OPERATING AND MAINTENANCE INSTRUCTION MANUAL MODEL 708 FM-STEREO GENERATOR __________
—– USER’S RECORD —– Model 708 - Serial No.
TABLE OF CONTENTS Section I - INTRODUCTION Model 708 Product Description ........................................................................................... 3 General - Features Audio Pre-Processing Requirements ................................................................................... 3 Pre-emphasized Transmission - Split-Spectrum Peak Control Internal Overmod Protection Model 708 Technical Specifications....................................................................................
Implications of Composite Processing ............................................................................. 17 Pilot Modulation - Out-of-Band Components Section IV - Calibration Calibration ..........................................................................................................................
Section I INTRODUCTION MODEL 708 PRODUCT DESCRIPTION General Inovonics introduced its first FM-Stereo Generator in 1992. The original Model 705 was the hallmark of simplicity, and was intended to answer the more basic of FM radio broadcasting needs. The Model 708 includes several additional features, and boasts performance superior to that of its predecessor.
transmission. This noise is generally worse for listeners who are some distance from the broadcasting station. If a low frequency tone of, say, 300Hz is applied to the transmitter at a level which yields full carrier modulation (100% or ±75kHz deviation), a high frequency tone of 10kHz applied at this same input level would overmodulate the carrier (400% or more!) because of the transmission pre-emphasis equalization.
MODEL 708 TECHNICAL SPECIFICATIONS Frequency Response (with preemphasis turned OFF, or through appropriate de-emphasis network): ±0.5dB, 20Hz–15kHz; –1dB or less at 16kHz, –60dB or better at 19kHz. SCA / RDS Inputs: 10K-ohms, unbalanced; accept SCA or RDS subcarriers at 0dBu or greater levels for nominal 5% to 10% subcarrier injection, relative to 100% modulation. Stereo Separation (LàR or RàL): >65dB, 20Hz–16kHz.
Figure 1 - Block Diagram, Model 708 FM-Stereo Generator — 6 —
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 check that the rear-panel voltage/fuse designation is properly marked.
Balanced Inputs Unbalanced Inputs Input Gain Range A balanced program audio feed to the 708 will use both the + and the – terminals, plus the associated G (ground) terminal for each of the two stereo channels. Since these are “bridging” (high impedance) inputs, they provide no termination for an audio processor or other equipment which feeds the 708. Please feel at liberty to connect a 600-ohm resistor across the input terminals should you feel this really necessary.
PRE-EMPHASIS SELECTION The Model 708 Stereo Generator supports 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 shipping destination is jumpered at the factory, but this is easily changed if necessary. Beneath the top cover and about two inches behind the input barrier strip are two jumper pin strips, each with a push-on jumper.
COMPOSITE OUTPUT The unbalanced COMPOSITE OUTPUT of the Inovonics 708 has a resistive source impedance of 75 ohms. This permits runs of moderate length (about 50 feet, maximum) to the exciter or microwave link input. If the cable length is more than just a few feet, the cable should, itself, 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.
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 multi-turn trim controls require fifteen to twenty complete rotations of the adjusting screw to cover their total operating range.
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. The green LED indicates normal operation, the red LED lights whenever the pilot has been turned off. MODE Under some circumstances it may be advisable to broadcast monaurally.
COMPOSITE PROCESSING PEAK VALUE OF CLIPPING PRODUCTS OUT >60dB below 100% 1dB 50dB below 100% 2dB 45dB below 100% 3dB 40dB below 100% Composite clipping is performed prior to injection of the 19kHz stereo pilot and any subcarriers. This averts pilot clipping or other pilot amplitude modulation. Nevertheless, crosstalk of the program audio into RDS or SCA subcarriers is exacerbated by composite clipping and its inevitable intermodulation and harmonic generation.
(10X) circuit gain re-scales the measurements for these lower-level signals. For these measurements, the 100% marking should be read as 10%, and 0dB as –20dB. NORMAL SETUP PROCEDURE This setup procedure presupposes a normal installation with the Model 708 PROGRAM LINE INPUT fed directly from the output of a properly adjusted audio processor.
to about 5%, SCA subcarriers to 10%. Again, the 100% mark on the meter should be read as 10% for these measurements. Output Pre-check The initial portion of the output adjustment procedure is performed under simulated “on-air” operating conditions. Feed typical program material to the stereo generator via the processing system, but stereo temporarily disconnected the generator from the exciter until the precheck verifies normal operation.
4. Switch the Modulation Monitor to display the injection level of the 19kHz stereo pilot and confirm 9% injection. Touch-up the PILOT INJ. control, if necessary, then re-confirm 100% carrier deviation. 5. If an RDS or an SCA subcarrier is to be broadcast, re-connect it to the 708 at this time. Using the Mod-Monitor, or other means of reading the auxiliary subcarrier injection level, verify proper injection. 6. Reset the Mod-Monitor to read total carrier modulation.
Out-of-Band Components Simple clipping of a symmetrical waveform will invariably generate odd-order harmonics of the fundamental frequency. The level of, and signal degradation by, these harmonics will depend in large part on the depth of the clipping action. Because of its complementary pre- and deemphasis, FM broadcasting can forgive a surprising amount of baseband clipping, insofar as audible distortion is concerned.
Figure 4c - 2dB Clipping Figure 4d - 3dB Clipping — 19 —
Section IV CALIBRATION “Routine” calibration of the Model 708 Stereo Generator is never required, its digital-synthesis circuitry is simple and stable. The only justification for confirming any of the internal adjustments is out-ofspecification operation, and only after all other possible faults have been considered and eliminated. It’s worth a call to the factory to discuss any anomalies before charging-ahead and possibly making things worse.
