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Re: SSTV Algorithms?



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. 

Hmm.  That's not quite true.  Consider a signal at 4kHz, being sampled 
at 8kHz.  What you'd see is a triangle wave, if you were sampling at the 
peaks of the incoming signal (if you were sampling at the crossover 
points, you'd see no signal at all!).  How do you tell what the waveform 
of the signal originally was?  You can't...

If you sample your 4kHz signal at 16kHz, you've got four points across 
each cycle, so at least you can start to get an approximation.  If the 
input signal was a sinewave you might see a sample at a crossing point, 
then a sample at a peak, then a sample at the next crossing point, then 
a sample at the next peak.  You'd get a roughly sinewave-y signal, if 
you squint a bit.

When I'm recording digital signals or SSTV I use 44.1kHz 16-bit mono 
.wav files.  Disk is cheap, and the ISS doesn't fly over that often...

> 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.

It's annoying how many really good pieces of software for amateur radio 
are closed-source Windows-only things.  I don't know about anyone else, 
but I think that rather goes against the spirit of amateur radio.  I'd 
rather play with something hacker-friendly that I can take apart and 
adapt than some big sealed-up box'o'tricks.  I wrote a bunch of music 
synthesis plugins and made them available free (free as in beer and Free 
as in speech).  Other people have sent me back little tweaks and 
improvements, that I've folded back into the code.  That way, *everyone* 
gets a better toy to play with, and we all get to learn something.

> If you are interested in sampling theory, check out 
> http://www.dspguide.com/ch3/2.htm

At some point I must finish my rewrite (for copyright reasons) of the 
Ensoniq Mirage sampling guide.  It's got a great explanation of it.

Gordon MM3YEQ
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