OPERATING AND MAINTENANCE INSTRUCTION MANUAL “DAVID-II” ——— MODEL 716 ——— FM-STEREO PROCESSOR / GENERATOR __________
—– USER’S RECORD —– Model 716 - Serial No.
TABLE OF CONTENTS Section I - INTRODUCTION Model 716 Product Description ........................................................................................... 3 General - Features Model 716 Technical Specifications..................................................................................... 3 Patent Notice ........................................................................................................................ 4 Block Diagram ....................................................
Section IV - Calibration Calibration .......................................................................................................................... 18 Equipment Required - Power Supply Check - Clock Set Low-Pass Filter Matching - Crosstalk Trim - Separation Trim Section V - Circuit Descriptions Circuit Descriptions ............................................................................................................
Section I INTRODUCTION MODEL 716 PRODUCT DESCRIPTION General Inovonics introduced the first “DAVID” (as in David vs. Goliath) FMStereo Processor/Generator in 1992. The original Model 715 was the hallmark of simplicity, and was intended to answer the more basic of FM radio broadcasting needs. The “DAVID-II,” Model 716, includes a redesigned pulse-widthmodulation audio processing section.
Crosstalk (MàS or SàM): Processing OUT: nonlinear crosstalk –55dB or better. Processing IN: overall crosstalk –50dB or better Compressor/Limiter: Fast-acting peak limiter with secondary “platform” time constant to compress dynamic range with average-value weighting. Independent high frequency limiter conforms to selected pre-emphasis characteristic. Subsequent program signal clipping is relegated to non-repetitive limiter overshoots 1ms or less in duration.
Figure 1 - Block Diagram, DAVID-II FM-Stereo Processor/Generator — 5 —
Section II INSTALLATION UNPACKING AND INSPECTION Immediately upon receipt of the equipment, inspect carefully for any shipping damage. If damage is suspected, notify the carrier at once, then contact Inovonics. It is recommended that the original shipping carton and packing materials be saved for future reshipment.
Voltage Selector A mains voltage selector switch is located beneath the top cover of the unit, adjacent to the AC mains connector on the circuit board. With primary AC power disconnected, slide the red actuator with a small screwdriver so that the proper mains voltage (115 or 230) is visible. You must always install an appropriate fuse, and should mark the rearpanel voltage/fuse designation. It is factory practice to cross-out the inappropriate marking with an indelible black marking pen.
channels. Since these are “bridging” (high impedance) inputs, they provide no termination for the console or other equipment which feeds the DAVID-II. Please feel at liberty to connect a 600-ohm resistor across the input terminals should you feel this really necessary. Most professional equipment nowadays features 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 the mystique of telephone practices.
PRE-EMPHASIS SELECTION The DAVID-II supports the 75-microsecond FM broadcasting preemphasis 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 shipping destination is jumpered at the factory, but this is easily changed if necessary. Beneath the top cover, locate the second vertical row of integrated circuits from the right-hand edge of the chassis.
reduction technique which employs a similar boost-before-cutoff at the high-frequency end of the spectrum. Figure 4 - “Bass-Enhance” Equalization Curve “Bass-Enhance” Jumpering As delivered from the factory, “Bass-Enhance” equalization is defeated. To enable the “Bass-Enhance” equalizer, remove the top cover and locate the first vertical row of integrated circuits closest to the righthand side of the chassis, the one just behind the PROGRAM LINE INPUT barrier strip.
SUBCARRIER INPUT Internal combining is provided for an RDS or an SCA auxiliary subcarrier. The rear-panel SUBCARRIER INPUT connector is an unbalanced input with a load impedance in the neighborhood of 10K-ohms. An RDS subcarrier normally accounts for 5% or less of the total composite multiplex output signal, making it 26dB or more below peak program modulation. SCA subcarriers typically have a higher injection level, about 10%, or 20dB below program peak deviation.
Section III SETUP AND OPERATION PANEL CONTROLS AND INDICATORS A brief description of the front-panel controls and indicators is given here. Do scan this section to at least verify that our terminology agrees with yours. PLEASE NOTE that all front-panel multiturn trim controls require fifteen to twenty complete rotations of the adjusting screw to cover their total operating range.
unnatural proportions. The green GATE OPEN indicator will remain continuously lighted during most music programming, but will blink off during pauses in speech. AVERAGE COMPRESSION This is, in essence, a “drive” control at the input of the compressor/limiter stage, affording the user control over program density. When fully counterclockwise (MIN) the broadcast signal will retain most of the dynamics of the source material.
