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[TenTec] Digital Speech Processing & Pegasus

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Subject: [TenTec] Digital Speech Processing & Pegasus
From: geraldj@isunet.net (Dr. Gerald N. Johnson, electrical engineer)
Date: Tue, 19 Feb 2002 09:31:41 -0600
Clipped audio converted to SSB RF has enormous amplitude spikes. That
makes audio clipping and audio compression work poorly for enhancing SSB
transmitter output. The whole reason for any compression is to allow
driving the PA to a higher average power without driving it into an
excessively nonlinear region (which produces excess intermodulation
products known as splatter). Since audio processing tends to introduce
the peaks the PA needs to not see, RF processing is necessary. And then
it has to be careful (such as by post envelope shaping bandpass
filtering) to not let the IMD products from the envelope shaping be
heard as splatter.

I've found that while the ear is insensitive to the relative phase of
harmonics, the SSB transmitter is very much so, complicated by the
relative phase response to harmonics of the microphone, the audio
circuits and especially the SSB transmit filter. I have found that
selecting the microphone can enhance the average power from the
transmitter (as limited by the peak detecting ALC). I suspect that the
optimum microphone will vary with the voice and especially the relative
phase response of the transmit SSB filter. I believe it is important to
use a microphone with limited response, similar to that of the SSB
transmit filter so that audio stages and balanced modulator are never
overdriven by audio components that can't pass through the SSB transmit
filter. Hence I dislike the D-104 for its peak response is outside the
SSB transmit filter. I get along well with the EV664 (its wideband
response is decently flat) and for HF SSB I use a Shure R-5
communications microphone element mounted in a D-104 case. I have a
battle ship Turner hand microphone that also has proven to work well
with my VHF Yaesu FT-726.

73, Jerry, K0CQ
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.

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