[amsat-bb] Re: SSTV Algorithms?
John B. Stephensen
kd6ozh at comcast.net
Fri Oct 17 02:18:27 PDT 2008
The filtering ensures that the shape is known. A triangle wave above half
the cutoff frequency becomes a sine wave as filtering removes the harmonics.
> 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.
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