Inovonics 230 User manual

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INSTRUCTION
MANUAL
MODEL
230
"MULTIBAND
AUDIO
PROC
E
SSOR"
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January
1977
503-B
VANDELL
WAY,
CAMPBELL,
CA
95008
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GENERAL
INFORMATION
ANO
SPECIFICATIONS
General
Specifications -
8-Band
Compressor
Specifications -
Peak
Limiter Specifications
• • 3
II.
DESIGN
PHILOSOPHY
AND
FUNCTIONAL
DESCRIPTION.
• • . • 6
III.
Multiband
Compression
-
Gated
Expansion -
Peak
Limiter -Asynnetrical Modulation -
Phase
Follower -
AM
Stereo -Frequency-Selective Limiter
INSTALLATION
•
...
Unpacking
and
Inspection -
Mounting
-
RFI
-
In/Out Connection -
Polarity
-Line Considerations
. • 9
IV.
OPERATION
AND
ADJUSTMENT
.•••...•••.....••
11
Basic Adjustments -
AM
Adjustments -
FM
V.
VI.
Adjustments -
Peak
Limiter Release
Timi
ng
-
Gated
E
xpa
nsion Function -
Program
Response
Shaping
CIRCUIT
DESCRIPTIONS
.••..•
. . . . .
14
Input Amplifier -
Bandpass
Compressors
-
Gating
Circuit
- C
ombining
Amplifier/Program
Phase
Detector -
Peak
Limiter Circuits -
Output Amplifier -
Power
Supply
CALIBRATION
·
••••
•••
••
••••
••
•
General
-
Equipment
Required -
Bandpass
Com
pressor -
Program
Amplifier -
Peak
Limiter -
H.F.
Limiter Calibration -
Rectifier
Balance
...
18
VII.
APPENDIX
•.•.••••••••..
·
••....••.••
22
Parts
Lists
-Schematics -Warranty
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- I. - GENERAL INFORMATION ANO SPECIFICATIONS:- ·- -~-- .. ~~-·<~_,.....;.-·-..
The Inovonics 230 is a broadcast audio orocessor for both AM and FM
service, designed specifically to enhance perceived signal loudness
and th us extend the effective ar ea of station coverage. When prop-
erly adjusted, the 230 will so process the audio program sfgnal as
to increase transmitter modulation density to a figure approaching
theo ret ical maximum, while still maintaining a high value of program
l is tenability.
General Specifications
The Inovonics 230 is comprised of two primary functional sections;
the gated 8-ba nd Comp res sor, and the Peak Limiter section which also
includes the Phase Fo l-1 ower (AM) ., and independent Freq uen cy-Selective
Limiter
circuitry {FM). Specifications relative to overall ·230 per-
fonnance , and data spe cific to the Compressor and Limiter sections
are tabulated separately.
Fre2uency Response (below Compressor thresho ld):
-ldB, 20Hz - 20kHz.
Noise Level: Better than 6Sd8 below 100% modulation.
Distortion:
<1% THO for any degree of compression and l_imiti~g
of steady-state signals 20Hz -20kHz.
Input: Balanced-bridging; accepts program levels between -20
and +lOdBm. .
Output: Balanced output feeds
600-ohm lines or terminating inputs
at levels between O and +20dBm, corresponding to 100% modulation.
Stereo Interconnection: Two units may be interconnected by means
of an accessory cable to duplicate compression on a band-for-band
basis. (Also see pg. 8,
11
AM Stereo")
Power Requirement: 10s ·- 130VAC .(230V available), 50/60Hz; l/2A.
Size
and Weight: 7
11
X 19" .X 9
11
; 16 lbs.
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8-Bsnd
Compressor Specifica
tions
The
input
aud
io
program
signal
is
first
divided in
to
eight
frequency
bands
. each approximately
one
octave
wide
with
12
dB
-p
er
-octave
skirts.
Within
each
band
the
si
gna
l undergoes r.m.
s.
compression w
ith
full
fl
exib
ility
of
control over c
ompress
i
on
pa
rame
ters.
·
User
adjustment
of
Compression
Th
reshold
and
Compression
Ratio in each
of
the
eight
bands
pennits
program
re
spon
se
to
be
dynamica
lly
shaped in. a
manne
r
similar
to
that
afforded
by
a separate
Graphi
c Equaliz
er
to
complement
a
particular
prog
ramm
ing fonnat.
Frequency Division
(Hz):
20-60. 60-150. 150-300, 300-700, 700-1.Sk,
1.5k-3.5k, 3.Sk-7.Sk, 7.Sk-20k.
