Inovonics 708 Manual

OPERATING AND MAINTENANCE
INSTRUCTION MANUAL
MODEL 708
FM-STEREO GENERATOR
__________

—–USER’S RECORD —–
Model 708 -Serial No. _______
Date Purchased _____________
Warranty Card Mailed —¨
OPERATING AND MAINTENANCE
INSTRUCTION MANUAL
MODEL 708
FM-STEREO PROCESSOR/GENERATOR
September, 1995
1305 Fair Avenue, Santa Cruz, CA 95060
TEL: [phone] –FAX: [phone]

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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..................................................................................... 4
Patent Notice ........................................................................................................................ 5
Block Diagram ...................................................................................................................... 6
Section II -Installation
Unpacking and Inspection ................................................................................................... 7
Mounting ............................................................................................................................... 7
Rack Requirements -Heat Dissipation
AC (Mains) Power ................................................................................................................. 7
As Delivered -Voltage Selector -Power Cord
Radio Frequency Interference (RFI) .................................................................................... 8
Location -Ground Loops
Line Input and Input Range Selection ................................................................................. 8
Input Connections -Balanced Inputs -Unbalanced Inputs -
Input Gain Range -Gain Jumpers
Pre-Emphasis Selection ....................................................................................................... 9
Subcarrier Inputs.................................................................................................................10
Composite Output ..............................................................................................................11
Output Ground Loops
Section III -Setup and Operation
Panel Controls and Indicators ...........................................................................................12
(All Controls and Indicators Defined According to Function)
Normal Setup Procedure ....................................................................................................15
Input Gain Calibration -Pilot and Subcarrier Adjustments -
Output Pre-Check -Output Level Adjustment

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Implications of Composite Processing .............................................................................17
Pilot Modulation -Out-of-Band Components
Section IV -Calibration
Calibration ..........................................................................................................................20
Equipment Required -Power Supply Check -Clock Set -
Input Level Set -Low-Pass Filter Matching -Crosstalk Trim -
Input Level Calibration -Output Level Calibration -
Stereo Pilot Distortion Null -Subcarrier Calibration -
Separation Trim
Section V -Circuit Descriptions
Circuit Descriptions ............................................................................................................25
Component Annotation -Input Circuitry -
Filter Overshoot Compensator -Low-Pass Filter -
Pilot and Subcarrier Generation -Bargraph Meter -
Power Supply
Section VI -Appendix
Parts Lists -Schematics -Warranty ..............................................................................33

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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. Through efficient
manufacturing techniques and automated calibration procedures, the
708 maintains the tradition of a value leader among FM Stereo
Generators.
Features Features of the Inovonics 708 include:
•Accurate front-panel metering of the input program,
externally-applied subcarriers, and output signal
levels.
•Built-in, metered combining for two external (RDS or
SCA) subcarrier sources.
•Extended-response low-pass filtering with proprietary
overshoot compensation.
•Digital synthesis of pilot and subcarrier for unexcelled
performance and freedom from drift and routine
adjustment.
•Built-in, adjustable Composite Processor.
AUDIO PRE-PROCESSING REQUIREMENTS
The Inovonics 708 Stereo Generator incorporates two separate circuits
to safeguard against carrier overmodulation (over-deviation).
Nonetheless, it is anticipate that the 708 will be preceded by some form
of “audio processing” system which places a ceiling on program peaks
with specific allowances for protection of the pre-emphasis
characteristic. It is essential that this need for split-spectrum audio
processing be understood and met.
Pre-emphasized
Transmission
FM broadcasting practice imparts a high-frequency pre-emphasis
(high-end boost) to the input program signal prior to transmission. At
the receiver a complementary de-emphasis (high-end rolloff) network
restores overall flat frequency response. The purpose of this exercise is
to reduce the high frequency noise which is invariably added in
transmission. This noise is generally worse for listeners who are some

