an audio signal balanced signals two equal and opposite signals run through a twisted pair The advantage of balanced signals is that common mode noise tends to be added equally to both portions of the pair it can be removed by subtracting the negative from the positive signal Common mode noise rejection cannot be performed on unbalanced signals which consist of an audio signal and a ground return bandwidth the arithmetic difference between the upper and lower cut off frequencies of an audio system bit binary digit the smallest unit of digital information equivalent to the result of a choice between two alternatives yes or no onor off Oor 1 A series of bits make up a binary number called a sample word common mode noise any signal which is common to both the negative and positive polarity of a balanced signal In a typical audio signal
28. escneeeeeeenneeeeeeeenaeeeeeeenaas Front Panel Controls cccccsssecceesescceeeeeeeeeeeeeeeseeeeesseneeeessenaes The Converter The Level Meters The Compressor Synchronization and Output Formatting Alternate Modes Rear Panel Connectors uunnssensensennnennnnnnnnnennnnnnnnnnnnnnnn nennen Audio Gonnecti nssu x seeckes castecieccodactazicsdashecioscatenbicecneastatiacdubaeties ca Connectors Cables Using the 20 20 AD uunsnennnssennnnannnnnnnnnnnnnnnnnnnnnennnnnnnnnnnnnnnennennnnn nennen Analog In D EEO PAE tee een Termination and Grounding via Internal Jumpers u Balanced and Unbalanced Input Signals nen Front Panel Adjustments 4444ssennnnnnnnnnnnnnnnnnnnnnnnnnn namen Synchronization and Sample Rate nnsssseennsnsennnnennnnnnnnnnnnn AES Sync Word Clock Internal and External Modes About Dithehia sian fiid ete AAG ite ete Psychoacoustically Optimized Noise Shaping PONS Automatic Offset Removal 20u4440snsnnonnnnnnnnnnnnnnnnnnnnnnnnnnn Compressor Modes ccceccceeeeeeeeeeeeeeeeeeeeaeeeceaeeeeeaaeeseeeeesnaaeeeaes Dynamic Range Compression Non Linear Transfer Functions Overload Protection Digitizing Analog Tape Link Display Digital Audio Outputs 200rsnsennnsennnnnennnnnnnnnnnennnnnnnnnnnnnnnnannn Realigning Two DAT Recordings 4444srnn nennen nennen nen Digital Sinewave Generator
29. ets the level at which com pression starts RELEASE This control allows adjust ment of the release time of the compressor Release times from OmS to are available The attack time of the compressor is automati cally set as a function of the signal rise time Controls and Connectors Synchronization and Output Formatting DITHER Selects the type of dither wide bandwidth 20 bit or High Frequency FORMAT Selects the format to be available at the output con nectors AES or S PDIF Note The format selected here will be sent to all out puts DITHER FORMAT a AES WC 4 SAMPLE RATE m gt e wn lt z oO AES WC In internal sync mode PRES indicates that either WC or AES is available at the inputs as a sync source In external sync mode PRES indicates that the selected sync source is present at the input connectors Alternate Modes ALT ATTEN Mutes the outputs Note that the level meters still work To unmute hold ALT and press ATTEN again ALT 4 CHAN Activates output of a digital sinewave at the AES ports To activate the alternate modes press and hold r 1dB SCALE ATTEN LOCK lights in external sync mode to indicate the unit has successfully locked to the in coming signal WC or AES then press one of these gt DISPLAY DITHER ALT MUTEC RMT LINK met 4 CHAN Repeat to
30. he LOCK light is off the selected sync source is present but is not within the 41 51kHz lock range AES PRES LOCK With WC selected as the external sync source the front panel SAMPLE RATE switch determines the setting ofthe AES and S PDIF Channel Status sample rate bits notthe actualsample rate which tracks the external sync source It is important to make sure that the sample rate selected at the front panel agrees with the external sync source as some digital tape recorders determine sample rate from the Channel Status bits rather than from the actual sample rate External Mode External Mode Error States AES PRES To operate in Internal mode press EXT SYNC until the WC and AES lights are both off Then press SAMPLE RATE until the desired sample rate lights In Internal mode the PRES light indicates that either WC or AES is available as an external sync source Internal Mode PRES Using the 20 20 AD Internal and External modes Using the 20 20 AD Lexicon About Dither Dither is a low level pseudo random signal which is added to digital audio to Psychoacoustically Optimized Noise Shaping PONS reduce quantization noise in effect by replacing it with a nicer sounding noise If the recorder or workstation which follows the 20 20 AD is capable of recording or processing 20 bits the DITHER switch should be set to 20 BIT If however the equipment can only record or process a 16 bit signal eit
