DAVID-III - Inovonics
Contents
1. sees 10 NORMAL SETUP PROCEDURE iue 12 INPUT GAIN Calibration MPX LEVEL Adjustment TRIBAND PROCESSING SETUP GUIDELINES 14 AGC Section Notes MASTER DRIVE BASS and BRILLIANCE FINAL 16 PIPP LIMITER COMP CLIPPING HOW DOES SOUND ia 17 Section IV CIRCUIT DESCRIPTIONS INTRODUCTION 5885 18 APWM PRIMER 18 PWM Gain Control Feedforward Processing Soft Knee The PWM Ramp Waveform PWM RAMP f CIBOGUIES bu dis 20 Master Clock Compressor Limiter Ramp AGC Ramp SIGNAL PATH OIROUITPY EA AEE AER EEE 21 INPUT STAGES EPOD RCM du 21 Line Input Balancing INPUT LEVEL Control AGC Stages AGC Gating AGC Metering Pre Emphasis Program Pre Clipping TRIBAND PROCESSING 23 Band Division Band Rectifiers Midband Time Constants Bass Processing Brilliance Processinge PWM Comparators Gain Reduction Indicators Combining Amplifier 27 LOW PASS FILTER 28 Filter Overshoots Overshoot Compensation The Low Pass Filter PILOT AND S2. AGC Metering Pre Emphasis Program Pre Clipping Six op amp sections monitor the output of IC45D and drive the seven front panel A G C GAIN indicators Each op amp section has gain and fixed bias values to ensure that the seven A G C GAIN LEDs fade evenly over the 5dB indicated steps of gain Jumping down to Page 39 IC49B has integration in its feedback path to effect proper program pre emphasis Jumper strip JP6 selects either the 75us or the 50us characteristic Pre emphasis selection is described on Page 9 The feedback path of IC49B also includes transistors Q32 and Q33 These serve as hard clippers for program signal peaks the clipping point determined by DC bias applied to the transistor bases Program pre clipping is keyed to the average value of the program signal This L R program average value is derived from the output of the AGC amplifier by IC45B Time constants of R277 and C96 are relatively quick so the PGM AVG voltage closely follows the envelope of the L R program signal The gain of IC45B maintains the PGM AVG DC voltage level at a point 10dB to 12dB above the actual program audio level at the output of IC49B This means that when program dynamics exceed an average to peak ratio typical of music and speech program peaks are hard clipped The effect of clipping at this point is seldom ever audible In the first place sadly few CD music tracks contain peaks more than 10dB above the true average level of th
3. 1 99 40 The Triband Processing section of the DAVID III operates in a feedforward mode rather than using the more traditional feedback method The amplitude of the input signal establishes circuit gain to hold the output constant at a 100 modulation ceiling value Feedforward limiting necessitates a conversion factor to give one dB of output attenuation for each dB that the input signal increases Put into linear voltage rather than log dB terms this function is expressed as 1 Figure 5 graphs this function x represents the circuit gain needed to hold the output at a constant 1 volt value with an input y that increases from 1 volt to 10 volts This technique becomes a bit precarious at small duty cycles attenua tions of 40dB or more but in the 0 to 30dB range typically required for audio processing PWM is easily implemented perfectly predictable and without sonic coloration 10 8 9 id us a E Figure 5 Voltage Transfer AY INE BR Function morirlo lt 0 0 2 0 4 0 6 0 8 1 0 CIRCUIT GAIN Assuming a linear relationship between PWM on off time and signal gain reduction the 1 y expression translates directly to the duty cycle that is 0 1 is a 10 ON time and 1 0 a 100 ON time In the example above 1 volt represents a threshold level below which gain will remain at full value In practice the threshold is adjusted to conf
4. Midband gain reduction voltage is integrated by R165 and C57 The voltage across C57 represents the long term average of gain reduction in effect This voltage is buffered by IC34A which has about a DC gain This voltage G R 3dB is connected to the top of the BASS and BRILLIANCE adjustments C65 integrates the DC peak value of the rectified bass channel input BASS RECT though integration is not instantaneous because of series resistor R213 This resistor does not have the expected effect of slowing down the bass limiter attack time as the rectified input is fed directly to the bass band PWM comparator Q23 Q22 and Q21 The charging of C65 is delayed to be sure which causes self modulation for the first few cycles of the bass waveform This 25 Brilliance Processing PWM Comparators Gain Reduction Indicators imparts a delightfully gutsy quality to the attack of bass instruments making the bass sound phatter BASS control R142 delivers variable amounts of gain spoiling DC bias to the gain reduction circuitry When set at 3 an amount of DC equal to average midband gain reduction plus an additional 3dB due to gain in IC34A becomes the reference for bass band gain reduction release Bass peaks exceeding midband program energy will be limited independently but gain in the bass band will be spoiled by the additional 3dB even at full release Set at OdB the wiper of R142 is at
5. The 200ns negative going pulse from IC44A labeled AGC CLOCK may also be picked up on Sheet 1 of the Schematic Page 38 This pulse turns Q26 to charge C109 This capacitor discharges through R304 and R305 to create a separate second ramp waveform for the AGC stage This discharge shape is a simple R C function As the AGC stage works in a feedback mode the R C ramp gives a constant dB per second control rate over the 304 range SIGNAL PATH CIRCUITRY INPUT STAGES Line Input Balancing INPUT LEVEL Control NOTE Left and right program channel circuits are identical so only the left channel will be described in detail A skilled technical professional should be able to extrapolate the circuit descriptions for the right channel Leafing to Page 38 1 48 is the program line input balancing amplifier JP2 and JP3 short circuit resistors R358 and R359 respectively to give IC48B additional gain for low level program inputs A single multiturn potentiometer R326 sets the DAVID III input sensitivity for both stereo channels IC54B and IC55A include both positive and negative feedback to create constant current sources for the left and right channel program signals feeding the two ends of R326 Despite appearances INPUT GAIN control R326 does not operate as an arm of a voltage divider Constant current feeds to the two ends of this pot makes its attenuation of the program signal directly proportional to the re
6. Why in the world would anyone build a comparator from discrete transistors instead of using an integrated circuit For a couple of reasons actually Most fast comparator ICs do not have sufficient input dynamic range for DAVID III applications many have a pathetic 5 volt restriction on the differential input range Comparators that have sufficient differential and common mode specifications aren t fast enough and those that are speedy tend to oscillate if you look at them wrong Finally three transistors cost a fraction of what the one or two single sourced ICs cost that we could have used in these spots G R gain reduction indicators utilize a simple differential amplifier driving a pair of LEDs In each of the three bands the DC control voltage is fed to one input of the differential amp and the compres sor limiter release level is fed to the other Gain and offset of the 26 stages is such that the green LED glows at full brightness with no reduction in effect fading to full brightness of the red LED with 12dB of reduction Midway through the display range both LEDs should be lit about halfway Combining Back on Page 39 IC33B is a current summing combining amplifier Amplifier and low pass filter The three frequency bands are summed through resistors R235 R234 and R233 with a CMOS analog switch sections in series with each Gain control PWM drives the analog switches the duty cycle effectively multiplying the 3k se
7. Variations from these conditions such as an intermediate STL or other link in either the input or the output path of the DAVID III may call for considerations not addressed here At this point the DAVID III should be installed in the program chain with power applied The MODE switch should be in STEREO and the PIPP LIMITER switched ON The single INPUT GAIN adjustment controls both stereo channels simultaneously It is set simply to center the AGC operating range with a nominal Zero VU program line input of typical program material If this procedure shows control range to be outside adjustment limits recheck circuit board jumpering for Line Input Range Selection as described on Page 8 With program from the console or link receiver feeding the DAVID III adjust the front panel INPUT GAIN control so that the A G C RANGE indicator hovers around the OdB mark most of the time If this procedure is performed properly and if the operator pays reasonable attention to console metering the AGC GAIN indicators should normally hover around and always remain in the safe operating zone between 10dB and 108 This part of the procedure is performed on air under normal operating conditions and using typical program material You must have access to the station s Modulation Monitor An RDS or SCA subcarrier should not be connected until Step 6 12 PLEASE be certain that you understand operation of the Mod Monit