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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. Normal voice and music programs have
comparatively low energy at the higher frequencies, and actual
statistics of program spectral composition were taken into account
when pre-emphasis characteristics were established years ago.
Nevertheless, occasional high energy, high frequency peaks (sibilants,
cymbals, etc.) can still cause carrier overmodulation, even when
program peaks are broadband-limited to 100%-modulation values.
This is especially true when modern-day recordings of contemporary
music constitute the program source.
Split-Spectrum
Peak Control A pre-emphasis network ahead of a broadband limiter, and a de- emphasis circuit following it, will deal with this situation, although the
overall level will “duck” whenever a high-end peak occurs. This
imparts a “choppy” sound to the program and reduces both
intelligibility and perceived loudness. What is instead required is a
program limiter operating as a dual-band device; a broadbandsection
to cope with normal program peaks, and an independent high-
frequency limiter (with proportionally faster time constants) to deal
with those program components accentuated by the transmission pre-
emphasis characteristic. A limiter of this type, now common in both
FM and AM broadcasting systems, has negligible audible effect on most
program material while providing absolute protection from carrier
overmodulation. Comprehensive FM audio processing systems are
available from Inovonics, and from a number of other suppliers.
Internal
Overmod
Protection
Overmodulation protection circuitry internal to the 708 starts with a
complex, active peak clipper integral with the patented low-pass filter
overshoot compensator. Because of its unique mode of operation, a
good deal more peak clipping may be tolerated than with more simple
audio clipping circuits. The 708 may, in fact, even be used “barefoot,”
or with minimal audio pre-processing, yet still yield acceptable
performance.
The 708 also features a built-in Composite Processor which can be
adjusted to clip as much as 3dB into the multiplex signal. This clipping
is performed prior to injection of the 19kHz stereo pilot and any
auxiliary subcarriers. While this “composite clipping” technique is
routinely used to increase perceived loudness, it is at the expense of
introducing spurious harmonic products above the 54kHz upper band-
edge of the composite stereo signal. Please refer to the further
discussion on Page 17.

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MODEL 708 TECHNICAL SPECIFICATIONS
Frequency Response (with pre-
emphasis turned OFF, or through
appropriate de-emphasis network):
±0.5dB, 20Hz–15kHz; –1dB or
less at 16kHz, –60dB or better at
19kHz.
Stereo Separation (LàR or RàL):
>65dB, 20Hz–16kHz.
Distortion (1dB below 100%
modulation):
<0.1% THD in demodulated
audio.
Noise:
Better than 80dB below 100%
modulation in demodulated audio,
20Hz–16kHz. 38kHz residual and
“digital” noise above 54kHz,
–70dB or better.
Crosstalk (MàS or SàM):
Linear and nonlinear crosstalk,
–50dB or better.
Stereo Pilot:
19kHz, ±1Hz; injection level
adjustable between 7% and 12%,
relative to 100% modulation.
Program Line Inputs:
Active-balanced/bridging;
accommodate input levels between
–10dBu and +20dBu, equivalent
to 100% modulation.
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.
Pre-emphasis:
May be jumpered for 75µs or 50µs
characteristic; may be switched
OFF at front-panel.
Composite Processor:
Adjustable between 0dB and 3dB
clipping of the 100%-modulated
multiplex waveform prior to
19kHz pilot insertion.
Composite (Multiplex) Output:
Adjustable between 2V p-p and
8V p-p; 75-ohm source impedance.
Digital Synthesis Sampling Rate:
608kHz (16X subcarrier
oversampling).
Power Requirements:
105–130VAC or 210–255VAC,
50/60Hz; 15 watts.
Size and Weight:
1¾”H x 19”W x 7”D (1U);
7 lbs (shipping).
BLOCK DIAGRAM
A simplified Block Diagram of the Model 708 is shown on the following
page. Generator circuitry is explained in detail under Circuit
Descriptions, Section V, which references Schematic Diagrams found in
the Appendix, Section VI.
PATENT NOTICE
Low-pass filter overshoot compensation circuitry used in the Inovonics
708 is protected under U.S. Patent No. 4,737,725.

