Donald Chester k4kyv at hotmail.com
Fri Jun 18 13:39:07 EDT 2004

>Any one out there got comments on the Comdel speech processor

They work on AM as well as SSB.  I built a homebrew version back it the late 
70's using a set of 64 kHz mechanical filters removed from telephone 
multiplexing equipment.  Mine had better audio quality than the original 
Comdel and was quite effective, but it was nothing to write home about.  The 
design eliminates the harmonic distortion of speech clipping, but does 
nothing to reduce intermodulation distortion.  Intermod is much more 
objectionable than harmonic distortion.

The purpose of the Comdel was to allow for speech clipping on SSB, since due 
to fundamental modulation theory, simply clipping the audio to flat peaks 
before applying it to a SSB generator generates high amplitude rf peaks, 
just exactly the opposite of what you are trying to accomplish with 
clipping.  The extreme case of applying squarewave audio to a SSB generator 
produces peaks of theoritically INFINITE amplitude.

The processor works by generating a low-frequency SSB signal (below 100 
kHz), clipping and filtering the SSB signal, and demodulating the clipped 
SSB and applying the resulting audio to the transmitter.  The result is a 
near 1:1 peak-to-average ratio with heavy clipping, but the intermod makes 
the audio sound mushy.


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