COMPOSITE PROCESSING In the fully-counterclockwise (OUT) position, an output “safety” clipper catches only those very fast peaks and overshoots which sneak by the earlier protection circuitry. As the COMPOSITE PROCESSING control is rotated clockwise, the level applied to the safety clipper is increased by the dB value indicated on the front panel. Composite clipping will increase apparent loudness, but at the expense of generating some spurious harmonic products. The following tabulation is approximate.
the AUDIO PROCESSING switch in the IN position unless outboard provision is made for pre-emphasis and limiting. Without external pre-limiting, program material will be driven hard into the clipping circuits and result in egregious distortion.. PILOT The PILOT switch lets the user manually turn off the 19kHz stereo pilot for certain tests. Since the 19kHz pilot is necessary for proper system operation, the switch should always be left ON.
1. To the left channel only, apply a 500Hz test tone from the console at a level 2.5dB above normal reference level. This is simply a console VU meter indication of +2.5VU. 2. Adjust the left channel (L) INPUT GAIN control so that both the 0dB and the –5dB AGC GAIN indicators light to equal brilliance. Remember that the AGC circuit responds slowly to level changes, so have patience in performing this step of the procedure. 3. Turn off the +2.5VU test signal to the left channel and apply +2.
subcarrier level control) for the proper injection. This is normally about 5% for an RDS subcarrier, 10% for an SCA. 7. Put the Mod-Monitor back into its “Total Peak Modulation” measurement mode and, if necessary, trim the OUTPUT LEVEL control so that the peak flasher(s) indicate only occasional 100% peaks.
Section IV CALIBRATION The upgrade to PWM processing has greatly simplified calibration of the DAVID-II when compared with its predecessor, the Model 715 DAVID. The number of trimmer adjustments under the top cover has been reduced from nineteen(!) to only eight. “Routine” calibration of the DAVID-II is never required. The PWM processing and digital-synthesis circuitry is simple and stable.
Low-Pass Filter Matching The low-pass filter for the left program channel is not adjustable; rather, it employs close-tolerance components to meet design parameters. Since close frequency and phase response matching between stereo channels is essential to acceptable Stereo Generator crosstalk performance, trim controls have been included in the right channel filter so that it may be tuned to match left channel response. 1. Set the AUDIO PROCESSING switch to the OUT position.
9. Adjust R65 (between IC21 and IC22) for a right-channel null. NOTE: The third filter null is at approximately 34.5kHz. Rather than to match frequency response between channels at this point, the third filter adjustment is used to help match phase response in the next procedure. Crosstalk Trim If your station Mod-Monitor includes a true “precision” FM-stereo demod (crosstalk/separation resolution of 70dB or better), it may be used in lieu of a Spectrum Analyzer for this part of the Calibration Procedure.
difference” to balance crosstalk measurements, sacrificing a null at one frequency to improve that at the other. When this part of the Calibration Procedure has been successfully completed, the sub-channel measurement should remain below –50dB (relative to 100% modulation) as the Audio Generator is swept from 20Hz to 15kHz.
6. Adjust the equalization and phase trimmers, R56 and R57, for best stereo separation. There is some interaction between these two adjustments, but there will be only one setting of each which will give best separation. Null one, then the other, and back-and-forth, etc., until no further improvement can be had. The final adjustments should yield best separation at all frequencies between 20Hz and 15kHz, left-into-right and right-into-left.
Section V CIRCUIT DESCRIPTIONS This section details the circuitry of the Inovonics DAVID-II. Circuit descriptions refer to the two pages of Schematic Diagrams contained in the Appendix, Section VI, Pages 39 and 40. Component Annotation DAVID-II Schematics may appear to be annotated in a somewhat haphazard manner, insofar as component reference designations are concerned.
Feedforward Control Switch ON Switch OFF Attenuation 100% 0% 0dB 50% 50% 6dB 25% 75% 12dB 10% 90% 20dB 1% 99% 40dB Both the wideband and the independent high frequency compression/ limiting circuits operate 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.
characteristic. This means that the program signal eases gently into compression and limiting, avoiding the abrupt change from a linear to a limited state which 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. The PWM “Ramp” Waveform Figure 7 shows a “ramp” waveform generated by the DAVID-II to effect the desired feedforward transfer characteristic.