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Compr
ession Ratio: 2:1,
~:1,
8:1;
switch-selectable in each band.
Compression
Th
reshold: Variable over lS
dB
range
in
each band.
Attack
and
Release
Timing:
Optimized
for
each
band
consistent
with rapid operation, r .
m.s
. response
and
low
distortion.
Gated
Expansion
(DEFEATABLE
FUNCTION):
(See
pg.
7)
·
Peak
Limiter Specifications
Following
compression
the
input
program
is
recombined
into
broadband
fonn
.
At
this
point the
fast
Peak
Limiter
acts
.
on
the signal
to
re-
duce
program
peaks
to
the desired
ab
so
lute
value
for
100%
modulation.
The
Peak
Limiter
can
be
adjus·
tei;I
for
Phase-Followin9,
Asymmetrical
operation in
AM
service,
-0r
an
independent Frequency-Selective Limiter
enabled
for
FM
broadcast
use
.
Compression
Ratio:
:>100:1
Limiting Threshold: Variable over
25d8
range.
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Attack
iime:
--=lus
for
any
degree
of
limiting:
~·:.
Release
Time:
Complex
function
of
program
peak
content; manually
adjustable over
10:l
range.
Peak
Syrr,netry
(DEFEATABLE
FUNCTION):
· Positive
peak
value can
be
adjusted
from
100%
to
150%
of
negative
peak
value.
Phase
Follower
(DEFEATABLE
FUNCTION):
(See pg.
7)
Frequency-Selective Limiter
(DEFEATABLE
FUNCTI
ON
):
Switchable
to
complement
either
75µs
or
25µs
FM
pre-emphasis
characteristics
to prevent overmodulation
from
high frequency
program
energy.
Circuit
provides
totally
independent limiting
of
high frequencies.
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II. DES I GN PHILOSOPHY ANO FUNCT I ONAL DESCRIPTION
Multiband Compression
The Inovonics 230 Multiband Audio Processor represents a significant
departure in design and operating phi losophies from the usual family
of broadcast compressor/limiters. The most obvious difference is the
individual processing of signals within multiple frequency bands.
The majority of audio level compressors act on the broadband signal,
regulating circuit gain as a function of program level. This is gen-
erally accomplished by deriving a DC control voltage from the audio
signal, and utilizing this voltage to regulate circuit gain.
In order to remain unobtrusive in its action, a compressor must be
relatively fast-acting; that is it must follow the envelope of the
audio program quickly enoug h to reduce sudden input increases, and
must release quickly to avoid audio "holes" and audible fade-up when
the input level returns to normal. Unfortunately, this fast action
is not consistent with low distortion, particularly at low frequencies,
as a necessarily
short rectifier filter time constant results in ampli-
tude modulation
of the program by its own harmonic products.
The obvious alternative is to independently compress high and low
· frequencies, tailoring the rectifier filter time constants to the sub-
ject portions of the spectrum. In this manner, low frequency distor-
tion could remain at an acceptably low level, while, at the same time,
compression could
be rapid at voice frequencies and above.
In i ts utilization of this multiband approach, the Inovonics 230 divides
the
au d io spectrum into eight separate bands. While this might seem
an unnecessary number of divisions, the advantage is twofold. First,
broadband energy can be concentrated and maintained at a high level,
as no loud, single tone or musi .cal note can reduce gain in any other
than its own band. Secondly, by providing manual adjus·tment over both
· the Compression Ratio
and Compression Threshold in each of the eight
bands, the spectral distribution of the program can be tailored to
impart a specific ''sound" to the program, almost regardless of the
nature of the program source. This can be nearly as important an ad-
vantage as perceived loudness
in an already loud, competitive market-
place.
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.,
Gated
Expansion
As
a
co
mpres
sor reduces the
dynamic
range
of
a program, so
it
a1so
reduces the program-to-noise
ratio.
Noises
that
wer
e
once
far
below
the average program level
(teletype
printers,
air
conditioners,
etc.)
are
now
clearly
audible
when
the average
program
level drops.
To
lessen
this
annoyance, the
230
features
an
optional
Gated
Expansi
on
mode,
which
simultaneously reduces the gain in
all
eight
bands
once
the
program
level
falls
below
a predetermined Gating Threshold value.
In
order
for
the gating
circuit
to
properly di
fferentiate
between
legitimate
program
material
and
spurious noise, frequency weighting
restricts
gating
sensitivity
to
mid-band frequencies.