8. users With all due diligence please avoid the temptation to use cross referenced hobbyist or TV VCR Repair Shop direct replacement parts Nearly any semiconductor IC capacitor resistor or connector used in the DAVID III will be available from one or more of these firms Each supplier maintains a Website and publishes a full line printed catalog which is free for the asking Minimum order restrictions may apply and export orders may prove somewhat problematical Mouser Electronics 1 800 346 6873 Digi Key Corporation 1 800 344 4539 Future Active Industrial Electronics 1 800 655 0006 Allied Electronics w alliedelec com 1 800 433 5700 37 9 0 abed rooe aA 002661 19 038318 305539034 4 6 817 TAGON SOINOAONI JILVIN3HOS A 324 NEE 9084 7068 OL V 324 6054 6 0 08 g eH 2 lt YOL 260 4608 yose 39 6661 ita 2200 way 1919 9983 volo 2200 6212 vesal lt 3989 ns du 8880 Sheu yore 4022 tel di Sik NIV9 6810 TO 9880 cAi 905 00591 Ns D 96831 N 1431 1023 4022 964 cet 1519 7200 68H 8212 38 9102 8084 00 661 _ y ENA T 1 889 01 301 954 YOL e om 650
9. Possible alternates are the CSF Thompson IRD series or the Vishay Roederstein KE 1808 or KT 1817 series 1 0uF and above are general purpose aluminum electrolytics with radial leads A safe voltage rating for any electrolytic in the DAVID III would be 25V but because of size and other considerations a replacement capacitor should always carry the same rating as the one being replaced 34 1 2 5 8 9 C14 16 28 29 31 35 41 44 50 51 60 61 70 71 81 91 114 130 CR1 6 57 1 1 11 12 19 13 14 5 16 8 10 12 18 17 9 11 14 15 16 113 17 1 1 IC3 44 1 4 12 13 14 16 17 22 25 28 33 35 40 48 55 1 5 9 IC8 5 18 IC19 1 26 41 59 1 27 84 45 46 79 3 4501 JMP1 6 L1 L2 Q1 Q2 Capacitor Y class Ceramic Disc 0047uF 440VAC Murata Erie DE7150 F 472M 1 Capacitor Variable 5 50 Mouser 24 024 Capacitor Electrolytic axial leads 1000uF 35V open Capacitor High Q Polypropylene 0 0033uF 2 5 100VDC WIMA MKP 2 series Capacitor Non Polar Electrolytic 22uF 25V open Diode Silicon Rectifier open 1N4005 Diode Silicon Signal open 1N4151 Fuseholder Littlefuse 0286067 The fuse itself is a 5mm normal fast blow type the value should match the specification stated on the rear panel Ferrite Bead Amidon 73 801 T1 Red LED Kingbright L934ID T1 Green LED Kingbright L934GD T1 Yellow LED Kingbright L934YD T1 Bi
10. Referring to the Sheet 6 of the Schematic on Page 43 IC3D is the DAVID III master clock running at 1 216MHz which is 64 times the stereo pilot frequency 1 8 is a buffer and binary dividers 1 21 1 48 4 derive the 152kHz PWM switching frequency This is the PROC CLOCK that is picked up on Sheet 3 of the Schematic Page 40 The 152kHz PROC CLOCK is buffered by IC44D The squarewave differentiated by C107 and R302 creates a 200 nanosecond pulse at the output of IC44C The positive going edge of the pulse turns on Q25 which brute forces the pedestal portion of the PWM ramp waveform 90 The negative going edge of the 200ns pulse is inverted by IC44A and differentiated by C93 and R267 The output of IC44B is a 100ns negative pulse that turns on Q24 to charge C95 instantaneously to about 6 volts C95 has a complex discharge path calculated to create the precise PWM reference ramp waveform The initial fast discharge is through R272 and diodes CR44 CR45 and CR46 After an initial discharge period CR46 is biased out of the equation and discharge continues through R273 When the voltage level falls below the turn on point of CR44 and CR45 the remaining discharge path is through R271 The action just described produces the ramp waveform of Figure 6 which is repetitive at the 152kHz rate The ramp is presented to one input of the three comparators that create the PWM switching squarewave
11. a nutshell positive and negative program peaks are limited independently of one another This ensures full modulation of the FM carrier and an audible increase in program perceived loudness Refer to the more detailed discussion under the FINAL PEAK CONTROL subhead on Page 16 11 COMP CLIPPING In the fully counterclockwise OUT position of this MODE control an output safety clipper catches only those occasional very fast peaks or overshoots that sneak by the earlier protection circuitry in the signal path As COMP CLIPPING is rotated clockwise the level applied to this safety clipper is increased by the dB value shown by the panel markings MODE simply switches the operation of the DAVID III between normal multiplex stereo transmission and forced monaural operation In the MONO mode both the 19kHz stereo pilot and the 38kHz subcarrier are turned off and left and right program channels are summed FUSE DISCONNECT The front panel fuseholder also provides a means of disconnecting AC mains power in an emergency Push the fuseholder cap down and pull it away from the panel to interrupt power NORMAL SETUP PROCEDURE INPUT GAIN Calibration MPX LEVEL Adjustments This setup procedure presupposes a normal installation with the DAVID III PROGRAM LINE INPUT fed directly from the output of the audio console and with a direct connection from the COMPOSITE MPX OUTPUT to the wideband input of a transmitter exciter
12. found white paper on the Inovonics limiting is protected by US Patent No 6 658 120 COMP Composite clipping has traditionally been the tool used to squeeze CLIPPING the very last bit of loudness from the processed FM program signal Too much clipping of the multiplex signal can generate spurious harmonic products that clutter the baseband spectrum interfering with the 19kHz pilot and compromising performance of RDS and SCA subcarriers In the DAVID III the multiplex waveform is clipped prior to the point at which the 19kHz stereo pilot is inserted This ensures that the pilot is not directly modulated though the pilot is not protected from harmonic products that are generated as the program waveform is clipped at fractional intervals mostly one third and one fifth of the pilot frequency The following tabulation is an approximation of the SG Spectral Garbage generated by composite clipping COMPOSITE PEAK VALUE OF PROCESSING CLIPPING PRODUCTS OUT gt 60dB below 100 Mod 1dB 50dB below 100 Mod 2dB 45dB below 100 Mod 3dB 40dB below 100 Mod Keep clearly in mind that crosstalk of program audio harmonic components into the RDS or SCA subcarrier region is exacerbated by composite clipping so use this feature sparingly Generally 1dB of COMP CLIPPING can be tolerated without harmful effect HOW DOES IT SOUND The foregoing overview of processor setup is intended only as a guide Final decisions
13. held between samples by C26 and buffer IC18A The stereo multiplex signal consists of 16 discrete sine value weighted steps When the front panel MODE switch 52 is set to MONO the counting sequence is halted with the counter preset at the center position count of 4 This stops the subcarrier generation and L R audio appears at the DAVID III output S2 also stops the stereo pilot in the MONO mode Diodes CR12 and CR13 are biased to the same 100 modulation level established by the PIPP Limiter and overshoot compensation circuits The buffered composite signal less the stereo pilot is pre sented to this clipper through front panel COMP CLIPPING control R36 When R36 is set to its fully counterclockwise position OUT the two diodes catch only the occasional program peak which somehow has eluded the earlier processing and clipping stages As R36 is cautiously turned clockwise program peaks at the 100 modulation value may be subjected to as much as 3dB clipping Even though the composite waveform is clipped prior to pilot injection good broadcasting practice demands judicious use of this feature See Page 17 IC11B is a combining stage for the main sub portion of the composite signal and for the 19kHz stereo pilot The front panel MPX LEVEL control R41 is in the feedback path of IC11B affording adjustment over the composite level delivered to the exciter Since all components of the composite output are digitally synthesi
14. product of the band splitting E45 architecture that actually gives an advantage the DAVID III Note that the dip falls in the lower voice range Thus when bass is boosted voices won t tend to become tubby FINAL PEAK CONTROL PIPP LIMITER As previously defined PIPP is an acronym for Polarity Independent Peak Processing This function separates the gain control action between the positive and negative excursions of the program waveform with the zero volt DC baseline of the incoming signal as the point of reference This elementary explanation will undoubtedly elicit a horrified response of But that adds distortion Well yes and no The only truly undistorted waveform that we can measure or perceive as a pure tone by ear is a textbook sinewave Because a sinewave is perfectly symmetrical the PIPP limiter acts equally on both the positive and the negative excursions and thus does not distort the wave shape in the slightest An asymmetrical waveform on the other hand most notably a solo musical instrument or speech will have asymmetry and peakiness removed by the PIPP limiter Does this imply that the waveform by definition already full of harmonics ie distortion will further be distorted Technically yes A saxophone may sound more saxophone like and speech will definitely have a different quality But after all we are talking audio processing here and sonic modification is par f
