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VE2HAR > MT63 12.03.05 23:31l 63 Lines 2604 Bytes #-7310 (0) @ WW
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Subj: [MT63] Re: Crest Factors
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VE2HAR
Sent: 050312/2212z @:VE2HAR.#MTL.QC.CAN.NOAM Laval #:41844 $:22548sentto
Greetings friends,
This term "Crest Factor" is a new one on me. I admit to doing my
Postgraduate School work way back shortly after WW2. I see Mark Miller has
clarified the term to be peak to RMS. That appears to be akin what we SSB
pioneers called the "Peak to Average" power ratio. We almost lost the SSB
campaign to Double Sideband Suppressed Carrier (DSB) because the Human Voice
has an extremely poor Peak to average Power ratio. One of the chief
proponents of DSB was a fellow whose last name was Kahn. Back in the AM
days one could greatly improve the Peak to Average ratio of the human voice
by using audio clipping to improve the ratio greatly. DSB could also employ
the audio clipping but the nature of SSB put the peaks back in. Kahn was
able to show that DSB was 3db better than SSB by using the audio clipping.
That is when we invented the "Voice Processor". We simply generated the SSB
signal first and then clipped and filtered. Worked great. When the going
got tough one just turned on the "Voice Processor". We found that the
optimum adjustment of the "Processor" was at about 6db peak to average
improvement. That was almost like turning on a linear. Nowadays 10db by a
linear is more or less standard. There was a time linears produced more
than 10db but grounded grid linears became the norm. My point is that, if
we are talking about the same sort of thing, then higher "Crest Factors"
indicate poorer "on the air" performance. The best producer would be a
"Crest Factor" of 0db. That would be a steady carrier. Practically no
information is derived from a steady carrier. I say practically no
information meaning nothing beyond knowing the presence of the carrier.and
the Doppler produced by reflections of the steady carrier.
Aloha -- Butch -- W6KAG -- Coos Bay Oregon
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