Peak
Limiter
The
230
employs
a
fast,
true
p·eak
limiter
rather
than a
clipping
cir-
·
cuit.
The
apparent increase in average modulation afforded by a
clipper,
a
result
of
the
greater
r.m.s. value of the
clipper's
squared output
waveform,
is
likely
to
suffer
substantial
loss through phase
shift
and
overshoot in the
AM
transmitter
modulator or
FM
stereo generator.
The
230
preserves signal
waveshape
as
much
as possible,
consistent
with
instantaneous reduction
of
fast
program
peaks.
The
advantage
of
true
peak
limiting in terms
of
audible signal
distortion
is
obvious, even in
casual
listening
situations.
Asyrm,etrical Modulation
In
AM
broadcast use
of
the 230, the value
of
positive modulation
peaks
may
be
continuously adjusted
to
assume
up
to
150
%
of
the negative
peak
value. Although positive modulation
is
legally
restricted
to
125%,
many
plate-modulated transmitters require drive in excess
of
+125%
to
reach the legal
maximum.
It
must
be
warned,
however,
that
transmitters
which
fall
into
th
is category
are
incapable
of
linear
operation
up
to
+125%
,
and
the very
slight
gain in perceived signal loudness must be
weighed
against probable high signal
distortion.
Phase-Follower
Asymmetrical
modulation .
of
pos
·
itive
and
negative
peaks
yields
a
modu-
lation
advantage only
when
the
program
material
is
a.symmetrical in
nature.
Most
music, because
it
is
a blend
of
complex
waveforms
which
are not phase-related,
has
a very
even
balance
of
peak
energies. Solo
instruments
and
the
human
voice,
on
the other hand, possess a
more
or
less fixed
relationship
between
the phase
of
the fundamental tone
and
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the harmonics. This leads to an appreciable and consistent imbalance
between positive and negative peaks, and can be used to modulation
advantage.
The 230 Phase-Follower circuit monitors mean peak energy balance and
instantly inverts program phase to maintain maximum positive AM trans-
mitter modulation. The prog r am inversion is effected only at signal
zero-crossings, however, to reduce audibility of the switching action.
Nevertheless,
an instantaneous second harmonic product is generated
each time the phase is inverted, and may be distracting with some types
of pr ogram material. Sensitivity of t he Phase -Follower circuit is
adjustable so that a c omprom ise between advantage and annoyance can
usually
be reached.
AM Stereo
As of this writing, the subject of ste reophonic AM broadcasting is
receiving a good deal of attention. Although no standard has been
adopted, the most promising system utilizes a combination of carrier
amplitude and phase modulation. In this system, a L+R audio signal
amplitude modulates the
transmitter in a conventional manner, assuring
monaural compatibility. The L-R audio signal phase-modulates the
carri e r, and is independently demodulated and matrixed by the AM stereo
receiver to yield Left and Right information.
When interconnected for FM stereo operation, the 230 Phase-Follower
circuit is automatically disabled. Nonetheless, when two 230
1s
are
interconnected, and the Phase-Fo·llower in one unit manually enabled,
the other Phase-Follower
circuit will be slaved to the first. If the
first 230 should process the L+R signal which amplitude-modulates the
carrier, and the second unit the phase-modulating L-R information, the
result will be a fully modulated AM carrier with the stereo matrix
phase properly maintained.
_Frequency-Selective Limiter
Standard
FM broadcasts utilize a 75µ s pre-emphasis time constant for
signal-to-noise improvement; . Dolby-encoded broadcasts a 25µs charact-
eristic. As a result of this signal pre-emphasis, the amount of trans-
mitter input drive to achieve 100% modulation is much less at high than
at low frequencies. An effective FM limiter mu st therefore effect
greater reduction of high frequency program prak s.
The Inovonics 230 incorporates a separate Pea k Limit er to control the
energy
of high frequency program peaks independently of the normal
broadband limiting action. The high frequency limiter can be switched
between 75~s and 25µs limiting characteristics .
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III. INSTALLATION
Unpacking and Inspection
Upon receipt of the equipment, inspect at once for shipping damage.
Should any such damage be observed, notify the carrier at once;·if
not, proceed as outlined below. It is suggested that the original
shipping carton and materials be retained should future re-shipment
become ne cessary. In the event of return for Warranty repair, ship-
ping
damage sustained as a result of improper packing for return may
invalidate the Warranty.
