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29
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. This lights the
GATE OPEN
indicator and
pulls the junction of R138 and R162 to a slightly negative value.
When the program falls below the gating threshold, the output of
IC29B goes positive. A positive DC bias from R138 and R162 passes
through CR24 to the program peak integrator comparator, IC30A. If
the last value of AGC gain is positive, this DC bias toggles IC30A
negative to slowly reduce AGC gain.
The divider at the base of Q12 is scaled to turn the transistor on when
AGC gain reaches 0dB. This will pull the bias from IC29 toward
ground when AGC gain reaches 0dB. If the last value of AGC gain is
negative, Q12 inhibits the DC bias from IC29B until gain reaches the
0dB value.
The gain-corrected program signal at the output of IC48B has a
nominal (sinewave) level of 0dBu at 400Hz.
WIDEBAND COMPRESSION AND LIMITING
Compressor/
Limiter Timing
As discussed in the description of the AGC circuitry, the output of
IC28C is the full-wave-rectified program signal. CR17 passes the peak
value of this signal to a fast integrator consisting of R105 and C53.
Integration is on the order of 1ms and represents limiter
attack
time.
Assuming the front-panel
AVERAGE COMPRESSION
control, R145, is
turned fully clockwise (wiper to the output of IC28C), integration
through R117 causes C54 to charge to the
average
value of the
program waveform. The peak integration capacitor, C53, discharges to
this average value through R106. This time constant, about 50ms,
establishes the initial peak limiter release to the average compression
“platform” value. Platform release, back through R117, is about one
second. In practice, C53 discharging into C54 elevates the platform
when the program material has numerous, repetitive peaks. The
resulting program-controlled, “floating platform” release characteristic
inhibits program self-modulation, or “pumping,” effects.
AVERAGE
COMPRESSION
Control
As R145 is turned counterclockwise, a positive DC bias is added to the
platform value. This introduces a variable attenuation which is
reflected as program signal gain reduction
ahead
of the
compressor/limiter circuit. Front-panel markings relate to an
equivalent figure of average level compression for typical program
material.
