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24
Switch ON
Switch OFF
Attenuation
100%
0%
0dB
50%
50%
6dB
25%
75%
12dB
10%
90%
20dB
1%
99%
40dB
Feedforward
Control
Both the wideband and the independent high frequency compression/
limiting circuits operate in a feed
forward
mode, rather than using the
more traditional feed
back
method. The amplitude of the
input
signal
establishes circuit gain to hold the output constant at a 100%-
modulation “ceiling” value.
Feedforward control necessitates a conversion factor to give one dB of
output
attenuation
for each dB that the input signal
increases
. Put
into linear (voltage), rather than log (dB) terms, this function is
expressed as X=1/Y. Figure 6 graphs this function. X represents the
circuit gain needed to hold the output at a constant 1 volt, with a 1-volt
to 10-volt input signal Y. This technique becomes a bit precarious at
small duty cycles (attenuations of 40dB or more), but in the 0 to 30dB
range typically required for audio processing, PWM is easily
implemented and quite predictable.
Figure 6
Voltage Transfer
Function
Assuming a linear relationship between PWM on/off time and signal
gain reduction, the 1/Y expression translates directly to the duty cycle;
that is, 0.1 is a 10% on time, and 1.0 a 100% on time.
“Soft Knee”
In actual implementation we require a “threshold” level, below which
gain will remain at full value.
Just
above threshold, the transfer
function will conform to what has become known as a “soft knee”
characteristic. This means that the program signal eases
gently
into