It
is important that the Warranty Registration card found at the
front of this manual be returned; not only does this assure coverage
of the equipment by the Warranty, but the user will automatically
receive
specific servicing or modification infonnation if and when
it should become available.
Mounting
The 230 is packaged to mount in a standard 19-inch equipment rack,
requiring 7 inches of rack space per unit. The 230 generates negli-
. gible heat, and itself is unaffected by wide variations in the ambient
operating temperature.
RFI
The 230 is specifically designed to operate in close proximity to
broadcast transmitters; nevertheless, care should be exercised in
locating the unit away from abnormally high RF fields.
In some installation situations, an RF ground loop may .be formed be-
~ tween ~he input or output cable shield grounds and the AC power cord
ground.
Use of a "ground-lifting" AC adapter should remedy any problem.
hfOut Connection
The 230 input and output.connections are brought out to a rear panel
screw-terminal
barrier strip. Both the input and output are balanced
(transformer-isolated), with
ground terminals provided for cable shields.
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The
input
and
output
of
the
230
are
each
marked
with+
and
-desig-
nations
to
aid
in
maintaining prope·r phase in stereo
applications,
or
proper asyrrmetrical modulation
of
program
peaks. In the
latter
case,
the+
output tenninal will
be
positive-9oing with positive modulation
peaks.
Line
Considerations
Should
the equipment
which
feeds the
230
require output loading,
an
external
600
ohm
resistor
should
be
placed across the
230
input tenni-
nals; otherwise
the
input is "balanced-bridging" with a
characteristic
input
impedance
of
lOK
or
gre
ater
.
The
source
impedance
of
the
230
output amplifier (ahead
of
the
line
output transfonner)
is
virtually
zero
. .
However,
as
the
output
trans-
former
has
some
reactan
ce,
it
is
important
that
the
230
output be
terminated in
600
ohms
to
minimize
output overshoot.
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IV. OPERATION AND ADJUSTMENT
The Inovonics 230 is a f undamen tally more complex device than most
other broadcast audio processors, a
nd hosts a full complement of ac-
cessible controls whi ch materially aff ect both the technical composition
and the subjective audio quality of the transmitted program s ignal.
The various adjustments are identified on the inside of the removable
front a
ti justment cover. Despite the seemingly obvious mean ings of the
control function descriptions,
it is highly recommended that the out-
lined
procedu re be carefully followed for initial setup. Subsequent
readjust
men t to secure a desired effect should be made only after it
has been est ablished that the 230 functions properly and predi ct ably
in accordance with the
initial setup procedure.
All adjusune nts are made "on the air..," utilizing a typical program feed .
and the station Modulation Monitor.
Basic Adjustments
1. Preset controls as follows:
COMPRESS ION THRESHOLD (a 11 8) - fully "up" ( OdB)
COMPRESSION RATIO (all 8) -4:1
FUNCTION - Ungat ed
LIMITER MODE -Flat
PEAK LIMITER RELEASE - fully counterclockwise
2.
With a n onn al program feed from the console, adjust the INPUT
GAIN (CO MPRE SSION) control for 10 - lSdB of .compression in the
most active bands.
3. Adjust
PEAK LIMITER DRIVE for 5 - lOdB Peak Limiting.
4. Adjust
OUTPUT LEVEL control for 100% modulation on peaks as
indicated
by the Modulation Monitor.
5. Proceed with
specific AM or FM adjustment~.
~~ Adjustments (Asymmet'.ical Modulation)
1. Set the
LIMITER MODE switch to Asymmetrical, and adjust the
POS. PEAK AMPLITUDE (ASYMMETRY) control for desired positive
peak modulation per the Modulation Monitor.
_2.
With the LIMITER MODE switch in the Phase-Following position,
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the 230 will invert the signal at zero-crossings to maintain
maximum positive modulation. The PHASE-FOLLO WE R SENSITIVITY
control can be left fully clockwise to enable inversion with
the
least amount of signal asymmetry, or rotated counterclock-
wise
to invert with correspondingly greater asynmetry, thereby
reducing inversion incidence.
(See wa rning on pg. 8)
FM Adjustments (Fr equ ency-Selective Limiting)
With the LI M ITER MOD E switch in either the 75µs (n ormal FM) or 25ps
(Dolby FM) positions, high frequency program peaks will be inde-
pendently attenuated
to prevent o vermodul ation due to transmit ter
pre-emphasis. The OUTPUT LEVEL control will require readjus tme nt
from the initial setting made under Basic Adjustments to insu re
full FM carr ier modulation.