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Figure 1 -Block Diagram, Model 708 FM-Stereo Generator

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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. In the event of return for
Warranty repair, shipping damage sustained as a result of improper
packing for return may invalidate the Warranty!
IT IS VERY IMPORTANT that the Warranty Registration
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 some
means of trace in the case of lost or stolen gear, but the user
will automatically receive specific SERVICE OR MODIFICA-
TION INSTRUCTIONS should they be issued by Inovonics.
MOUNTING
Rack
Requirement Inovonics’ Model 708 is packaged to mount in a standard 19-inch equipment rack and requires only 1¾ inches (1U) of vertical rack
space. The use of plastic “finishing” washers is recommended to
protect the painted finish around the mounting holes.
Heat Dissipation Consuming no more power than a bedside clock radio, heat generated
by the 708 is insignificant. The unit is specified for operation within
an ambient temperature range from freezing to 120°F/ 50°C. Because
adjacent, less efficient equipment may radiate substantial heat, be sure
that the equipment rack has sufficient ventilation to keep the
temperature below the stated maximum.
AC (MAINS) POWER
As Delivered Unless specifically ordered for export shipment, the Model 708 is set at
the factory for operation from 115V, 50/60Hz AC mains. The rear-
panel designation next to the fuseholder will confirm both the mains
voltage selected and the value of the fuse supplied.

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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. It is factory
practice to cross-out the inappropriate designation with an indelible
black marking pen. You can remove this strikethrough with solvent to
redesignate.
BE SURE that the mains voltage selector setting and pri-
mary fuse value are appropriate for the mains supply before
plugging the 708 into the wall outlet.
Power Cord The detachable power cord supplied with the 708 is fitted with a
North-American-standard male plug. The individual cord conductors
are supposed to be color-coded in accordance with CEE standards:
BROWN = AC “HOT” BLUE = AC NEUTRAL GRN/YEL = GROUND
If this turns out not to be the case, we offer our apologies (cord vendors
vary) and advise that U.S. color coding applies:
BLACK = AC “HOT” WHITE = AC NEUTRAL GREEN = GROUND
RADIO FREQUENCY INTERFERENCE ( R F I )
Location Although we have anticipated that the 708 will be operated in close
proximity to high-power transmitters, you should exercise care in
locating the unit away from abnormally high RF fields.
Ground Loops In some installations a mains frequency or RF ground loop may be
formed between the input or output cable shield grounds and the AC
power cord ground. Use of a “ground-lifting” AC adapter should
remedy the situation, though the chassis ultimately must be returned
to earth ground for safety. Generally, being screwed-down in the
equipment rack will satisfy this requirement.
LINE INPUT AND INPUT RANGE SELECTION
Input
Connections The Inovonics 708 has electronically-balanced (transformerless) left- and right-channel PROGRAM LINE INPUTS. These are brought out to
a screw-terminal barrier strip on the rear panel and include chassis
ground connections for cable shields. Please note that the screw-
terminal barrier block can be unplugged from the chassis! This makes
initial connection easier and permits quick removal of the 708 from the
rack should maintenance ever be required.

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Balanced Inputs 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. 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 engineering. More often than not, audio line
impedance matching is ignored by today’s enlightened broadcasters
(you) and leading-edge equipment manufacturers (us).
Unbalanced
Inputs When the generator is fed from a test oscillator or other equipment with unbalanced outputs, the single center conductor of the shielded
input lead should be connected to the + terminal, and the shield to G.
In addition, a jumper wire should be installed between the –terminal
and G.
Input Gain
Range The 708 can accommodate line-level program inputs with a nominal “Zero-VU” value between –10dBu and +20dBu. This 30dB range is
divided into two 15dB ranges by internal jumpering.
As shipped, the generator is jumpered for “professional” input levels
between +5dBu and +20dBu, corresponding to 100% modulation.
Most processor and STL receiver outputs will fall into this range,
+6dBu, +8dBu and +10dBu being typical levels.
Lower levels, between –10dBu and +5dBu, may be encountered with
feeds via lossy studio-transmitter telephone line circuits. The extra
gain for the low-level range is enabled by changing jumpers beneath
the top cover.
Gain Jumpers Under the top cover, and just behind the PROGRAM LINE INPUT
barrier strip, you will find two jumper pin strips, each with a “push-
on” jumper. The strips are identified as JMP2 and JMP3 in the circuit
board legend, and each has an Hand an Lmarking to indicate the
proper jumper placement for high level and for low level inputs,
respectively. Figure 2 illustrates jumpering options.
HIGH LEVEL INPUTS
(+5dBu to +20dBu) LOW LEVEL INPUTS (–10dBu to +5dBu)
Figure 2 -PROGRAM LINE INPUT Range Selection

