Jim,
I remember attending a lecture by Dr. Lee of Pac Bell, who was testing the
new CDMA modulation being used at US West in Denver. The new scheme was
very bandwidth efficient and had great intelligibility but less than stellar
quality. He said with some of the military type modulation schemes, where
syllables were encoded and then re-constructed at the other end with lookup
tables, you could be speaking with your own mother and understand every word
she said, but you would not recognize her voice!
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of jimlux
Sent: Thursday, April 30, 2020 7:41 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 1dB more RF Power?
On 4/30/20 6:18 PM, David Gilbert wrote:
>
> I confess to having generated those files, although it was several
> years ago that I put them on my web site. They just get referenced
> here or there about once a year when a similar topic surfaces.
>
> http://www.ab7e.com/weak_signal/mdd.html
>
> One dB was about all I could claim was distiguishable, but a
> difference two dB was surprisingly so.
>
> 73,
> Dave AB7E
>
this is known as the "just noticeable difference" thing and ties into the
Weber-Fechner law which is really more that perceived differences are log
scaled (that is, a 3 dB change from 3 to 6 dB is perceived as the same jump
as 10-13 dB)
for speech it's around 2-3 dB in SNR
(https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4335553/ was the first hit in
general, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871229/ is another -
apparently the hearing aid business really cares about this)
However, for detecting tones in noise, or constrained speech, the JND is
smaller. And, JND isn't necessarily related to intelligibility or
detectability.
I used to work with tactical comm radios just as digital coding was coming
into play and there were tons of arguments about various schemes of various
computational complexity: is CVSD better or worse than mu-law with
companding at data rate X? What about LPC-10?
It is *highly* situation dependent. What's ok in a tactical situation where
you just want to call in artillery fire to the correct grid square is
different than whispering sweet nothings to your sweetie.
This kind of analysis, of course, is why modern cellphone audio quality is
so bad. Back in the 80s, if someone had said that random consumers would be
happy with 8kbps encoding of speech they would have called them crazy. What
we used as the "standard of comparison" was so-called "toll quality" which
is about 56 kbps, sampled at 8kHz.
This is sort of interesting and describes the "signal detection"
problem, which is related to the "CW detection" problem
https://www.weizmann.ac.il/neurobiology/labs/ulanovsky/sites/neurobiology.la
bs.ulanovsky/files/uploads/behavneurosci_lecture13_dovsagi_psychophysics_05j
uly2018.pdf
And of course, the real problem in CW detection is not just a random tone in
noise, it's a "structured tone sequence" in the presence of other
"structured tone sequences".
From a psychophysics standpoint it's an interesting problem
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