S
UB JECTIVE ADJUSTMENTS
Peak Limiter Release Timing
As preset under Basic Adjustments, the Peak Limiter release
time is at maximum, resulting in s low est recovery from program
peak s. Average modulation can be increased significantly by
turning the PEAK L IM ITER RELEASE contol clockwise from its
present full CCW position, thus operating with a shorter re-
lease time. The final setting must · be based on a judgement
of program quality, compromising between a sound that's just
"loud," and one that can be listened to for an extended period
without fatigue.
Gated Expansion Function
With the FUNCTION switch in the Gated Expansion position, Com-
pressor gain will reduce by a nominal lOdB figure when no legit-
i mat e input signal is present. This serves to reduce background
noise
by the same lOdB •. The GATING THRESHOLD contral sets the
point
at which an ·input signal will open the gate and return
Compresscr gain to maximum. The control should be advanced suf-
ficiently clockwise to keep the GATE OPEN indicator illuminated
constantly during music, but not so
far as to prevent gating
action during pauses in speech.
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Program
Response
Shaping
Since the
230
has independent control over both Compression
Threshold
and
Compression
Ratio in each band, frequency char-
acteristics
may
be
tailored
to
complement
a
specific
program-
ming
format in
much
the
same
manne
r as with a Graphic Eq~alizer.
Regardless
of
THRESHOLD
and
RATIO
setting,
230
response·
at
low
levels
(below
Compression
Threshold) will remain
flat.
The
control
settings
will,
however,
directly
influence the
amoun
t
of
energy in e
ac
h
band
at
normal
program
levels.
When
all
eight
bands
are adjusted
similarly,
the
230
will impart a "bright"
sound
to
the
program;
this
is
due
to
spectral energy
distribution
in typical
music
and
speech
signals,
and
the tendency
of
the
230
to bring
up
the higher frequency
bands
which
are
by
nature lower
in energy
level.
If
it
is
desired
to
maintain spectral balance
and
prevent response
coloration
by
the 230, the
COMPRESSION
RATIO
switches should
be
set
identically,
and
the
THRESHOLD
controls adjusted
for
approxi-
mately equal compression in
all
bands
with typical
program
material.
Response
may
be
tailored
by
adjusting
either
the
RATI
O switch
or
the
THRESHOLD
control (or both),
to
yield
the desired "sound."
It
is
advis~d,
however,
that
the following precautions
be
observed:
1.
Make
all
changes in small
steps,
listening
carefully
to
the
results
each time.
2.
If
it
is
desired
to
e
mphasi
ze
certain
bands,
keep
the
THRES-
HOLD
controls
of
these
bands
at
OdB,
and
the Compression Ratio
lower than in the others.
3.
Avoid
differences
of
more
than
SdB
in the
THRESHOLD
settings
of
adjacent bands. Violation of
this
warning.
can
lead
to
very
undesirable "phasing"
or
"swishing"
effects
.
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V. CIRCUIT DESCRIPTIONS
--
Input Ampli fier
The 230 program input is transf ormer-iso lated by Tl on the "mother"
circuit board and routed to the P rogram Amplifier assembly and -Input
Amp lifier !Cl. Current gain stages Ql and 2 provide additional output
drive to feed
all eight Bandpass Compressor assemblies, the inputs of
which are paralleled at the Compressor Input Bus . Gain of the Input
Amplifier
can be adjusted over a 30d8 range by the INPUT GAIN (COMPRES-
SION) cont't ol R4 to accommodate different input line levels.
Bandpass Compressors
In each of these assemb 1i es, si gna 1s from the Compress . or Input Bus .are
fed
to ICl, a voltage-variable gain stage. Th is circuit, the •santana•
configuration, utilizes FET Ql as an active variable resistance across
the inv
erting input of the operational amplifier. Q2, a similar
11
durrmy
11
FET, is placed across the non-inverting amplifier input and cancels the
c
hann el non-linearities of Ql. Th is permits control of the aud io signal
at levels which would otherwise be above the low-distortion operating
range
of the FET's •
. Q3 and associated . components form the high-pass section of the bandpass
filter, Q4 the low-pass section. Both sections are second-order active
filters with 12d8-per-octave rolloff. The output of the filter circuit
is fed to the common Combining Bus through the BAND GAIN calibration
control R25 . Amplitude and phase characteristics of the eight individual
filters are such that the combined outputs yield a flat signal with min-
imum ph ase distortion.