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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. These are
identified as JMP1 and JMP4 in the circuit board legend, and each is
designated with a 75µs and a 50µs marking to indicate proper jumper
placement for the two available pre-emphasis characteristics. Figure 3
illustrates the jumpering options
75-MICROSECOND 50-MICROSECOND
Figure 3 -Pre-emphasis Jumpering
SUBCARRIER INPUTS
Internal combining is provided for two auxiliary subcarriers for SCA
and/or RDS services. The rear-panel SUBCARRIER INPUT connectors
are unbalanced inputs 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. Subcarrier
inputs at a level of 0dBu or greater may be fed to the SUBCARRIER
INPUT connectors, and can be regulated to the proper injection level by
the front-panel SUB INJ. level controls.
Front-panel metering of the RDS or SCA subcarrier levels allows
accurate adjustment of the injection level(s), even in the absence of a
Modulation Monitor with a specific display of subcarrier injection.

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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. This is a “flat” input,
rather than a monaural program input with signal pre-emphasis.
Program signal pre-emphasis is imparted by the 708.
Output Ground
Loops Because the Stereo Generator output and the exciter input are both unbalanced, it is best to locate the 708 near the exciter and power both
from a common AC mains circuit. This will help avoid ground loops
and mains-related hum in the transmitted signal.

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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. Depending on the trim-pot manufacturer, the end
of the range may, or may not, be identified by a “click-click” sound or
other audible/tactile sensation.
INPUT GAIN These two controls at the far-right end of the front
panel adjust the 708’s input sensitivity for the nominal
program level applied to the LEFT and RIGHT channel
PROGRAM LINE INPUT barrier strip terminals on the
rear panel. The controls have a 15dB range which is
multiplied by input gain range jumpering to an input
acceptance range of 30dB (see Page 9). The INPUT
GAIN controls are adjusted so that the 100%-
modulation “ceiling” imposed by Model 708 internal
circuitry coincides with the input program peak level
which corresponds to 100% modulation.
PRE-EMPHASIS System pre-emphasis is imparted by the Stereo
Generator in most situations. With the front-panel
PRE-EMPHASIS switch IN, the selected characteristic
(see Page 10) is enabled and the attendant green LED
will light. If, on the other hand, pre-emphasized audio
is applied to the 708, the switch should be set to the
OUT position to avoid “double” pre-emphasis. This
alternate mode is indicated by the red LED.
O’SHOOT COMP. The Model 708 incorporates a patented overshoot
compensation circuit which inhibits overshoots in the
primary 16kHz audio low-pass filter sections. The
switch is normally left in the IN position, indicated by
the green LED. Compensation may be switched OUT
for system tests which require carrier deviation in
excess of 100% (maximum); this will light the red LED.
The yellow LED to the right of the green one displays
circuit action and will flash almost continuously when
“bright” music is being broadcast.

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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. For example, should the
station suddenly suffer an emergency power loss,
temporarily reverting to mono transmission would
help regain some of the lost coverage until full power is
restored.
When switched to MONO, the MODE switch removes
both the stereo subcarrier and the 19kHz pilot. The
transmitted signal will consist of L+R, the algebraic
sum of the left and right program channels. The
MODE switch does not remove RDS or SCA
subcarriers. Green and red LEDs indicate the MODE
selection.
COMPOSITE
ADJUSTMENTS The PILOT INJ. and SUB 1/SUB 2 INJ. controls set the
injection levels of the 19kHz stereo pilot and
externally-applied SCA or RDS subcarrier(s). The
OUTPUT LEVEL control varies the overall composite
output level delivered to the exciter or STL. OUTPUT
LEVEL is a “master” adjustment and does not affect
the relationships (level ratios) between the stereo
program, pilot and RDS/SCA subcarrier.
MPX EQUAL. is a composite (multiplex) equalization
trimmer in the output stage of the 708. This has been
factory-set for best stereo separation into a resistive
load, but can be used to compensate for frequency
response deficiencies in a composite STL or in the
input section of the exciter.
COMPOSITE
PROCESSING In the fully-counterclockwise OUT position, an output
“safety” clipper acts on only those very fast program
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
tabulation at the top of the next page is approximate;
please refer to the discussion about additional
implications of composite clipping on Page 17.