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16. BCARRIER SYNTHESIS Clock Pilot Generation Center Sampling Subcarrier Generation Turn if you will please to the last sheet of the Schematic on Page 48 As previously described in the PWM discussion IC3D is a crystal controlled oscillator at 1 216Mhz 1 8 buffers the clock and IC21A the first binary divider furnishes the 608kHz digital synthesis sampling frequency IC10 is an up down BCD counter clocked at the 608kHz sampling frequency 1 of 10 decoder IC9 OR gate IC8B and binary divider IC7A work together to keep IC10 continually counting from zero to 8 back down to zero etc Counting logic is decoded by a 1 of 9 de multiplexer IC18 and IC20A that samples a resistor string with sine weighted values This generates one half a sinewave for each counting cycle IC21B reverses the DC polarity applied to the top of the resistor string for every other up down count forming the complete 19kHz pilot sinewave from 32 discrete steps R68 introduces an offset to compensate for any difference between the power supply rails This is adjusted to null the 38kHz second harmonic component The segmented Stereo Pilot is buffered by IC17B Analog switch IC20C is controlled directly by the 1 216MHz clock It turns on for one half of one clock cycle precisely at the center of each stepped pilot waveform sample This charges C27 to the sample voltage value which is held by buffer stage IC17A until the next center sample is taken Ce
17. Color LED Mouser 512 MV5437 T134 Red LED Kingbright L53ID 1 Green LED Kingbright L53GD T1 Yellow LED Kingbright L53YD Integrated Cct open LM317 T Integrated Cct open LM337 T Integrated Cct open CD4011 Integrated Cct open LF353N Integrated Cct open CD4028B Integrated Cct open CD4071B Integrated Cct open CD4051B Integrated Cct open CD4081B Integrated Cct open CD4066B Integrated Cct open LM324N Connector AC Mains Switchcraft EAC303 Connector XLR Female Mouser 568 NC3FAH 0 Connector BNC chassis mounting male Mouser 528 81 221 75 Shorting Shunt for 0 1 inch header strips open Inductor 560uH Molded Inductors Inc CTS3 561J Inductor 220uH Molded Inductors Inc CTS2 221K Transistor NPN open MJE340 Transistor PNP open MJE350 Q3 5 8 9 12 Transistor NPN open 2N3904 18 25 80 88 Q4 6 7 10 11 14 15 18 Transistor PNP open 2N3906 21 24 26 29 31 32 Q16 17 19 20 22 Transistor NPN open 2N5088 23 27 28 Except at noted by reference designation all resistors used in the DAVID III are the value specified on the schematic qualified per the following a Fixed Resistors with values carried to decimal places implying 1 tolerance example 232 3 01K 10 0K 301K are watt 1 metal film type b Fixed Resistors with values typical of a 5 tolerance example 220 3 3K 10K 270K are 5 carbon film type Single Turn
18. Inputs Jumpers for LOW LEVEL Inputs Figure 2 Input Gain Range Jumpering 8 PRE EMPHASIS SELECTION The DAVID III supports both the 75 microsecond FM broadcasting pre emphasis standard common to the Western Hemisphere and parts of the Orient and the 50 microsecond standard used in Europe and Asia Pre emphasis appropriate to the delivery destination is set at the factory but this can be changed if necessary Beneath the top cover locate the two three pin jumper strips labeled JP1 and JP6 These are located in the rows of components behind the left channel XLR connector and just either side of IC49 The push on jumper shunts may be moved between the 50 and 75 positions and both jumpers must be set identically as shown in Figure 3 below Jumpers for 75us Jumpers for 50us Figure 3 Pre Emphasis Jumpering COMPOSITE MPX OUTPUT The unbalanced COMPOSITE MPX OUT BNC connector on the rear panel of the DAVID III has a resistive source impedance of 75 ohms This allows cable runs of moderate length to about 50 feet to the exciter or microwave link input If the cable is more than just a few feet the cable itself should have a 75 ohm characteristic impedance and be terminated in 75 ohms at the far end A Stereo Generator must always be connected to an exciter or link STL input specifically intended for multiplex stereo This is a wide
19. OPERATING AND MAINTENANCE INSTRUCTION MANUAL DAVID IIT MODEL 718 FM STEREO PROCESSOR GENERATOR INOVONICS INCORPORATED USER S RECORD Model 718 Serial No Date Purchased Warranty Card Mailed OPERATING AND MAINTENANCE INSTRUCTION MANUAL DAVID III MODEL 718 FM STEREO PROCESSOR GENERATOR June 2004 e 1305 Fair Avenue Santa Cruz CA 95060 N OVO N 831 458 0552 831 458 0554 INCORPORATED Visit our Website www inovon com TABLE OF CONTENTS Section INTRODUCTION MODEL 718 PRODUCT DESCRIPTION 3 History Features MODEL 718 TECHNICAL SPECIFICATIONS sese 4 BLOCK DIAGRAM eT 5 Section Il INSTALLATION UNPACKING veo rri rtr bh rea oae baa re 6 DON WINN C DEPONERE P I CE ECC MN e PH 6 Rack Requirement Heat Dissipation MAINS POWER i lena ede MORI n est lef 7 Fuseholder Mains Voltage Selector Power Cord RADIO FREQUENCY INTERFERENCE 7 Location Ground Loops LINE INPUT AND RANGE 8 Input Connections Unbalanced Inputs Input Grain Range Gain Jumpers PRE EMPHASIS SELECTION a5 ipa rer 9 COMPOSITE MPX OUTPUT ette eene e rne Fo bm 9 Section SETUP AND OPERATION PANEL CONTROLS AND INDICATORS
20. Trimming Potentiometers circuit board are Tocos GF063U1 series d Multi Turn Trimming Potentiometers front panel adjustable Tocos RJC097P series or equivalent cermet types R36 142 183 243 Potentiometer 10K Piher PT10MH01 103A2020 with Figure 15 spindle Inovonics P N 0580 51 Switch Voltage Selector ITW 18 000 0022 52 8 Switch SPDT Toggle C amp K 7101 M D9 A B E 1 Transformer Power Signal LP 20 600 or direct cross ref 1 Crystal 1 216MHz Inovonics P N 1242 PARTS SUPPLIERS Inovonics strives to maintain factory stock of all parts used in the products we manufacture A large proportion of the components in the DAVID III is generic and may be obtained from a wide variety of sources A few parts can be more or less proprietary These either may be manufactured specifically for Inovonics or purchased directly from a manufacturer that sells only in production quantities Inovonics does not depend on parts sales to fatten our coffers Nor do we impose a minimum charge for parts In some cases we will elect to supply nuisance parts at no charge rather than generate the necessary paperwork Always check with the factory we may well prove the best source for your replacement component needs 36 The electronic component distributors listed below have proven themselves reputable suppliers for small quantities of component parts for broadcasters and for other commercial or professional
21. UBGARRIER SYNTHESIS 29 Clock Pilot Generation Center Sampling Subcarrier Generation Composite Clipper Combining Amplifier Output Filter Output Phase and Equalization Trimmers POWER SUPPLY 31 YOUR NOTES AND DOODLES 5 5 HAM dr pA rai ERE DR RM EREMO 31 Section V APPENDIX EXPLANATION OF PARTS LISTINGS 34 PARTS LISTINGS Sieen aor 34 PARTS SUPPLIERS 36 SCHEMATIC DIAGRAMS 38 INOVONICS WARRANTY INSIDE BACK COVER Section INTRODUCTION MODEL 718 PRODUCT DESCRIPTION History Features Inovonics introduced the first DAVID as in David vs Goliath FM Stereo Processor Stereo Generator in 1992 The original Model 715 was the hallmark of simplicity having been developed to address the more basic of FM radio broadcasting needs The next generation DAVID II came out in 1995 It included redesigned pulse width modulation audio processing section which helped to expand the applicability of this updated product to broadcast situations where perceived loudness and modulation density were considered germane to the broadcaster s success This third generation DAVID III brings even more features and functionality to the modestly priced DAVID series along wi
22. band flat input not a monaural program line input with its own pre emphasis network Program signal pre emphasis is imparted by the DAVID III Section Ill SETUP AND OPERATION PANEL CONTROLS AND INDICATORS INPUT GAIN GATE OPEN A G C GAIN A brief description of the front panel controls and indicators follows Explanations are arranged right to left which is the order of the signal path through the DAVID III Please at least skim over this section to verify that our terminology agrees with your understanding More detailed notes on DAVID III operation follow this initial descriptive information The screwdriver adjusted front panel multiturn trim controls require fifteen to twenty complete rotations of the adjusting screw to cover their total operating range Depending on the manufacturer the end of the range may or may not be identified by a click click sound or other audible tactile sensation A single control adjusts input sensitivity for both the left and the right program channel inputs The single control makes DAVID III setup easier but provides no means of correcting for channel imbalance Stereo imbalance should be addressed at the source of this problem The INPUT GAIN control has a 12dB range which is extended by the input gain range jumpering options described on Page 8 INPUT GAIN is adjusted using normal program material to maintain the indicated AGC gain in the center of its operating regi
