[amsat-bb] Re: SSTV Algorithms?
Gordon JC Pearce MM3YEQ
gordonjcp at gjcp.net
Fri Oct 17 01:15:58 PDT 2008
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
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.
More information about the AMSAT-BB