Instruction manual

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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 feedforward mode, rather than using the
more traditional feedback 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