Karl Larsen wrote:
Hi Geoff, cut your weeds and we can both think hard on what "Speech
Compression" really is. I got to thinking and about all it can be is
clipping of the high voice peaks, and increasing audio drive. I will
look at the ARRL Hand Book and see what they have.
In the AM days, many hams incorporated simple diode audio clippers in
their modulators to clip the negative-going peaks of the audio signal,
followed by lowpass filters to clean up the high-frequency distortion
products produced by clipping. This permitted a higher level of
modulation in the positive direction without splatter, thus increasing
the average power in the signal. Some amount of compression was often
used as well in order to increase the average modulation percentage.
Processed AM signals sounded much louder without significant widening
if the processing was done properly.
When SSB came along, it was found that audio peak clipping did not have
the desired result, because the height of the peaks in the RF envelope
did not correspond directly to the audio envelope. So RF clipping came
into use in which the clipping and filtering takes place after the
generation of the SSB (or at least DSB) signal. AS Geoff points out, if
you want to do this in the audio chain, you need to generate an RF
signal, clip and filter it, and then convert it back to audio.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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