— —
28
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. This causes gain control for the stereo program to
respond to whichever channel happens to have the greater energy.
Slaving the two channels in this manner is essential to preserving the
stereo image.
“PPM”
Response
The full-wave-rectified peak level of the program audio is buffered by
IC28C. For AGC control, the peak value is captured by CR20 and
integrated by R141 and C67. The discharge path of C67 is back to the
buffer output through R116. The net response of this circuit
approximates the ballistics of the European Peak Programme Meter, or
“PPM,” generally judged superior to an American “VU” meter in
accurately representing program dynamic range. With the AGC
emulating a conscientious (but slow-moving!) operator who manually
rides gain while watching a PPM, subsequent processing stages are
presented a uniform level for much more consistent results.
Servo Stage
IC30A compares the integrated peak value of the program signal with a
DC level corresponding to “0dB” AGC gain. When the program level is
too low, the output of IC30A toggles positive. When the program level
is too high, IC30A toggles negative.
Integrator IC30B translates the instantaneous need for more or less
AGC gain into a very slow, constant-slope linear ramp. The output of
IC30B ramps down for a low program level, and up for a high one.
This DC is applied to the – input of AGC PWM comparator IC42B and
compared with the 152kHz PWM ramp from C84. The duty-cycle-
modulated squarewave from IC42B switches IC46C to set AGC gain.
The AGC ceaselessly chases the input program peak value in its
attempt to hold the level steady. Though the AGC never actually
reaches equilibrium in normal operator, circuit action is so slow that
this constant correction is not audible.
AGC Metering
The linear DC ramp from IC30B is also routed to IC43. These four op-
amp sections drive the five front-panel
AGC GAIN
indicator LEDs.
Each section is configured for gain and offset bias so that the five LEDs
fade smoothly, one to the next. This gives the indicator string more
display resolution than might be expected for only five LEDs, since
intermediate values of gain become proportional to the difference in
brightness between adjacent indicators.
Gating
AGC in the
DAVID-II
is “gated.” When the input program falls below
a predetermined level, the AGC returns to its normal 0dB “resting”
value, rather than increasing gain and bringing up background noise.
The gating threshold is fixed at about –20dB with respect to the
corrected average program level. Moreover, the gating threshold is