Input Level Set 1. Preset the 708 front-panel switches as follows: POWER — ON MODE — MONO PILOT — OFF O’SHOOT COMP. — OUT PRE-EMPHASIS — OUT 1. The input gain range should be jumpered for high-level inputs; this is the H position on jumpers strips JMP2 and JMP3. Apply the output of the Audio Generator to both the right- and the leftchannel PROGRAM LINE INPUTS. Feed these two inputs in parallel, and in phase, by connecting the test signal “hot” lead to both + terminals, and the test ground lead to G.
3. When the Audio Generator has been fine-tuned to null the right channel, adjust R116 (next to IC31) for a null in the left channel filter. Both channels should null to the same relative amplitude. 4. Increase the Audio Generator frequency to the second filter null, approximately 21.8kHz. Again, fine-tune the Audio Generator frequency for the deepest null in the right channel signal. 5. Adjust R122 (next to IC33) for a left-channel null. 6.
Input Level Calibration NOTE: This portion of 708 calibration must follow the preceding Crosstalk Trim procedure directly. 1. Re-apply 400Hz to both channels and monitor the filter buffer stage at R128 and R135, just as described at the beginning of this Procedure. 2. Turn the O’SHOOT COMP. switch to the IN position. 3. Make no changes to the 708 INPUT GAIN controls. Note that as the output level of the Audio Generator is increased, the 9-volt p-p waveforms will clip, or flatten, top and bottom.
8. Reset the COMPOSITE PROCESSING control to the fullcounterclockwise OUT position. NOTE: when composite clipping is turned off, front-panel MPX metering will read –1dB when a steadystate, 100%-modulation input signal is applied. Stereo Pilot Distortion Null NOTE: This portion of 708 calibration must follow the preceding Output Level Calibration procedure directly. At this point the Modulation Monitor should indicate 90% Total Modulation of the 400Hz Audio Generator test signal, and 10% pilot injection. 1.
transmission system; for instance, in a “composite” STL, or even in a long run of coax cable. The following procedure leaves the 708 adjusted for its own best stereo separation. Nevertheless, the front-panel MPX EQUAL. control will give a user the option of optimizing the entire transmission system, though he should keep in mind that a chain is only as strong as its weakest link. It’s far wiser to fix the weak link than to accommodate for it at another point. 1.
Section V CIRCUIT DESCRIPTIONS This section details the circuitry of the Inovonics Model 708 FM Stereo Generator. Circuit descriptions refer to the three pages of Schematic Diagrams contained in the Appendix, Section VI, Pages 38, 39 and 40. Component Annotation The schematics may appear to be annotated in a somewhat haphazard manner, insofar as component reference designations are concerned.
filtering, helped define the signal peak amplitude. Even a fully phasecorrected filter will exhibit overshoots, and a 7-pole “elliptic” filter, as used in 708, 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.
Components in the right channel filter have fixed values. The resonant frequency of each of the left 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 which might prove valuable to the broadcaster, is the Electronic Filter Design Handbook by Arthur B. Williams, published by McGraw-Hill.
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 IC26 is clocked at the 608kHz sampling rate. The output is decoded by IC25. With gating provided by IC20C and CR19, IC26 counts continuously from zero to 8, back to zero, etc.
Output Filter Since all components of the composite output are digitally synthesized at a 608kHz rate, only a simple filter is required to eliminate the sampling frequency from the Stereo Generator output. L2, L1, C25 and C26 form a 4-pole filter with quasi-Butterworth response. This filter is exceptionally flat to about 100kHz, but effectively prevents any higher-order digital noise components from appearing at the 708 output.
comprise the Model 708 display. The front-panel meter is scaled both in percent-modulation and in dB. This is similar to a standard VU meter except for the peak-response ballistics. Each of the three LED bargraph modules is series-connected. This is a power-saving technique since each module, rather than each individual segment, draws only 5 milliamps.
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Section VI APPENDIX The following section of this Manual contains Parts Lists for the Inovonics 708 Stereo Generator, 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 708 FM-Stereo Generator. These are listed either en-masse, 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. If a particular component is not listed at all, this means that we do not consider it a typical replacement item.
CR10 F1 FB1 I1-9,16 Diode, Zener, 22V; (open mfgr.) 1N5251 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.
J1 AC Mains Connector, PC-mounting; Switchcraft EAC303 J2 6-Position Barrier Block; PCD Co. ELFH06210 PC-mounting header with ELFP06210 plug-in screw-terminal block. J601-603 JMP1-4 Connector, BNC chassis-mounting male; Amphenol 31-221 Shorting “Shunt” for 0.1-inch header strip (open mfgr.) L1 Inductor, fixed, 560µH; ACT Co. PA2-561K L2 Inductor, fixed, 220µH; ACT Co. C4-221K Q1,5,8,9 Transistor, NPN; (open Mfgr.) 2N3904 Q2,6,7 Transistor, PNP; (open mfgr.
MAIL-ORDER COMPONENT SUPPLIERS The following electronic component distributors have proven to be reputable suppliers of both large and small quantities of parts. Any semiconductor, IC, capacitor, resistor or connector used in the Model 708 is probably available from one or more of these firms. Each supplier publishes a full-line catalog available free for the asking. Mouser Electronics — Call (800) 346-6873 Digi-Key Corporation — Call (800) 344-4539 ACTIVE (div.
