# [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.

73

John
KD6OZH

>
> 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.

```