23. comes the final limiting point This action describes a closed loop feedback peak limiter for positive program peaks CR18 ensures that this part of the circuit has no effect over the negative going program components The emitter of Q5 is biased to a DC level that cancels the forward drop of CR18 and the saturation voltage of Q5 A small amount of program signal current is applied to the base of Q5 to linearize the transistor s dynamic conduction resistance over the range of the collector voltage swing 7 911 Q13 and Q6 serve the same function for negative program peaks Control voltage for the positive and negative limiters is summed by IC27D which with the help of its companion inverter drives a bi color LED to indicate PIPP Limiter action When the PIPP Limiter is turned off by S3 R103 is switched into the circuit This attenuates the program signal by an amount equal to the typical value of PIPP Limiter gain reduction so that the incoming program signal will not be clipped excessively by Q12 and Q11 when the PIPP circuit is not enabled LOW PASS FILTER Filter Overshoots Overshoot Compensation The Low Pass Filter low pass filters exhibit a certain amount of overshoot and ringing when presented with complex input waveforms Generally the sharper the cutoff the more pronounced the effect Overshoots result from the elimination of higher order input signal components Prior to filtering these componen
24. d alongside high power transmitters please practice reasonable care and common sense in locating the unit away from abnormally high RF fields Because the unbalanced COMPOSITE MPX OUT connector of the DAVID III is referenced to chassis ground a mains frequency or RF ground loop could be formed between output cable shield ground and the AC power cord ground A ground lifting AC adapter may well remedy such a situation though the chassis somehow must be returned to earth ground for safety Generally being screwed down in the equipment rack will satisfy the safety requirement LINE INPUT AND RANGE SELECTION Input Connections Unbalanced Inputs Input Gain Range Gain Jumpers DAVID III has electronically balanced transformerless left and right channel PROGRAM LINE INPUTS These appear as XLR female connectors on the rear panel As these are bridging high impedance inputs they provide no termination for the console or other equipment that feeds the DAVID III Most professional equipment nowadays has low output impedances and high input impedances The concept of 600 ohm line matching dates from the age of transformer coupling and is rooted in ancient telephone practices With few exceptions audio line impedance matching is ridiculed by today s hip broadcasters you and equipment manufacturers us The DAVID III may be fed from gear with unbalanced outputs Disco mixers have proven perfectly servic
25. designation MAX will be noted at full clockwise rotation of the MASTER DRIVE control Between 12dB and MAX Triband Processing release timing is jumped up to its fastest rate This n degree adjustment can yield a loudness increase but at the expense of audible artifacts with some program material notably solo voice Please use discretion with this maximum setting A good starting point for MASTER DRIVE is 6dB Listening tests with a variety of program material may suggest an alternative 14 BASS and BRILLIANCE setting Also adjustment of BASS and BRILLIANCE may dictate a change in the MASTER DRIVE setting Circuit action of BASS and BRILLIANCE is threefold First the setting dictates to what extent midband gain reduction simultaneously controls the amount of reduction at the frequency extremes that is the amount of coupling between the bands Secondly as BASS and BRILLIANCE are rotated clockwise not only do these bands decouple from the midband section but they also are allowed to release beyond the final midband release point Thirdly clockwise rotation of these controls speeds up the release timing in the subject band These three functions work together to create what we call Spectral Loading best defined as an artificial buildup of energy at either or both of the frequency extremes This is quite different from static fixed equalization When BASS and BRILLIANCE are set to OdB the Triband Processing section wor
26. e designation those that are considered generic may have qualification notations however Descriptions may or may not specify a particular manufacturer When no manufacturer is called out the term open advises that any manufacturer s product carrying the given part number or the same description in the case of a generic part is acceptable If a part is not listed at all this means that we do not consider it a typical replacement item Should you need to order an unlisted part call write fax or e mail the factory with a brief description of what it is that you need We ll then do our very best to figure out what to send you Components with reference designations below 500 are contained on the main printed circuit board and those in the 600s are chassis mounted components PARTS LISTINGS Unless specifically noted by component reference designation capacitors are specified as follows a Under 100pF are general purpose disc ceramic capacitors with no specific technical specification The letter following the value indicates picofarads b 100pF to 0 47uF are of the metallized Mylar or polyester variety Whole number XXp values are picofarads decimal values are microfarads capacitors in this category have 596 tolerance and are rated at 50VDC or better The style used in the DAVID III is the minibox package with lead spacing of 0 2 inch The preferred manufacturer is WIMA their FKS 2 or MKS 2 series
27. e music Secondly clipping is in the pre emphasized domain This means that clipping artifacts are attenuated by receiver de emphasis Finally the program derived DC value is a simple average and not based on an r m s average of the program signal Repetitive peaks add to the average value further to help bias the clippers out of the program For the most part only transient non repetitive program peaks are pre clipped Nonetheless this action is quite effective in reducing unnecessary Triband Processing action TRIBAND PROCESSING Band Division The DAVID III works on the program audio signal in three frequency bands These are nominally BASS MASTER midband and BRILLIANCE graphic representation of band splitting is shown in Figure 7 on the next page Band division is obviously asymmetric The three bands certainly don t look as though they would recombine properly to give flat overall response but they do Here s why IC35A is a second order low pass filter with rolloff beginning at about 100Hz The output of this stage is the bass band This output is subtracted from the program input by IC36A which also has a second order low pass function starting at about 4kHz The output of IC36A has the midband band pass response shown in 23 Band Rectifiers Midband Time Constants Figure 7 including the 1dB hump at 175Hz Note that the high pass characteristic of the midband is only first order even thou
28. e to 100 modulation Composite Multiplex Output Adjustable between 1V p p and 8V p p 75 ohm source impedance Power Requirements 105 180 or 210 255 50 60Hz 15 watts Size and Weight 134 H 19 W 8 D 10 8 lbs shipping Figure 1 below is a simplified Block Diagram of the DAVID III full set of schematic diagrams appears in the Appendix Section V u3xdiN E E 32019 W31SAS La NIVIN ov 21901 SIS3HLNAS 39019 9915 u3 40d TANNVHO WOH SSVd MO1 91 WVY90u d 9NISN3S 3AILISOd 834419 ONISNAS 3ALLV53N ddld 834412 THY JANG YILSYN SSv8 TANNVHO T3NNVHO OL N39 390197 WMd 21901 SNILV9 80193130 ANTWA JOWYSAY TANNVHO 769 8444119 ay SIAGNI ONY SISVHdW3 LAdNi WVY50dd 034315 039 1 8 BLOCK DIAGRAM Figure 1 Block Diagram DAVID II Model 718 Section Il INSTALLATION UNPACKING AND INSPECTION MOUNTING Rack Requirement Heat Dissipation As soon as the equipment is received inspect carefully for any shipping damage If damage is suspected notify the carrier at once and then contact Inovonics We recommend that you retain the original shipping carton and packing materials just in case return or reshipment becomes necessary In the event of return for Wa
29. eable for an installation on a tight budget For unbalanced lines the single center conductor of the shielded input lead should be connected to Pin 2 of the XLR connector and the shield connected both to Pin 1 and to Pin 3 The DAVID III can accommodate line level program inputs with a nominal Zero VU level between 15dBu and 104 This 25dB range is divided into two more manageable overlapping gain ranges that are selected by a series of four internal jumpers As shipped the DAVID III is set up for professional level inputs that lie in the OdBu to 10dBu range Most console and STL receiver outputs will fall into this range 4dBu and 8dBu are typical program line levels Lower levels between 15dBu and OdBu may be encountered when interfacing with semi pro gear or with feeds from lossy studio transmitter telephone line circuits Extra gain for the low level range is enabled by changing jumpers beneath the top cover Under the top cover and just behind the left channel input connector you will find four 3 pin jumper strips These are identified as JP2 through JP5 in the circuit board legend and each has a push on jumper shunt LO and HI designations identify the jumper position for low level and high level inputs respectively and all four jumpers must be set identically This is shown in Figure 2 below JP2 129 430 0129 130 Jumpers for HIGH LEVEL
