[Elecraft] Speech Processors

Mon Jan 27 13:30:01 2003

Julian, based upon my experience in designing, building and using
"speech processors" on a variety of rigs, I agree with you that the
advantage of outboard r-f processing with the K2 is  arguably small.
In my case, I leave well enough alone, although I have two
processors - the Comdel and the Vomax - that I could install in a few
minutes' time for an evaluation.

Frankly, the issue for me is whether or not the finals can stand the
increased duty ratio involved in producing 3 to 6 dB more average
power output. If they can, then the addition of an rf-based processor,
such as the Comdel design I described (also used by Drake and Ten Tec)
or the Vomax audio split-band design, could make a favorable
difference in the received signal strength and perceived
intelligibility and "loudness" of the audio.

The effect would be approximately as if the ten watts of the K2 peak
output were increased to 20 or 40 watts at the receiving end. One
"S-unit" makes little difference on the meter, of course, but the
enhanced articulation and intelligibility is key in allowing our weak
SSB signal to be read against a noisy background. Use of any speech
processing when the received signal is strong and easily understood is
merely an unnecessary abuse of the radio and the listener's ears.

The K2 "compressor" is a VCA  audio integrated circuit aimed at
maintaining a fairly constant and high level of modulation. The effect
is for the signal to sound louder at the other end, even though the
increase in average power is probably small. Such processors were
widely used, and probably still are, with AM and FM rigs to ensure a
more constant modulation level.

 But with SSB, we are dealing with a peak to average power ratio of
the unprocessed typical male voice of about 14 dB at baseband. This
accounts for the typical low average power output reading we see with
unprocessed speech - with a clean transmitter that is not being
overdriven - even though the peak power output is much higher.

A two-tone signal, for example, has a 2:1 power ratio of peak to
average power. (A one-tone signal - keydown CW for example - has a 1:1
ratio, of course.) That is why the two-tone is a convenient test
signal for evaluating power response of an SSB  transmitter. If the
transmitter is "clean" the average power reading of the output signal
will be one-half the peak power reading. With a complex voice signal,
we have to start talking about statistical measures and that is where
Bell Laboratories measured the 14 dB peak/average ratio of typical
male voices in connection with telephone system design.

 Audio compression does a little to improve this deficit and it *does*
make the audio at the receiving end sound louder since the audio level
can be modified on an almost syllabic basis if desired. (The K2 does
not appear to do this, however.) The trick, of course, is to modify
the average audio level without so compressing the signal that in-band
distortion products are generated. The K2 seems to find a happy medium
in this regard, although maximum compression frequently provokes the
response that less sounds better. A lot depends upon the operator's
voice and the mic being used, of course.

So, Julian, I agree with your conclusion:  the K2 already sounds
"punchy" but clean - meaning that Wayne and Eric did the audio
processing "right"  with the SSM 2165 chip - and there would appear to
be little merit in subjecting the finals to the higher duty ratio and
average power requirements of significantly processed speech using an
outboard rf-based processor. Such a processor *could* enhance the
output signal - perhaps considerably - but I seriously doubt that the
finals are up to the strain. I do not intend to find out with my K2!

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #3235

----- Original Message -----
From: "Julian (G4ILO)" <[email protected]>
To: "Elecraft List" <[email protected]>
Sent: Monday, January 27, 2003 3:46 AM
Subject: Re: [Elecraft] Speech Processors

> Does anyone think the K2 actually needs an RF speech processor? The
K2's
> built-in compressor seems to be extremely effective and seems to
allow the
> K2 to put out a much punchier signal than you'd expect from a 10W
radio. I
> can't see that you'd gain anything by using an outboard compressor.
>
> 73,
> --
> Julian, G4ILO. (RSGB, ARRL, G-QRP, K2 #392)
> G4ILO's Shack: http://www.qsl.net/g4ilo
>
>
>
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