31. her WIDE or HF dither should be selected This is true even if the compressor is on the dither generator always sends the appropriate signal for a 16 bit recorder regardless of the compressor settings All three of these basic dither types have the same quantization noise reducing effect but differ in the spectral distribution of the replacement noise WIDE dither produces white noise HF high frequency dither produces noise whose amplitude is lower than WIDE at low frequencies but higher at high frequencies Theoretically this is less audible than WIDE dither but just as effective 20 BIT dither has the same spectrum as HF dither but with a lower amplitude This setting should be used when recording to 20 bit media It is possible to filter the quantization noise in such a way that almost all of the noise within the frequency range where human hearing is most acute is shifted to higher frequencies This is called psychoacoustically optimized noise shaping PONS because it takes advantage of the variations in the ear s sensitivity to noise within the 20 Hz 20 kHz range To activate this dither press and hold the ALT button then press DITHER The three dither lights will go on WIDE 20BIT and HF to indicate that you are in PONS mode Note that due to limitations in processing power the compressor 4 channel mode and the ALT MUTE function are disabled while PONS mode is active To deactivate PONS simply press DITHER
32. ibrations per unit of time ex pressed in cycles per second Hz harmonic distortion the appearance of harmonics of the input signal at the output of a device link the tying of two channels of the 20 20 AD compressor to a shared gain curve lock two or more audio signals which have the same frequency and phase mS millisecond 1 1000 of a second oversampling a method of sampling an audio signal at many times the audio bandwidth typically 64 256 times in order to reduce the complexity of the anti aliasing filter The oversampled data is then digitally filtered to reduce the sample rate period the interval of time required for a waveform to complete one cycle and begin to repeat itself Period 1 frequency phase the angular relationship between two signals meas ured in degrees where 360 the period of the waveform When two identical signals are always of the same polarity they are said to be in phase or to have a phase of 0 When they are always of opposite polarity they are said to be out of phase or to have a phase of 180 polarity the positive or negative direction of an electrical force quantization the process of converting an infinitely varying waveform into a finite series of discrete levels The inaccuracy introduced by this process is called quantization error rms amplitude root mean squared amplitude a way of assigning a DC power equivalent to an AC waveform For a sine
33. in dB which is being reduced as the input level exceeds the threshold The 20 20 AD provides two sets of digital audio output connectors with three connectors in each set In four channel mode the first set contains digitized CH1 and CH2 the second set contains CH3 and CHA In two channel mode both sets contain identical copies of digitized CH1 and CH2 The unused set may be used to drive a backup recorder or to synchronize another 20 20 AD Each set of digital outputs contains three connectors an XLR connector which transmits AES3 digital audio an RCA connector which transmits an S PDIF compatible digital audio signal and a TOSLINK compatible optical transmitter All outputs are always active The front panel FORMAT switch selects between AES and S PDIF compatible Channel Status bits but it doesn t enable or disable the actual transmitters Make sure that the FORMAT setting is set for AES when using the XLR connector and S PDIF when using the RCA or optical connectors Pin 1 of the AES outputs is connected to system ground through jumpers W26 and W27 If the shield is connected to ground at the receiver end these jumpers should be opened Note that because of the transformer or optical isolation it isnotnecessary to connect the ground between the 20 20 AD andthe equipment receiving the digital audio data regardless of which output type you choose As mentioned above only appropriate high frequency cables should be used for digital
34. itute parts or perform any unauthorized modification to the instrument DANGEROUS PROCEDURE WARNINGS Warnings such as the example below precede potentially dangerous procedures throughout this manual Instructions contained in the warnings must be followed Dangerous onp WARNING death are pres ent in this instrument Use extreme caution when han dling testing and adjusting SAFETY SYMBOLS SAFETY SUMMARY The following general safety precautions must be observed during all phases of operation service and repair of this instrument Failure to comply with these precautions or with specific warnings elsewhere in these instructions violates safety standards of design manufacture and intended use of the instrument Lexicon assumes no liability for the customer s failure to comply with these requirements General definitions of safety symbols used on equipment or in manuals Instruction manual symbol the product will be marked with this symbol when it is necessary for the user to refer to the instruction manual in order to protect against damage to the instrument Indicates dangerous voltage Terminals fed from the interior by voltage exceed ing 1000 volts must be so marked The WARNING sign denotes a hazard It calls attention to a procedure practice condition or the like which if not correctly performed or adhered to could result in injury or death to personnel The CAUTION sign denotes a hazard I