30. ell as adjusting the program level this control also affects dynamic compression release characteristics which become more aggressive as the control is rotated clockwise Additional details are presented under TRIBAND PROCESSING SETUP GUIDELINES on Page 14 The three sets of G R Gain Reduction LEDs provide a visual display of the approximate amount of peak plus average gain reduction in effect in each of the three processing bands as well as giving an indication of the compression release characteristic The green OdB indicator remains lighted at full brilliance with full gain no processing in the attendant band When the 12 LED is lighted fully 12dB or more limiting and compression is in effect Equal brightness in both LEDs would suggest about 6dB of gain reduction With a little practice it should not be difficult to visually interpolate for intermediate G R values as well as to differentiate the fast limiter release from the slower compression platform timing characteristic Because the effect of these two controls is largely dependent on dynamics of the program source they cannot be thought of as simple EQ equalization adjustments What these afford is leverage over the relative amount of total energy at the frequency extremes of the program audio spectrum Refer to the more detailed discussion under the TRIBAND PROCES SING SETUP GUIDELINES on Page 14 PIPP is an acronym for Polarity Independent Peak Processing In
31. gh it was derived from a second order subtraction But wait that s not all 0dB ES di 30Hz 50 100 200 500 1kHz 2k 5k 10k 15k Figure 7 Triband Frequency Division is yet third subtraction stage It subtracts both the midband and the BASS band from the program input and its output is the brilliance band Now as all three of the frequency bands were derived by subtraction from the full range program signal there is no way that they can t add back up to flat response and they certainly do Moreover not only is the recombined amplitude response as flat as the proverbial pancake but the phase response is linear as well Even as the gain of a particular band is changed through Triband Processing action this circuit architecture is distinctly free of phase funnies and amplitude peaks at the crossover frequencies Each of the three band splitting stages has a companion phase inverter to provide full wave rectification of each band output DC control voltages from each band are common to both the left and the right channels to preserve stereo imaging The DC control voltages are identified in the schematic as BASS RECT MSTR RECT and BRILL RECT Flipping over to Page 40 C55 charges to the instantaneous peak value of the midband program signal through R202 peak derived DC is fed directly to the PWM comparator Q17 Q16 and Q15 The ramp waveform of Figure 6 is p
32. io established by the pre clipper ahead of the Triband Processing section This means that at the setting the average responding AGC is largely responsible for overall program signal leveling Expected program peaks 10dB to 12dB above the average program level will just drive transmitter modulation to 100 With the MASTER DRIVE pot at there will be very little if any processing of the incoming program aside from gain riding AGC As MASTER DRIVE is rotated clockwise less and less of the gain spoiling voltage is applied to C55 As result program peaks become more and more subject to processing in this section When MASTER DRIVE is at 12 only the quiescent 10dB worth of gain spoiling from R174 is in effect and processing is at maximum so far as levels are concerned As MASTER DRIVE is turned further clockwise from 12 to MAX something else happens The wiper of R183 begins to go negative because of bias through R181 This has two effects First the quiescent gain spoiling voltage through R174 is reduced somewhat which has the effect of further increasing midband processing Secondly the midband gain reduction release platform developed by the charge on C54 is eliminated because of negative bias through CR27 and R163 This causes a fast release over the entire midband gain reduction range which gives an apparent loudness increase with some types of programming but at the risk of some material sounding choppy
33. ks almost like a single band compressor Tonal balance will be much the same as the unprocessed program input though the program dynamic range will be restricted based on how MASTER DRIVE has been set Rotating BASS and BRILLIANCE toward 3dB will cause a reduction of energy at the ends of the spectrum The BASS control gives up to 9dB of low end Spectral Loading BRILLIANCE however affords only a maximum 6dB BRILLIANCE is purposely restricted because of FM broadcast system pre emphasis With high frequencies already accentuated by 10dB or more attempts to create additional top end sparkle or crispness in the program runs the risk of causing a sound that is either brittle clipped or mushy excessively HF limited Exercise discretion with extremes in Spectral Loading particularly at the top end Figure 4 below illustrates the frequency range and leverage of the BASS and BRILLIANCE controls Keep in mind that Spectral Loading is a dynamic function and that maximum effect will depend largely on the tonal balance of the program material 10 8 eet pee 5 1 4 b earn at RR RO FEDA A 30Hz 50 100 200 500 1 2 2k 5k 10k 15k Figure 4 Spectral Loading Range and Leverage Note in Figure 4 that the BASS control shows a second order Chebychev filter characteristic with the expected dip prior to boost and a peak prior to cut This is a
34. m Lj 007661 974 _ 4 008 399105 3982 3106 Lu 6 4 ALFE ALOE 3106 544 9 9 NINN 01 001 991 6 9 5500 823 wee OO X001 wool gela 18 8819 42 7000 L 9280 004661 qu a a 439 ino 00 1 9900 029 19 000 H 096 S HNE 006 led Wd 6 034313 4 41 133 6 43 44 1 INOVONICS WARRANTY TERMS OF SALE Inovonics products are sold with an under standing of full satisfaction that is full credit or refund will be is sued for products sold as new if returned to the point of purchase within 30 days following their receipt provided that they are returned complete and in an as received condition CONDITIONS OF WARRANTY The following terms apply unless amended in writing by Inovonics Inc A The Warranty Registration Card supplied with this product must be completed and returned to Inovonics within 10 days of delivery This Warranty applies only to products sold as new t is extended only to the original end user and may not be transfer red or assigned without prior written approval by Inovonics This Warranty does not apply to damage caused by misuse abuse accide
35. may be permitted to exceed the customary 100 modulation figure when SCA and RDS subcarriers are broadcast The station or consulting engineer should be aware of legal guidelines in effect and assume responsibility for any adjustment that results in a total modulation reading in excess of 100 10 Decrease the COMP CLIPPING and MASTER DRIVE controls to their normal operating values As a starting point set COMP CLIPPING to 1dB and MASTER DRIVE to 6aB 13 TRIBAND PROCESSING SETUP GUIDELINES AGC Section Notes MASTER DRIVE Processor setup is obviously very subjective and will depend entirely on the station s format the desired sonic signature and the needs wants and whims of various station personnel A few hints are provided here some of which are redundant to information that has already been given with the identification of the various front panel appointments The slow gain riding AGC action of the DAVID III departs from earlier versions in this series of processors The response to program material is average responding rather than peak responding The rationale behind our about face on this issue has to do with the tendency toward greater and greater processing of music source material Although the popular CD is only a consumer music format it is nonetheless capable of a full 90dB dynamic range Sadly a statistical analysis of most CDs on the market will show total dynamics of 20dB or less except f
36. must be made based on extensive listening and not on the basis of numbers The dB markings around the front panel controls correlate well with what s going on inside the DAVID III but not necessarily with what you may expect to hear Please make every effort to arrive at a setup that sounds good to you and to other station personnel for the majority of program content and over the long term Resist the temptation to change settings because of how one or two specific music tracks sound some songs simply sound awful with any processing or without it for that matter 17 INTRODUCTION Component Annotation A PWM PRIMER PWM Gain Control Section IV CIRCUIT DESCRIPTIONS This Section details the circuitry of the Inovonics DAVID III Circuit descriptions refer to the six sheets of Schematic Diagrams contained in the Appendix Section V Pages 38 to 43 DAVID III schematics may appear to have component reference designations assigned in a haphazard manner Rather than to annotate the schematic in a logical sequence we have instead chosen to designate the components on the circuit board in a logical top to bottom and left to right manner following the physical placement of the parts in their neat little rows It is our expectation that this practice will make any necessary troubleshooting easier as a component can physically be located quickly following analysis of the schematic The DAVID III utilizes Pulse Width Modula