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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. Use this feature sparingly!
“MULTIMETER” An up/down momentary toggle switch cycles the front-
panel metering among the various display options;
LEDs identify the function monitored. The metered
values are:
LLeft program input channel.
RRight program input channel.
L+R The sum of the left and right channels.
L–RThe difference of the left and right
channels.
MPX The composite multiplex output.
PILOT The 19kHz stereo pilot.
SUB 1 Subcarrier #1
SUB 2 Subcarrier #2
POWER (Sorry, the function of this switch is classified.)
BARGRAPH METERING
The front-panel bargraph meter is useful in initial level-setting and to
verify proper operation of the 708 Stereo Generator. The up/down
switch to the right of the display cycles the meter through its various
measurement functions.
The meter is peak-responding, and is scaled both in percent-modulation
and in dB, with 0dB corresponding to 100% modulation. When switched
to show measurements of PILOT, SUB 1 or SUB 2, an additional 20dB

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(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. As explained on Pages 3 and 4, the processing
system must maintain program peaks at a ceiling value corresponding to
100% modulation, and incorporate “pre-emphasis protection”
(independent high frequency) limiting in addition to broadband peak
control. This procedure further supposes a direct connection of the
COMPOSITE OUTPUT to the wideband input of an exciter/transmitter.
Variations from these conditions, such as an intermediate STL or other
link, analog or digital, in either the input or the output path of the 708,
may call for considerations not addressed here.
At this point the 708 should be installed in the program chain with
power applied, and have all front-panel function switches ON.
Input Gain
Calibration 1. Double-check PC board jumpering options for proper line input range selection and for proper pre-emphasis selection.
2. Feed the audio processor left channel input, only, with a 500Hz
sinewave test signal which yields 6dB to 10dB of indicated signal
limiting. This should drive the processor left channel line output to
its “ceiling” value and present the left channel input of the 708 with
a signal representing 100% modulation of that channel.
NOTE: Good engineering practice prescribes a standardized audio line
level at any equipment interface. This facilitates setup,
troubleshooting, and “patching” alternate gear in and out of the
program signal path. Take this opportunity to set the output level
of the processing system at a preferred “zero-dB” level: +6dBm,
+10dBm, or any other value common to station operation.
3. Set the bargraph meter to measure left channel (L) level. Adjust L
INPUT GAIN for a 0dB, or 100%, indication.
4. Repeat steps 1 -3 for the right program channel. Be sure that only
the right channel input of the processing system is driven.
Pilot and
Subcarrier
Adjustments
1. Set the bargraph meter to measure PILOT. Adjust the 19kHz stereo
pilot (PILOT INJ.) control for desired injection; typically 9%. Keep
in mind that the 100% point on the meter is actually 10% for this
measurement.
2. Cycle the meter to read any subcarriers applied to the 708. The
SUB 1 INJ. and SUB 2 INJ. controls may be adjusted for the desired
injection levels for subcarriers. Typically, RDS subcarriers are set
to about 5%, SCA subcarriers to 10%. Again, the 100% mark on the