A portion of the bandpass filter output also passes through THRESHOLD
control R13 to IC2, a voltage gain stage. QS and 6 comprise a "Baxandall"
full-wave
rectifier. The positive-going portion of the input waveform is
,amplified by corrnnon-emitte r stage QS, the negative by co!llTion-base stage
Q6. Thus equal positive and negative information cause similar currents
at the junction of the Q5 and 6 collectors. Q7 is a non-linear current-
source collector load for Q$ and 6, and imparts a logarithmic character-
istic to the resultant DC voltage. This log tr ·ansfer function creates
the desired linear compression of signals exceeding the preset Compression
Threshold. · .
Further
DC amplification of the rectified bandpass signal is performed
by Q8 and IC3. The combined gain of these stages establishes overall
Compression Ratio which can be varied with COMPRESSION RATIO switch
S1 and calibration pot R22 . Compression is inhibited for test purposes
..
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-.
•
when the Program Amplifier FUNCTION switch is placed in the
11
Proof"
position. This
clamps the anode of CRl slightly negative, shunting
Q8 collector current to ground. .
A positive current
is fed through R27 to the inverting input of IC3,
generating a negative
DC offset at the output of this stage. It is
- --
this off set which ultimately appears across R36 and is adjusted by
this calibration control to "pinch off" FET Ql. As rectified signal
reaches the non-inverting input
of IC3, the DC offset is reduced, thus
causing signal compression.
The offset current through R27 can be
varied by the Gating circuit to simultaneously reduce gain in all eight
channels, lowering
background noise when no input signal is present.
The components between the output of IC3 and the base of Q9 filter the
DC control voltage. Compressor Attack and Release timing are a function
of these component values, which are optimized for the range of fre-
quencies specific to each band to insure rapid operation, low distortion
and minimum audibility of Compressor action. Complementary emitter-
followers Q9 and 10 isolate the filter from metering and other loads.
Gating Circuit
A portion
of the "raw
11
230 program input signal present on the Program
Amplifier assembly is routed through GATING THRESHOLD control Rl to
. voltage amplifier . IC2. This stage is bandwidth-limited, with -3d8
points
at 300Hz and 3kHz. This insures that the signal at the output
will consist mostly
of legitimate program material, rather than ~purious
noises.
IC3, a unity-gain inverter, and diodes CRl and 2 provide full-
wave rectification of the weighted signal.
With FUNCTION switch S1 in the Gated position, capacitor C8 charges to
the positive supply through R12. When the weighted and rectified input
signal reaches the predetermined threshold value,
Q3 begins to conduct,
discharging
C8. IC4 adds DC gain and hysteresis to the circuit, and its
output toggles negative when program is present, positive when the pro-
,
gram level falls below the threshold level. Transistors Q4 and 5 and
the RC combination of R18 a nd ClO integrate the gating logic to provide
a
smooth level transition between the gated and ungated, or "gate open"
states .
Combining Amplifier ( Program Phase Detector
The outputs of the eight Bandpass Compressors are combined by the oper-
ational
summing amplifier !CS on the Program Amplifier assembly. Drive
to the Peak Limiter (and thus the amount of program peak limiting em-
ployed) is set by the gain of this stage, variable over a 25dB range by
...
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l
'
,·
(
.I
-·-
·
·--
-
. .
....
PEAK
LIMITER
DRIVE
control
R26.
Positive
and
negative output excursions
of
JCS
are also independent
ly
rectified
by
diodes
CR8
and
7,
respectively;
the
peak
values held
by
capacitors
C16
and
15.
Pol
arity
of
the
DC
voltage across
Cl?
indicates
the predominant
polarity
of
program
peaks
and
controls the automatic
program
phase reversal feature
to
maintain
maximum
positive
AM
carrier
modulation.
IC7
adds
DC
gain and
hysteresis
to
the switching signal .
FET
Q8,
normally
off,
prevents
IC7
from
changing
state
except
when
m
om
entarily
turned
on
at
signal zero-crossings. A zero-crossing de-
tector
is
com
prised
of
amplifier
st
ages
IC6
and
Q7,
and
diodes
CR9
and
10.
PHA
SE
FOLLOWER
SENSITIVITY
adjustment
R34
selects
the degree
of
peak
po
larity
imbalance required
to
effect
a
program
phase
reversal.
Peak
l
im
iter
Circuits
Input signals to
the
Peak
Limiter assembly
are
fed
to
ICl,
a variable
gain stage
of
the Santana configuration (See pg.14). ICl,
acting
as
the
br
o
adband
peak-controlling
ele
m
ent,
drives
IC2,
a second Santana
circuit.