35. l 1125 937 5 750 eo 9000 e 110625 140625 _ AS 562 5 17062 5 375 A EN 20062 5 GAIN Frequency in Hz 48 kHz Sample Rate 1895 1378 1033 5 N 861 5 o 689 Eu FE 4651 74 ZT 63 eo 8269 e 10163 5 AC 12920 me 2 2 X 24 o E 26 A N z Ne HIN THRESHOLD Level in dBfs z Ng 517 15676 344 5 ae ab 18432 5 GAIN Frequency in Hz 44 1 kHz Sample Rate 3 To turn off the sinewave generator press and hold down 1dB SCALE ALT and press 4 CHAN The 20 20 AD will resume normal operation Note that in sinewave generator mode only the compressor GAIN and THRESHOLD knobs have any effect Pressing any dual key combinations other than ALT 4 CHAN may invoke undocumented diagnostics routines If you accidentally get into diagnostics mode the best way to exit is to turn the power off then on again Definition of Terms AES Audio Engineering Society aliasing a form of distortion which occurs when an A D converter s sample rate is less than twice the highest fre quency component of a signal amplitude the maximum departure of the value of a wave form from its average value attenuation a reduction in the amplitude of an audio signal balanced signals two equal and opposite signals run through a twisted pair The advantage of balanced signals is that common mode noise tends to be added equally to both portions of the pair it can be removed by subtracti
36. le clicks If for artistic reasons you wish to turn the compressor off during a recording do so by increasing the release time to oo Using the compressor on off switch may produce a click 20 20 AD User Guide Using the 20 20 AD When RELEASE is set to 0 the compressor acts like an amplifier with anon Non linear linear transfer function For example setting GAIN to 6dB and THRESHOLD to Transfer Functions 14dB produces a close approximation of a generic tape saturation curve This mode is also useful for overload protection of some types of percussive instruments Note This function is not available in 4 channel mode When recording live music onto 16 bit media the recording engineer tries to Overload Protection maximize dynamic range by keeping the signal as close as possible to full scale without actually clipping Peak levels are typically set a few dB below full scale because losing a few dB of dynamic range is far preferable to clipping The digital compressor eliminates this tradeoff To use it first set the input levels to give yourself a comfortable margin between peak level and clipping For example to give yourself 4dB of margin push 1db SCALE and PK HOLD and adjust the input level until the peak reads 4dB the LED labeled 18 Now turn on the compressor Set GAIN to 4dB This increases the signal level by 4dB restoring OdB peak level Set THRESHOLD to OdB to automatically select the maximum practical ratio
37. nd CH4 This allows you to take advantage of the extended dynamic range of two channel mode when using only a single stereo pair When digitizing stereo pairs the left channel of each pair should be sent to CH1 and CH3 the right to CH2 and CHA The 20 20 AD accepts balanced or unbalanced signals with the following peak input levels Attenuator Signal Type balanced balanced unbalanced 2 to 22 8 to 28 4to 16 0 to 20 6 to 26 6 to 14 Unbalanced signals cannot be correctly attenuated with the front panel ATTEN button Input levels of unbalanced signals must be lowered before entering the 20 20 AD The analog XLR connectors should be wired as follows Signal Type balanced screen return shield screen and return positive polarity unbalanced positive polarity open or return In a typical studio environment each piece of equipment is connected to a star ground through the safety ground on the AC connector or through a ground strap tied to the chassis In these cases connecting the shield of the audio cable s to pin 1 at both ends of the cable will cause the 20 20 AD to be grounded to the audio source through two paths the shield of the audio cable and the safety ground or ground strap This will result in a ground loop which manifests itself as hum at the power line frequency A common solution to this problem is to connect the cable shields only at the source end and to leave
