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PPM3 Page 7
the far-end phone. So the solution is to use a high amount of compression in
order to amplify the far-end, weaker signal much more than the local stronger
signal. In this way both sides of the conversation are closer to each other in
signal strength when applied to the main mixer. A 100K ohm resistor was
used to set the compression level around 7:1. The same chart that was used
for the mic circuit can also be used with the phone circuit if a different com-
pression ratio is desired.
The clip detect circuit that powers the clip LED is basically constantly com-
paring the signals from both the left and right channels against a set threshold
that is close to the mixer amp’s upper voltage rail, which is where it clips.
There is a little amount of hysteresis to enable the LED to be lit up long
enough to visibly see it during short signal spikes. The core of the peak detect
circuit is D3, D7, R25, and C24. Basically, an averaging DC peak voltage
sample is created, held with a charge on C24, and discharged with R25. R20
and R17 control the charge time of C24.