In
this
instance, however,
capacitors
C3
and
4
are
introduced
in
series
with the
variable-resistance
FET
Q4
to
im
part
a frequency-
selective
nature
to
the gain reduction
characteristic.
This
totally
independent control
of
high frequencies
is
utilized
in
the
75µs
and
-
~5µs
FM
limiting
mqdes
.
In
AM
use,
IC2
functions only as a unity-gain
.inver
ter.
.
A broadband peak-reducing
DC
control voltage
is
derived
from
the
output
signal
of
the
Peak
Li
miter assembly. In
AM
processing
applications
this
can
be
either
the output
of
ICl,
or
the inverted output
of
IC2.
In
the Phase-Following
mode,
the
two
switching
FET
1
s
Q5
and
6
are
controlled
by
logic
from
the
Program
Amplifier assembly,
selecting
the
program
phase
which
results
in
maximum
positive
modulation. In
the
ot
her operating
situations,
Q6
is
kept
on
to
maintain overall
230
phase
integrity
and
to
permit frequency-selective limiting
for
FM
by
the
high frequency limiting
circuit
of
IC2.
Transistors
Q9
and
10
comprise a Baxandall full-wave
rectifier
config-
uration (See
pg.
14) driven
by
buffer stage
IC3.
Q9
and
10
can
be
balanced
for
symmetrical
pe~k
rectification
wit~ a variable
DC
offset
from
calibration
control
R25.
When
asymmetrical
rectification
is
desired,
the
sensitivity
of
the
rectifier
to
negative-going program
peaks
is
decreased by biasing the base
of
QlO
negative with
POSITIVE
PEAK
AMPLITUDE
control
R30.
Since buffer stage
IC3
inverts,
this
decreased
sensitivity
of
the
rectifier
to
negative
peaks
corresponds
to
an
increase in the ievel
of
positive
program
peaks
at
the
230
out-
put.
The
value
of
positive
peaks
can
be
varied
from
10
0%
to
150%
of
the negative
peak
value. Transistor Qll multiplies the gain
of
the
..
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. _,,.,,;___·-----=---
......
Baxandall rectifier, resulting in a very high com pr ession ratio (on
the order
of 50:1 or more) above the peak limiting threshold.
The time constant of the peak rectifier filter is a function of peak
incidence and duration. The first half-cycle of a program peak, or
isolated single peaks in excess of the mean peak reduction value, pass
through
CR3 to the gate of QI and effect instantaneous peak reduction.
Repetitive peaks,
on the other hand, cause CB to charge through- R39 and
CR4, introducing a longer-term gain reduction. This longer-term re-
duction of repetitive peaks prevents the audible distortion attendant
to peak "clipping."
The Peak Reduction meter is driven by a sample-and-hold circuit consist -
ing
of !C's 4 and 5 and transistor Q13. Peak holding time of the
metering
circuit is fixed by the time constant of Cll and R44, and at
about two seconds permits an accurate display of even the fastest pro-
gram peaks.
The independent high frequency limiter f un ctions similarly to the broad-
band peak reduction circuit just discussed. The output of IC2 is equ al-
ized to the 75µs or 25µs characteristic by C12 and R48 and 49. IC6 is
a gain stage driving the Baxandall rectifier comprised of Q14, 15, 16
and 17. The resultant DC is applied to Q4 tc reduce the higher fre-
quencies. CRll, Cl4 and R61 and 62 form a dual release filter similar
to that used in the broadband circuit.
Output Amplifier
The Output Amplifier is part of the Program Amplifier assembly and
consists of IC8, with Q9 and 10 as current-gain stages. Gain is set
by OUTPUT LEVEL control R43. The 230 line output is isolated by the
chassis-mounted transformer
T2.
Power Supply ·
'
Power transformer T3, rectifier diodes CRl-4 and filter capacitors Cl
and 2 deliver "raw" DC to the two "three-terminal" voltage regulators,
IC's 1 and 2. Dua l operational amplifier 1C3 is connected to provide
additional
AC feedback around the two regulators, further reducing
ripple and noise on the ±18-volt regula·ted supplies.
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VI. CALIBRATION
Genera l
The Inovonics 230 has been carefully calibrated at the factory and
"burned-in" to verify circuit stability prior to shipment. There is
no requirement for routine instrument calibration, and the only reason
which might necessitate adjustment of the various calibratio n controls
would be a catastrophic failure and subsequent replacement of critica l
components. A
calibration procedure is nonethele ss given, but should
be attempted only by qualified individuals who have read the C ir cuit
Descriptions and who understand the various circuit functions.