37. nt or neglect This Warranty is voided by unauthorized attempts at repair or modification or if the serial identification label has been removed or altered TERMS OF WARRANTY Inovonics Inc products are warranted to be free from defects in materials and workmanship A B Any discrepancies noted within ONE YEAR of the date of delivery will be repaired free of charge or the equipment will be replaced with a new or remanufactured product at Inovonics option Parts and labor for factory repair required after the one year Warranty period will be billed at prevailing prices and rates IV RETURNING GOODS FOR FACTORY REPAIR A Equipment will not be accepted for Warranty or other repair without a Return Authorization RA number issued by Inovonics prior to its return An RA number may be obtained by calling the factory The number should be prominently marked on the out side of the shipping carton Equipment must be shipped prepaid to Inovonics Shipping charges will be reimbursed for valid Warranty claims Damage sustained as a result of improper packing for return to the factory is not covered under terms of the Warranty and may occasion additional charges Revised Feb 2003
38. nter sampling eliminates integration of switching noise that is concurrent with leading and trailing edges of the waveform steps The FM multiplex waveform consists of a main channel conveying the L R stereo sum in its normal audible frequency range and a sub channel with L R stereo difference information in the form of 99 Composite Clipper Combining Amplifier Output Filter 38kHz double sideband suppressed carrier subcarrier The 19kHz pilot is added to enable stereo receivers to reconstruct and re introduce the 38kHz carrier for difference signal demodulation Digital synthesis of the composite waveform is similar to the generation of the stereo pilot previously discussed but with sinusoidal commutation between the left and the right stereo program channels at a 38kHz rate Up down counter IC6 is clocked at the 608kHz sampling rate The output is decoded by IC5 and with gating provided by IC8D CR11 and IC7B IC6 counts continuously from zero to 8 back to zero etc A Pulse from IC8A presets the counter to its center position count of 4 each time the pilot reaches the proper phase relationship This ensures proper and constant synchronization between pilot and subcarrier IC15 and IC20D also decode the count sequentially sampling each tap of the resistor divider string which bridges the left and right program audio signals Sampled program audio is buffered by IC16B center sampled by IC20B and
39. on DAVID III AGC is gated meaning that during brief pauses in the program the gain is locked to prevent background sounds from slowly increasing to unnatural proportions The green GATE OPEN indicator will remain continuously lighted during most music programming but will blink off during pauses in speech This series of seven LEDs displays the amount of correction in effect for long term variations in the program level The AGC circuit is average responding and is left right correlated that is the overall circuit gain is controlled by the greater of the left or the right channel signals This measure is incorporated to preserve the program s stereo image AGC correction rate is intentionally quite slow With a conscientious operator at the console the indicated A G C GAIN should always hover near the OdB indicator 10 MASTER DRIVE G R BASS and BRILLIANCE PIPP LIMITER The safe operating zone of the AGC circuit is within the central three green LEDs Consistent operation at 10dB or 100 suggests that the INPUT GAIN control should be readjusted If either the 15dB or the 15dB indicator becomes lighted to full brightness the input program signal is definitely out of limits and the reason should be found and corrected at once This is the first adjustment for audio signature in the processing chain It represents the input level control to the Triband Processing section of the DAVID III As w
40. or and that it is properly calibrated and connected to the correct transmitter RF sample point or has a solid off air signal 1 Put the Mod Monitor in its Total Peak Modulation measurement mode with peak flasher s set to 100 2 Just for this calibration procedure turn the MASTER DRIVE and COMP CLIPPING controls fully clockwise maximum 3 With normal program fed to the DAVID III at a level that maintains A G C GAIN in the center of its range adjust the MPX LEVEL control for a Mod Monitor indication of frequent 100 modulation peaks 4 Back down the MPX LEVEL control slightly so that the 100 peak flasher s light only occasionally 5 Switch the Modulation Monitor to display the injection level of the 19kHz stereo pilot 6 Adjust the PILOT INJ control for a 9 Mod Monitor reading of pilot injection 7 OPTIONAL If an RDS or an SCA subcarrier is to be transmitted connect it to the proper combining point at this time Using the Mod Monitor or other means of reading the auxiliary subcarrier injection level adjust the subcarrier level control for the proper injection This is normally about 446 for an RDS subcarrier and 1046 for an audio SCA 8 Put the Mod Monitor back into its Total Peak Modulation measurement mode and if necessary trim the MPX LEVEL control so that the peak flasher s indicate only occasional 10096 peaks 9 NOTE Depending on current operating rules and practices total peak carrier deviation
41. or fade ins and outs An averaging response to program dynamics encourages the downstream Triband Processing section of the DAVID III to work harder on source material with a greater dynamic range as indeed it should Under our previous position pre squashed music would get just as much compression and peak processing as wide dynamics live announce certainly more than it needs This would constitute a case of overprocessing of the musical content of the program The correction rate of the AGC stage has been speeded up from previous DAVID versions by a factor of four The present value is about 2dB second just a bit slower than would result in audible dynamic alteration of the program given the averaging response to music and speech This sets the amount of overall processing When set at OdB there is little if any dynamic gain reduction ever in effect except with program material having a high average to peak ratio an unprocessed announce microphone channel for instance Normal MASTER DRIVE range extends from OdB to 12dB These figures reflect the approximate amount of program dynamics compression called into play and to a subjective extent the amount of increase in loudness Loudness increase will depend largely on the dynamic range of the unprocessed program material Obviously if the source CD is already highly compressed there s not much that can be done to further decrease the average to peak ratio The
42. or the course Another possible concern might be that mucking about with waveform symmetry could introduce a sufficient DC component to the program that the exciter frequency reference PLL would unlock Take comfort in the fact that audio program material is constantly changing and DC offsets are short lived Not to worry The primary advantage of the PIPP concept is full negative and positive deviation of the carrier with all types of program material This adds up to a louder sound with a special added bonus of beefier bass Now if the PIPP concept worries or offends you or if you think it doesn t sound quite right we have provided a little switch to disable the feature quickly and easily The two LEDs associated with the PIPP limiter show circuit action in the separate and independent left and right channel limiters The resting state of the LEDs is yellow When positive excursions are being limited the LED goes red when negative excursions are limited the LED turns green It is interesting to watch the constant color change of the two PIPP LEDs as the program is worked on and especially to observe the symmetry difference between stereo channels By operating on each channel independently the PIPP limiter actually introduces a 16 small additional amount of stereo difference information This gives a little bit of stereo enhancement at no extra charge Further discussion of the PIPP limiting concept can be
43. orm to what has become known as a soft knee characteristic This means that the program signal eases gently into 19 The PWM Ramp Waveform compression and limiting to avoid the abrupt change from a linear to a limited state that characterizes traditional feedback topology The soft knee is normal behavior for feedforward gain control and is credited for the smoother less fatiguing sound attributed to this method Figure 6 shows the waveform that is generated by the DAVID III to affect the desired feedforward transfer characteristic Note the similarity between this waveform and Figure 5 but with the addition of an extended droop to the right This droop gives the soft knee compression limiting threshold A negative going pedestal has also been added to keep the CMOS analog switches active even when no gain reduction is in effect tet E E semi Figure 6 DAVID III PWM Ramp Waveform V 2V div 1us div A f ee WE This ramp runs continuously at a 152kHz rate It is applied to input of a comparator and a DC voltage derived from the input signal is fed to the other comparator input The comparator output is the duty cycle modulated squarewave that chops the program audio signal E PWM RAMP CIRCUITS Master Clock Compressor Limiter Ramp
