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25
characteristic. This means that the program signal eases
gently
into
compression and limiting, avoiding the abrupt change from a linear to
a limited state which characterizes traditional feedback topology. The
“soft knee” is normal behavior for feedforward gain control, and is
credited for the smoother, less fatiguing sound attributed to this
method.
The PWM
“Ramp”
Waveform
Figure 7 shows a “ramp” waveform generated by the
DAVID-II
to
effect the desired feedforward transfer characteristic. Note the
similarity between this waveform and Figure 6. Allowance has been
made for the “soft knee” compression and limiting threshold, and a
negative-going “pedestal” has been included to keep the CMOS analog
switches active, even when no gain reduction is in effect.
Figure 7
Ramp Waveform
This ramp runs continuously at a 152kHz rate. It is applied to one
input of a comparator, and a DC voltage derived from the input signal
is fed to the other comparator input. The comparator output is the
duty-cycle-modulated squarewave which “chops” the program audio
signal.
PWM RAMP CIRCUITRY
Master Clock
Referring to the second sheet of the Schematic on Page 40, IC8C is the
DAVID-II
master “clock” running at 1.216Mhz, 64 times the stereo
pilot frequency. Binary dividers IC7A, IC3A and IC3B derive the
152kHz PWM switching frequency. This is buffered by IC8A and
IC8B, and is capacitor-coupled to the base of Q10. Q10, held in
saturation by R103, is momentarily turned off with each negative tran-
sition of the switching waveform. When Q10 turns off, Q9 turns on for
the same brief period. With Q8 serving as an “active pull-up resistor,”
Q9 delivers a stiff negative-going 200ns pulse at the 152kHz rate.
