[amsat-bb] Re: SSTV Algorithms?
Curt Nixon
cptcurt at flash.net
Fri Oct 17 05:52:26 PDT 2008
Hi Joe:
I beg to differ with you on the sampling information. Having been a
digital signal acquisition and instrumentation specialist for a number
of years, I'm quite familiar with the nyquist criteria and all.
Sampling at 2f, you know the frequency and something about amplitude of
the signal and thats about all. That stuff about knowing ALL about it
is just not true--"if" you knew ahead of time, that the signal in
question was nothing but sinusoidal, then yes, you are correct--but how
often is that the case. That is what has driven highly oversampled AD
conversion. If you meant twice the maximum frequency COMPONENT of the
signal, then thats close, but is still based upon sin theory (fourier
series) and leaves alot of detail out of non-sinusoidal parts of the
signal unless one considers all of the harmonic content of the source.
For investigative or diagnostic work, especially characterizing
waveshape leading edges of clocks, etc, we used to recommend sample
rates of 5-8 times the highest frequency component of interest.
I wasn't so interested in the actual SSTV algorithms as I was in seeing
if there is any visible difference in the result of different algorithm
use--or is everyone using the same decoding kernel?
Thank you Sir, I suspect you know all this anyway--thought I would
clarify it as I came to understand it.
Curt
KU8L
Joe Fitzgerald wrote:
> Curt Nixon wrote:
>> I expect that the faster the soundcard samples, the better?
>>
>>
> It's counterintuitive, but through the magic of sampling theory, once
> you sample at twice the maximum frequency of the signal you are
> interested in you know _everything_ about it! As a practical matter,
> most ham gear audio rolls off sharply above 3 kHz (and SSTV is lower
> than that) , so sampling at a measly 8 or 11.025kHz rate is plenty.
> Howard Long wrote:
>> Do you know of detailed texts that are public domain? If
>> so let's have 'em!
>>
>>
>
> And it's not public domain, but it is available as free software:
> check out QSSTV for source code.
>
> If you are interested in sampling theory, check out
> http://www.dspguide.com/ch3/2.htm
>
> -Joe KM1P
>
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