44. ping costs low The cap is reseated by reversing the removal process This fuse also serves as a front panel emergency power disconnect for the DAVID III Unless specifically ordered for export shipment the DAVID III is set at the factory for operation from 115V 50 60Hz AC mains This can be confirmed by checking the designation beneath the mains connector on the rear panel The inappropriate voltage and fuse value will have been crossed out at the factory with an indelible felt marker To change the mains voltage first remove the top cover of the unit A clearly marked slide switch is next to the AC mains connector on the circuit board With power disconnected use a small screwdriver to set the switch for 115VAC or 230VAC operation Be sure to install the appropriate fuse listed on the rear panel You can remove the factory strikethrough with a carcinogenic solvent and then cross out the inappropriate marking with an indelible felt pen The detachable IEC type power cord supplied with the encoder is fitted with a North American standard male plug The individual cord conductors may be color coded in either of two ways 1 In accordance with US standards BLACK AC WHITE AC NEUTRAL GREEN EARTH GROUND 2 To European CEE standards BROWN AC BLUE AC NEUTRAL GRN YEL EARTH GROUND RADIO FREQUENCY INTERFERENCE RFI Location Ground Loops Although it is natural for the DAVID III to be installe
45. resented to the other input of the comparator The output of the comparator is a squarewave labeled MSTR PWM on the schematic The duty cycle of this squarewave is inversely proportional to the gain reduction needed to maintain midband program level at a constant value This describes the feedforward gain control action C55 discharges through R162 into C54 and both capacitors discharge through R164 to ground The purpose of C54 is to provide a release platform which is defined as 1 a secondary level to which fast peak reduction can release quickly and 2 level representing the average value of gain reduction that can release 24 55 Processing more slowly This dual time constant separates the peak limiting and average compression functions of DAVID III processing C55 is never allowed to discharge completely to ground however R174 and R175 provide a fixed bias which is delivered to C55 through CR30 This bias maintains about 9dB of gain reduction in the midband section at all times an essential requirement of the Spectral Loading feature The MASTER DRIVE control also factors into the equation At full counterclockwise OdB an additional amount of fixed DC bias is fed to C55 through CR26 and R161 about another 12dB worth of gain reduction in fact This serves to further reduce midband and as we shall see overall gain through the system This gain spoiling is about equal to the average to peak rat
46. ries resistor When the duty cycle is 50 in the bass band R235 looks like 6k At 10 the effective resistance is 30k The current summing configuration of IC33B eliminates interaction between the bands and the low pass response removes the 152kHz switching frequency THE PIPP LIMITER The operating theory of the PIPP Limiter is detailed on Page 16 Though the subject US Patent covers various analog and digital implementations of the fundamental concept the configuration used in the DAVID III is absurdly simple On Page 41 the program audio signal from the Triband Processor section is routed to IC32B through R199 and R198 C68 and C69 turn this stage into a unity gain low pass filter The bases of Q11 and Q12 are connected to the junction of R199 and R198 The emitters of these transistors are clamped at DC levels representing the absolute output ceiling of the DAVID III Thus Q11 and Q12 will always serve as hard clippers for program peaks that exceed the assigned ceiling value But each of these two transistors is part of a voltage amplifier as well Using the Q12 side of the circuit for an illustration as the program signal approaches the final clipping point Q12 begins to conduct which turns on Q14 to begin charging C40 Voltage developed at the top of C40 turns on Q5 which is connected to the program signal path through R102 and CR18 As Q5 conducts positive waveform excursions are maintained at the clipping threshold which be
47. rranty repair shipping damage sustained as a result of improper packing for return may invalidate the Warranty IT IS VERY IMPORTANT that the Warranty Registra tion Card found at the front of this Manual be completed and returned Not only does this assure coverage of the equipment under terms of the Warranty and provide a means of tracing lost or stolen gear but also the user will automatically receive specific SERVICE OR MODIFI CATION INSTRUCTIONS should the factory issue them The DAVID III mounts in a standard 19 inch equipment rack and requires only 1 inches 10 of vertical rack space The use of plastic washers is recommended to protect the painted finish around the mounting holes Consuming about the same power as a couple of night lights the 718 itself generates negligible heat The unit is specified for operation within an ambient temperature range extending from freezing to 120 F 50 C But because adjacent less efficient equipment may radiate substantial second hand heat be sure that the equipment rack is adequately ventilated to keep its internal temperature below the specified maximum ambient AC MAINS POWER Fuseholder Mains Voltage Selector Power Cord The fuseholder is at the far left of the front panel Apply downward pressure and pull the cap outward to access the 5mm mains fuse Note that the cap has space for a spare fuse as well though a spare has not been included due to our ongoing diligence in kee
48. sistance to the grounded wiper This means that as the left channel signal decreases the right channel level increases and vice versa But the backward level controlled right channel program is combined out of phase with the raw right 21 AGC Stages AGC Gating channel input by IC54A causing the right channel to track the left channel output of IC54B Many thanks to W Stephen Woodward and Electronic Design magazine s Ideas section for this clever one The AGC amplifier is made up of two identical cascaded stages IC53B and are configured as second order low pass filters with gain There is a CMOS analog switch at the input of each stage part of the 4 section IC52 The duty cycle or on versus off time of this switch determines the gain of the circuit as tabulated at the top of Page 19 The AGC output is inverted by IC50B to afford full wave rectification by CR56 and CR57 The cathodes of these diodes are common to those in the right channel to make AGC action dependent on the greater of the two channels thus preserving the stereo image The rectified AGC output is integrated by R277 and C96 the output of IC45B representing the average value of the program signal This derived DC voltage is identified as PGM AVG and is also employed in the pre emphasis stages IC45C compares the average program level with a DC reference from the R310 R311 divider When the program level exceeds the reference the outp
49. splacement of certain program overtones is not discernible to the listener but is most effective in inhibiting filter overshoots The 7 pole elliptic function Cauer low pass filter is an active version of the classic L C designs worked out in Germany during the late 1940s probably with a slide rule The particular active configuration used in the DAVID III is sometimes called the FDNR because each of the legs to ground simulates a Frequency Dependent Negative Resistance Referring back to the classic L C 28 design resistors in series with the signal replace series inductors and each of the active circuits to ground replaces an inductor capacitor series resonant element Components in the left channel filter have fixed values The resonant frequency of each of the right channel legs is variable over a small range so that the two channels may be precisely matched in amplitude and phase response A great cookbook with this and other filter circuits that might prove valuable to the broadcaster is the Electronic Filter Design Handbook by Arthur B Williams published by McGraw Hill The reader is kindly directed to this source for a more informed discussion of how the FDNR circuit works than we could possibly muddle through here 1 14 buffers the output of the low pass filter and includes gain both to compensate for the 6dB filter loss and to present the proper amplitude to the digital synthesis circuitry PILOT AND SU