The negative-going pulse from Q9 triggers a similar event with Q6 and Q7. On the positive transition of the pulse, Q6 is turned off, allowing Q7 to turn on. C49 and R99 establish the off time for Q6, hence the on time for Q7. Compressor/ Limiter Ramp When Q7 turns on, C52 is charged to very nearly the +9-volt power supply rail through CR18. As soon as Q7 turns back off, C52 begins to discharge through three distinct paths.
SIGNAL PATH CIRCUITRY NOTE: Left and right program channel circuits are identical, so only the left channel will be described here. A skilled technician should find it within his capability to extrapolate for the right channel. INPUT STAGES Line Input Balancing “Bass Enhance” IC44B is an “active-balancing” stage for the left channel program line input, affording rejection of unwanted common-mode signals.
DC Control Signal Rectifier IC49B is a simple unity-gain inverter. Along with diodes CR45 and CR46, it forms a full-wave rectifier for the left channel program signal. The right channel has an identical rectifier, and the resulting rectified voltage represents the higher level of the two channels. This voltage is used by the feedback-mode AGC and the feedforward compressor/ limiter. The common-cathode rectifier configuration is the equivalent of an analog OR gate.
corrected average program level. Moreover, the gating threshold is frequency-weighted for greatest sensitivity in the voice range. Gating is based on L+R energy. The left and right channels are summed by IC36B through R276 and R255. CR30 and CR29 comprise a full-wave rectifier at the input of comparator IC29B. The output of IC29B is pulled to ground so long as the corrected program level is above the gating threshold.
IC28D buffers the DC gain-controlling voltage and delivers it to comparator IC42A. The PWM squarewave from IC42A drives PWM attenuator IC46D. IC45B and IC37A comprise another 4-pole low-pass reconstruction filter to remove the 152kHz PWM switching component. Gain Reduction Indicators IC28C is a slope detector sampling the wideband compressor/limiter DC control voltage. The output of this op-amp toggles positive, lighting the W.B. LED, when a rapid voltage increase indicates the reduction of a program peak.
FILTER OVERSHOOT COMPENSATOR Sources of Overshoot All 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 which, prior to filtering, helped define the signal peak amplitude.
the DAVID-II 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 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.
PILOT Switch Subcarrier Generation In its OFF position, the front-panel PILOT switch, S3, is arranged to interrupt the pilot sampling sequence and preset IC6 to a zero count. This stops the stereo pilot at its zero crossing. The FM-multiplex “composite” 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 a 38kHz double-sideband, suppressed-carrier subcarrier.
Combining Amplifier Output Filter IC18A is a combining stage for the main/sub portion of the composite signal, the stereo pilot, and an optional RDS or SCA subcarrier applied to the rear-panel SUBCARRIER INPUT jack. The front-panel OUTPUT LEVEL control, R13, is in the feedback path of IC18A, affording adjustment over the composite level delivered to the exciter.
Section VI APPENDIX The following section of this Manual contains Parts Lists for the Inovonics DAVID-II, Schematic Diagrams of all electronic circuitry, and an explanation of Inovonics’ Warranty Policy.
PARTS LIST EXPLANATION OF PARTS LISTINGS This section contains listings of component parts used in the Inovonics DAVID-II FM-Stereo Processor/Generator. These are listed either enmasse, or by schematic component reference designation, and may, or may not, specify a particular manufacturer. When no manufacturer is called-out, the term “open mfgr.” advises that any manufacturer’s product is acceptable.
CR1-6 CR7,10-47 CR8,9 F1 FB1 Diode, Silicon Rectifier; (open mfgr.) 1N4005 Diode, Silicon Signal; (open mfgr.) 1N4151 or equiv. Diode, Schottky; (open mfgr.) 1N5711 Fuseholder, PC-mounting; Littlefuse 345-101-010 with 345-101-020 Cap for ¼-inch (U.S.) fuses, or 345-121-020 Cap for 5mm (European) fuses. (Fuse is normal “fast-blow” type in value specified on rear panel with reference to mains supply.
L2 Inductor, fixed, 220µH; ACT Co. C4-221K Q1 Transistor, PNP; Motorola MJE350 Q2 Transistor, NPN; Motorola MJE340 Q3,4,8,9,10,11 Transistor, NPN; (open Mfgr.) 2N3904 Q5,6,7 Transistor, PNP; (open mfgr.) 2N3906 Q12 Transistor, NPN; (open mfgr.) 2N5088 Unless specifically noted by component reference designation below, resistors are specified as follows: a) Fixed resistors with values carried to decimal places implying a 1% tolerance (example: 3.01K, 10.0K, 15.