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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 pre-
check verifies normal operation.
The processor, itself, should now indicate normal values of gain
reduction, including a minimum of 6dB peak limiting of program audio
peaks. Under this condition the 708 bargraph display will show
consistent left (L) and right (R) channel signal activity peaking at 0dB.
A display much in excess of 0dB indicates limiter overshoots and a
possible deficiency of the processing system in maintaining a consistent
output “ceiling.” Occasional overshoots up to +1dB or +2dB are to be
expected, but anything above these values is cause to question
processing effectiveness.
Temporarily set the front-panel COMPOSITE PROCESSING control
fully clockwise to the 3dB point. With typical program audio applied,
check bargraph metering of the composite multiplex output (MPX).
Peak modulation should remain within the limits shown:
Stereo program modulation only —100%
Stereo program and RDS only —105%
Stereo program and one SCA only —110%
Stereo program, RDS and SCA —115%
Stereo program and two SCAs —120%
NOTE:MPX metering relates to the total composite output voltage
peak excursion before the OUTPUT LEVEL control, not the actual FM
carrier deviation. The 100% reference point represents a normal stereo
signal without subcarriers; actual measurements will reflect the
algebraic addition of any subcarriers.
OUTPUT LEVEL
Adjustment PLEASE be certain that you understand operation of the station Mod- Monitor, and that it is properly calibrated and connected to the correct
transmitter/exciter RF sample point. The remainder of this procedure
is performed “on-air.”
1. Turn the 708 OUTPUT LEVEL control fully counterclockwise and
connect the generator’s COMPOSITE OUTPUT to the input of the
exciter.
2. Turn off or disconnect any subcarrier inputs to the 708. With only
the stereo programming applied, advance the OUTPUT LEVEL
control for a Mod-Monitor indication of frequent 100%-modulation
peaks.
3. Back-down the OUTPUT LEVEL control slightly, so that the 100%
peak flasher(s) light only occasionally.

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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. If
necessary, trim the OUTPUT LEVEL control to maintain carrier
deviation within legal limits. Depending on current operating rules
and practices, when either an SCA or an RDS subcarrier is
broadcast, total peak carrier deviation may be permitted to exceed
the customary 100%-modulation figure. The station (or consulting)
engineer should be aware of legal guidelines in effect and assume
responsibility for any adjustment which deviates from this
procedure.
7. Reset the COMPOSITE PROCESSING control to the full-
counterclockwise, OUT position.
IMPLICATIONS OF COMPOSITE PROCESSING
In the FM “loudness war,” the ultimate tool in the broadcaster’s arsenal
is composite processing. This is more appropriately called composite
clipping since linear gain-reduction techniques cannot easily be applied
to the “interleaved” multiplex stereo signal. The historic justification
for composite clipping reflects the need to control the overshoots of
uncorrected low-pass filters in early stereo generators.
Despite refinements in filter technology, which all but eliminate
overshoot as a potential overmodulation problem, it seems that a small
gain in perceived loudness can always be achieved by clipping peak
excursions of the multiplex signal just before it is fed to the exciter.
This clipping action is not without certain tradeoffs, however, among
which are modulation of the 19kHz stereo pilot and the generation of
out-of-band distortion components.
Pilot Modulation The 19kHz stereo pilot, already at a level some 20dB below 100%
modulation, is subject to amplitude perturbations by program peaks
when overall composite clipping is employed. With more than about
1dB of clipping, the pilot can actually be lost for the duration of the
clipped peak. The receiver’s stereo decoder may even lose phase-lock;
this was a serious problem with early add-on composite clippers.
More sophisticated stand-alone composite clipping devices strip the
19kHz stereo pilot from the composite signal, clip the remaining
multiplex stereo components, then re-apply the pilot. The Inovonics
708 simply performs its clipping function prior to initial pilot injection.

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18
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 de-
emphasis, FM broadcasting can forgive a surprising amount of
baseband clipping, insofar as audible distortion is concerned.
Nonetheless, clipping products can clutter that part of the baseband
spectrum reserved for RDS and SCA subcarriers, potentially creating
crosstalk into those services. With one or two dB of clipping, distortion
products will probably be tolerable. Much more clipping than this will
not only compromise subcarrier services, but can interfere with
adjacent stations as well.
Figures 4a through 4d reveal out-of-band components generated by
composite clipping. These “worst case” examples were created by
feeding a stereo generator from a CD player. No overmod-protection
audio processing was employed, stereo generator input gain was
adjusted such that the internal clippers associated with the pre-filter
overshoot compensation circuit were active more than 90% of the time.
The stereo generator output was monitored by a Tektronics 7L5
Spectrum Analyzer which accumulated data for the duration of the 6-
minute rock music selection.
Figure 4a -Clipping OFF Figure 4b -1dB Clipping