38. ng antenna Relocate the computer with respect to the receiver Move the computer away from the receiver Plug the computer into a different outlet so that the computer and receiver are on different branch circuits If necessary the user should consult the dealer or an experienced radio television technician for additional suggestions The user may find the following booklet prepared by the Federal Communications Commission helpful How to identify and Resolve Radio TV Interference Problems This booklet is available from the U S Government Printing Office Washington DC 20402 Stock No 004 000 00345 4 This triangle which appears on your component alerts you to the presence of uninsulated dangerous voltage inside the enclosure voltage that may be sufficient to constitute a risk of shock This triangle which appears on CAUTION your component alerts you to important operating and main RISK OF ELECTRIC SHOCK i in thi S DO NOT OPEN tenance instructions in this ac companying literature Copyright 1992 Lexicon Inc 3 Oak Park Bedford MA 01730 Tel 781 280 0300 Fax 781 280 0499 All Rights Reserved This publication is protected by copyright and all rights are reserved No part of it may be reproduced or transmitted by any means or in any form without express prior consent in writing from Lexicon Lexicon Part 070 09090 Printed in the United States of America GROUND THE INSTRUMENT
39. ng the negative from the positive signal Common mode noise rejection cannot be performed on unbalanced signals which consist of an audio signal and a ground return bandwidth the arithmetic difference between the upper and lower cut off frequencies of an audio system bit binary digit the smallest unit of digital information equivalent to the result of a choice between two alternatives yes or no onor off Oor 1 A series of bits make up a binary number called a sample word common mode noise any signal which is common to both the negative and positive polarity of a balanced signal In a typical audio signal common mode noise is introduced over the length of the cable by outside interference A typical example is line hum CMRR Common Mode Rejection Ratio measures the ability of a system to reject common mode noise It is expressed as the ratio of common mode noise entering the system to the common mode noise leaving it dBfs decibel full scale this is the unit used to describe the amplitude of a digital audio signal where O equals digital full scale digital headroom does not exist see dBfs distortion an unwanted change in a waveform as it passes through an electronic component or from one medium to another dither a low level signal which is added to digital audio to reduce quantization noise in effect by replacing it with noise which is perceived as less offensive dynamic range when used to des
40. nverters do not mute when they lose lock The AES SYNC IN signal can be any AES3 compatible signal running at the correct sample rate It does not require a Digital Audio Reference Signal DARS The 20 20 AD is shipped with a 110 ohm termination resistor across pins 2 and 3 of the XLR connector as required by the AES3 specification If you intend to daisy chain several 20 20 ADs from the same AES source you will have to open jumper W17 on all units except the last one in the chain Jumper W18 connects the cable shield to system ground The 20 20 AD is shipped with W18 open assuming that ground is supplied from the generator of the sync signal Note that unlike the analog inputs the AES SYNC input is transformer isolated it is not necessary to have a ground connection between the 20 20 AD and the sync source All AES connections require twisted shielded pair cable with a controlled impedance between 100 and 125Q and less than 15 pF ft of capacitance Microphone cable will not work reliably Refer to Page 7 for recommended cables The WORD CLOCK IN signal should be a logic level logic high 3V 5V square wave When shipped the 20 20 AD terminates the signal with a 75Q resistor through the leftmost jumper in J13 If the clock source is not capable of driving high through 75Q most TTL and CMOS drivers are not it may be necessary to open this jumper In this case it is important that the cable be kept as short as possible As with AES
41. pin 1 of the 20 20 AD end floating If this is not practical you can open the ground jumpers W1 W9 W11 and W13 on the 20 20 AD s analog inputs Using the 20 20 AD Analog Inputs Termination and Grounding via Internal Jumpers Using the 20 20 AD XLR CONNECTOR Lexicon CAUTION These instructions are for use by qualified personnel only Before opening the 20 20 AD REMOVE THE POWER CORD You may be exposed to hazardous voltages even if power to the unit is turned off Turn off power and REMOVE THE POWER CORD before opening the unit Take a moment to study the figure below This diagram shows the terminating and grounding of input signals for the unit and explains the function of most of the jumpers on the 20 20 AD Main pc board The configuration shown illustrates the jumper settings when the 20 20 AD is shipped from the factory CHASSIS GROUN ANALOG IN BD A a ie SHIELDED TWISTED PAIR w10 AES OUT wi2 wi4 O OPW ES XLR W2 600 OHM CONNECTOR 38 AES SYNC IN SHIELDED TWISTED PAIR RCA CONN 38 CONNECTOR XLR CONNECTOR BNC CONNECTOR W17 110 OHM S PDIF OUT L u N 75 OHM COAXIAL WORD CLOCK IN SYSTEM GROUND
