— —
26
The negative-going pulse from Q9 triggers a similar event with Q6 and
Q7. On the positive transition of the pulse, Q6 is turned off, allowing
Q7 to turn on. C49 and R99 establish the off time for Q6, hence the on
time for Q7.
Compressor/
Limiter Ramp
When Q7 turns on, C52 is charged to very nearly the +9-volt power
supply rail through CR18. As soon as Q7 turns back off, C52 begins to
discharge through three distinct paths.
The cathodes of CR21 and CR22 are held at two different DC levels by
the attendant divider strings. This means that C52 initially discharges
through R134, R135, and the series connection of R113 and R114.
When the charge falls to about +6 volts, CR21 is biased out of the
circuit, removing R135 from the discharge path. As the charge
appr1 volt, CR22 removes R134, leaving the R113 and R114 to
discharge C52 the rest of the way. The effect is a quick initial
discharge of C52, slowing as the DC level drops. This yields the
modified X=1/Y waveform slope shown in Figure 7.
“Pedestal”
Because the LM319 fast comparators cannot deal with an input voltage
range which approaches the supply rails, the ramp is taken from the
R113/R114 divider. A fixed amount of the negative-going 200ns pulse
is coupled through R115 to this same point, creating the “pedestal”
mentioned earlier. Q11 is an emitter-follower buffer for the
“composite” ramp waveform, applying the ramp to the + inputs of
comparators IC42A and IC29A. These generate the PWM squarewave
switching waveform for the broadband and the independent high
frequency limiters, respectively.
AGC Ramp
The AGC function of the
DAVID-II
is a very slow “gain servo”
operating in a feed
back
rather than a feed
forward
mode. The AGC
requires a PWM ramp of a different shape; not for a proper gain
transfer characteristic, but to maintain a constant
rate
of AGC
correction.
Each time Q7 turns on, C84 is charged through CR40. Unlike the
complex discharge path of C52, C84 has a simple RC discharge through
R204 and R234. Since the AGC operates in a feedback mode, the shape
of this ramp is far less critical than the one for the wideband and high
frequency feedforward limiters. The AGC ramp is applied directly to
the + input of the third comparator, IC42B. Though Q7 initiates the
two ramp waveforms, CR18 and CR40 ensure that each can assume its
characteristic discharge shape.
Processor
Bypass
S4, the front-panel
AUDIO PROCESSING
switch, allows the user to
completely bypass the
DAVID-II
audio processing system. When S4 is
switched
OUT
, binary divider IC3B is inhibited. Lack of drive holds Q7
off which stops the AGC and limiting PWM ramps. This drives the
outputs of all three comparators low, opening the signal path at each
PWM attenuator. S4 also switches IC40A and IC40D to route the
program input signal around the processing circuitry.