Equipment .Required
1. Printed
circuit Extender Card: Inovonics A/N 137300
(Available from Inovonics, $20, postpaid)
2.
Audio Oscillator: Hewlett-Packard 200CD or equivalent.
3.
AC Voltmeter: Hewlett-Packard 400H or equivalent.
Bandpass Compressor
Preliminary
1. T
he Bandpass Compressor assemblies -are adjusted one-at-a-time
in
their own respective sockets, with all other Bandpass Com-
pressor assemb lies removed.
2. AC voltmeter access to the Compressor Input Bus and the output
of the Combini ng Amplifier is made by temporarily soldering
sh
ort "pigtail" leads to the INPUT GAIN (COMPRESSION) and PEAK
LIMITER DRIVE controls on the Program Amplifier assembly. In
both cases connection is made to the control pin nearest the front
edge
of the board. .·Hereafter, these monitor poi nts are referred
to as
11
input
11
and "output," respectively.
3. Set the
Program Amplifier FUNCTION switch to Ungated, and rotate
the PEAK LIM!TER D~IV E control fully CCW. Settings of other
controls are unimportant.
4. Connect the oscillator to the 230 input.
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FET Balance/ Threshold Cal.
1. Insert the assembly to be calibrated into the Extender Card,
and the Extender Card into the proper socket.
-----
2. Set the oscillator to the midband frequency for the assembly
under
test. Adjust the oscillator output level for a reading
of -lOdBm at the "input" monitor point. ·
3.
Connect the voltmeter to the "output" monitor point. Turn R36
and RlS fully CCW, and set the THRESHOLD control fully "up .
11
Adjust R2 for a minimum reading (null) on the AC voltmeter.
4.
Turn R36 fully CW. Adjust the BAND GAIN control for a volt-
meter reading of -19. 6dBm.
5. Slowly rotate R36 CCW until the voltmeter drops 0.2dB. to
-19 . SdBm. ~ · -· · .
6. Turn Rl5 slowly CCW, setting for an additional drop of 0.2dB,
to -20dBm. At this point set R37 for a front-panel Compression
me ter reading of zero.
Ratio Cal.
1. Set the
RATIO switch to 2:1. With the AC voltmeter connected
to the "input" monitor
point, increase the oscillator output
level
for a reading of +6d8m.
2. Reconnect the AC voltmeter to the "output" mo nitor point. Ad-
just R22 for a voltmeter reading of -12dBm.
3. Set the RATIO switch to 4:1; the output should fall to -16dBm .
At 8:1 the reading should fall to -18dBm.
Repeat this procedure for each Bandpass Compr essor assembly. It
is suggested that the calibration pots be resealed with a small
amount of white glue (Elmer's, etc.). When all assemblies have
been adjusted, remove the two monitor access leads from the Pro gram
Ampli fier assembly.
Program Amplifier
There are no calibration ad ju stments on this assembly.
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--
.,
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Peak Limiter
Preliminary
1. Plug
the Peak Limiter assembly into the Extender Card, and the
Extender
Card into the Peak Limiter assembly socket.
2. This assembly is calibrated with all other assemblies installed,
the Program Amplifier FUNCTIO N switch in the
11
Proof
11
position
and the OUTPUT LEVEL control fully CCW.
3. The oscillator should be connected to the 230 in pu t, the AC
voltmeter to the 230 output.
4. Turn both the POS. PEAK AMPLITUDE (ASYMMETRY) and PEAK LIMITER
RELEASE controls fully CCW.
01/,Qg_ Balance/ R34 Adjustment
1. Set the Peak Limiter MODE switch to Flat. Set the oscillator
to a frequency of 500Hz and adjust the oscillator output level
for a reading of -lOdBm on the AC voltmeter.
2. Rotate
R34 fully CCW. Adjust RI for a minimum reading (null)
on the AC voltmeter.
3. Turn R34 fully CW and note the output reading. Slowly rotate
R34 CCW until the output decreases by O.ldB.
4. Set R45 for a front-panel Peak Reduction meter reading of zero.
mfM Balance/ R58 Adjustment
1. Increase the osci 11 a tor ·frequency to 20kHz.
2. Rotate R58 fully CCW. Adjust RS for a minimum reading (null)
on the AC voltmeter.
3. Turn R58 fully CW and note the output reading. Sl owly rotate
R58 CCW until the output decreases by O.ldB.
_ ,,.,_
7