42. t calls attention to an operating procedure practice condition or the like which if not correctly performed or adhered to could result in damage to or destruction of part or all of the product The NOTE sign denotes important infor mation It calls attention to procedure practice condition or the like which is essential to highlight NOTE Electrostatic Discharge ESD Precautions The following practices minimize possible damage to ICs resulting from electrostatic discharge or improper inser tion e Keep parts in original containers until ready for use e Avoid having plastic vinyl or styrofoam in the work area Wear an anti static wrist strap e Discharge personal static before handling devices Remove and insert boards with care Whenremoving boards handle only by non conductive surfaces and never touch open edge connectors ex cept at a static free workstation e Minimize handling of ICs Handle each IC by its body Do not slide ICs or boards over any surface e Insert ICs with the proper orientation and watch for bent pins on ICs e Use anti static containers for handling and transport To make a plastic laminated workbench anti static wash with a solution of Lux liquid detergent and allow to dry without rinsing Introduction Controls and Connectors 0nn2u00000000nna000000nnnnnnunnnunnnnnnnnnnnnnnnn Unpacking Power Mounting cccccceee
43. ter grow down during the peaks and stay down 3 Rewind the tape turn on the digital recorder and play the tape all the way through The compressor will add just enough fixed gain to boost the peak to OdBfs Because the gain doesn t change there are no compression artifacts If you are in 4 channel mode remember that you have less dynamic range available so you should try harder to set the input levels as close to OdBfs as practical only relying on the 20 20 AD for the last 2 or 3dB 20 20 AD User Guide In 2 channel mode when the LINK switch is off each of the two channels is compressed independently When the LINK switch is on the louder of the two channels sets the amount of compression This serves the same function as stereo link in analog compressors When you are recording a stereo signal and want to maintain the stereo image LINK should be on However if you have the compressor set up to protect against short overloads or if you have the release time set to 0 keeping LINK off will probably sound better When recording two mono signals LINK should be off In 4 channel mode when the LINK switch is off channels 1 and 2 and channels 3 and 4 are linked in stereo pairs When the link switch is on all four channels are linked There is no way to compress four mono signals When the DISPLAY switch is on the level meters indicate the amount of compressor gain reduction in each channel This shows the amount that gain
44. verters for a number of reasons In the 20 20 AD these typically include temperature changes changes between 2 channel and 4 channel mode change of sample rate or sync source or large adjustments of input level controls The DSP chips in the 20 20 AD automatically remove these DC offsets by passing data from the converters through a digital high pass filter with a corner frequency of less than 0 1 Hz This high pass filter will reduce output offsets to less than 80dB within one minute of a change in input offsets In order to minimize output offsets wait at least one minute after any of the above changes before recording DC offsets created outside the 20 20 AD are removed by blocking capacitors located between the analog input connectors and the first amplifier stage These capacitors have a corner frequency of less than 1 Hz which guarantees less than 0 1dB of loss at 10 Hz referenced to 1 kHz It is possible to extend the low frequency response down to less than 1 Hz by shorting these blocking capacitors This should only be done if the offset of the input signal is relatively constant and less than 1 of full scale approximately 150 mV with a 22 dBu input Transformer isolated sources are generally safe but non isolated sources should be checked with a voltmeter Offsets greater than 1 may take several minutes for the digital high pass filter to remove To short out the blocking capacitors remove C5 C6 C30 C31 C51 C52 C80
45. wave this is 0 707 times the peak level sample rate the periodic interval at which an analog audio signal is sampled e g 44 1 or 48 kHz signal to noise ratio the ratio between the maximum peak signal and the wideband noise in the absence of signal This number becomes more meaningful when distortion is added It is then expressed as S N D S PDIF Sony Phillips Digital Interface Format synchronize to arrange events to occur simultaneously In audio the events are both the period and the phase of a signal THD N the ratio of the rms amplitude of the distortion and noise that is added to a signal to the rms amplitude of the signal itself This can be expressed in percent or decibels S N D 1 THD N time constant the amount of time it takes for a signal to rise or fall 63 of its full scale This only pertains to signals which rise and fall exponentially such as the 20 20 AD meter time constants or compression gain curves word clock a square wave used as a synchronization source Its frequency equals the sample rate 27 lexicon 3 Oak Park Bedford MA 01730 USA Tel 781 280 0300 Fax 781 280 0499
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