50. stalk through processing and filters is better than 40dB L R Program Line Inputs Active balanced bridging XLR inputs accept nominal program line levels between 15dBu and 10dBu AGC Amplifier Unobtrusive average weighted correction for long term input level variations 15dB capture range Pre emphasis 75us or 50us selected by circuit board jumpers 3 Band Processing Tight peak control in three asym metrical frequency bands with secondary platform time constants to compress program dynamics with average value weighting Crossover frequencies at 100Hz and 4kHz See Figure 7 Page 24 MODEL 718 TECHNICAL SPECIFICATIONS Processing Control Master processing drive is adjust able between 04 and more than 12dB equating to a corresponding reduction in program dynamics and apparent loudness increase Equalization Adjustment range relative to original program balance BASS 3dB to 9dB BRILLIANCE 3dB to 6dB See Figure 4 Page 15 PIPP Final L R Limiter Patented Polarity Independent Peak Processing circuitry assures full modulation of the FM carrier despite abnormalities in the program waveform geometry Digital Synthesis Sampling Rate 608kHz 16X subcarrier oversampling Composite Processor Adjustable from 04 to 3dB clipping of the 100 modulated multiplex waveform prior to pilot injection Stereo Pilot 19kHz 1Hz injection level adjustable between 6 and 12 relativ
51. th a bigger bolder and brighter sound Wider AGC capture multiband processing equalization adjustments and a proprietary final limiting system make the DAVID III the rival of far more expensive alternatives Leading features of the DAVID III include e Comprehensive PWM audio processing with gain riding AGC three bands of compression and limiting and adjustable bass and brilliance EQ Inovonics unique PIPP final limiter which assures full carrier modulation with all types of program material e stereo generator section that utilizes digital synthesis techniques for best stereo separation and rock stable operation e A built in user adjustable composite clipper e A product that is simple to install and very easy to use the DAVID III is built with easy to find generic components for easy servicing anywhere in the world Frequency Response through appropriate de emphasis network 0 5dB 20Hz 15kHz 60dB or better at 19kHz Noise Better than 70d B below 100 modulation in demodulated L R audio 20Hz 16kHz Individual digital noise components above 54kHz are 70dB or better Distortion 0 596 THD in baseband and subcarrier at 95 modulation with PIPP limiter defeated lt 1 5 at any level of modulation with PIPP limiter engaged Stereo Separation L5R or R gt L gt 55dB 20Hz 16kHz Crosstalk M58 or S23M Nonlinear crosstalk is better than 504 50Hz 15kHz Linear cros
52. the same DC level as average midband gain reduction Bass peaks are independently limited as required but this section releases back only to the midband average release level At this setting the bass band is coupled or slaved to midband gain reduction As BASS is turned progressively clockwise the bass band is allowed to release to a DC resting point that is actually lower than the release point of the midband section This brings Spectral Loading into play which increases the overall energy in the bass band As the midband has a full release level that is spoiled by 9dB program bass can actually be enhanced by this same figure at full clockwise rotation of the BASS control Primary bass band release is through R203 and is realized when the BASS control is at 9 As gain spoiling bias is introduced through CR25 release slows down Because the brilliance band involves frequencies of 4kHz and up gain control action can afford to be very fast Attack is near instantaneous as C75 charges through R208 The integrated DC is applied directly to PWM comparator Q20 Q19 and Q18 Spectral Loading for the brilliance band is identical to that described above for the bass band The only difference is R214 inserted in the clockwise leg of BRILLIANCE control R243 This limits the amount of Spectral Loading to 64 for the top end of the audio spectrum a figure in keeping with restraints imposed by FM transmission system pre emphasis
53. tion techniques rather than VCAs voltage controlled amplifiers This technology as applied to broadcast audio processing was pioneered by Inovonics in the early 1980s and has been refined over the years PWM isa simple colorless quasi digital method of linear gain control that avoids several hassles associated with more expensive and possibly single sourced VCA integrated circuits Our implementation of PWM utilizes the popular 4000 series CMOS parts that are dirt cheap and available in any civilized area PWM sets the gain of an analog circuit simply by sampling or chopping the analog signal that is turning it on and off at a rapid rate Consider an audio program signal which can be turned on and off with a toggle switch When the switch is on attenuation is zero When off attenuation is infinite If we satisfy Dr Nyquist s axiom and toggle this switch at a rate at least twice that of the highest audio frequency linear signal attenuation becomes directly proportional to the OFF time as depicted in the chart at the top of the next page The DAVID III sampling rate is 152kHz about ten times the highest program audio frequency Since this is well above the Nyquist rate anti aliasing and reconstruction filters can be simple and gentle 18 Feedforward Processing Soft Knee PWM GAIN CONTROL ACTION Switch ON Switch OFF Attenuation 100 0 OdB 5096 5096 6dB 2596 7596 12dB 1096 9096 20
54. ts helped to define the signal peak amplitude Even a fully phase corrected filter will exhibit overshoots and the 7 pole elliptic filter used in the DAVID III can overshoot 3dB or more Other systems of overshoot control permit the primary low pass filter to overshoot then isolate and re introduce the overshoots to cancel themselves in the signal path The patented overshoot compensator in the DAVID III on the other hand pre conditions the limited program signal ahead of the filter so that there is little tendency for the filter to generate overshoots Thumbing to Page 42 IC31B includes a phase lag network that displaces the fast leading and trailing edges of steep waveforms in time This means that the primary time domain characteristic of a program waveform that would normally excite filter overshoots is instead added to the waveform amplitude CR29 and CR28 are biased to the limited clipped level of the input program signal and strip these displaced and added components from the phase lagged program waveform IC29B is a differential stage that compares the input and output of the stripper to recover the stripped off components As these components contain much of the program harmonic high frequency information we cannot afford simply to throw them away By recombining these stripped off program components out of phase with the stripped program signal the spectral integrity of the program is maintained This 180 degree di
55. ut of IC45 goes high charging C97 through R298 and R299 When the program level is below the reference CR51 is reverse biased and C97 discharges through R299 and R297 Integration by R299 and C97 determine the AGC correction rate DC buffered by IC45D is presented to one input of a comparator comprised of Q27 Q28 and Q29 The AGC ramp is fed to the other comparator input and the output from Q29 is the PWM control for the four IC52 analog switch sections AGC in the DAVID III is gated meaning that gain does not slowly increase out of proportion when no valid input signal is present R356 and R357 sum the left and right channels into IC46A This stage is also a first order band pass filter that favors voice frequencies and desensitizes the gating circuit to air conditioner rumble and squeaky chairs The L R sum is full wave rectified by CR49 CR50 and IC46D A short time constant of this rectifier is set by R318 R314 C110 and C111 When the input is low below the gating threshold the output of IC46D goes positive CR53 biases the DC reference for the AGC out of the picture and CR52 sets a fixed DC bias through R309 R298 and R297 that corresponds to the AGC OdB resting point Thus when there s a pause in the program AGC gain slowly returns to zero When the program is active the output of IC46D goes negative This lights the GATE OPEN indicator reverse biases CR52 and CR53 and enables normal AGC action 22
56. zed at a 608kHz rate only a simple filter is required to eliminate the sampling frequency from the DAVID III output L2 L1 C21 and C20 form a 4 pole filter with quasi Butterworth response This filter is exceptionally flat to about 100kHz but effectively attenuates higher order digital noise components in the DAVID III output 30 Phase Equalization Trimmers POWER SUPPLY The output amplifier consists of IC11A IC4B and transistors Q1 92 R19 affords variable high frequency tip up and R13 varies the phase response These two controls are factory adjusted to optimize stereo separation DAVID ITI circuitry operates from the bipolar 9 volt supply diagrammed on Page 42 The two supplies are regulated by linear three terminal IC voltage regulators IC1 for the 9 volt supply IC2 for the 9 volt supply The power transformer has dual primary windings that may be switched in parallel or in series for 115V or 230V mains respectively YOUR NOTES AND DOODLES are welcome here and on the following page 32 Section V APPENDIX This section of the DAVID IIl Manual contains Parts Listings Schematic Diagrams and an explanation of Inovonics Generous and Liberal Warranty Policy 33 PARTS LIST EXPLANATION OF PARTS LISTINGS This section contains listings of component parts used in the Inovonics DAVID III Not all components are listed by schematic referenc
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