The frequency estimator works by taking an FFT of the sample buffer,
averaging it with the FFT of the last few sample buffers, and finding the
four (or two for 2FSK) frequency peaks. It's pretty simple, but allows us
to track a good deal of frequency drift and possible weird spacing from
different modulators. If there isn't enough of one of the symbols, it will
throw the estimator off. If you make a few assumptions about the frequency
spacing and drift, it wouldn't be too difficult to re-work the estimator to
handle a situation with less of each of the four tones. We're not too
worried about it for 2400A, as the UW and frame padding provides enough
variety to not knock the freq. estimator out of whack.

If you want to mess around with it yourself, the frequency estimation
happens in fsk_demod_freq_est

On Wed, Mar 23, 2016 at 5:35 PM, Steve <coupaydevi...@gmail.com> wrote:

> I've been playing with the 4FSK modulator. Every once in awhile I would
> get some bad decodes when I throw random data bits at it.
>
> For example here's the frequency peaks decoded from a random 96 bit
> pattern:
>
> 1218.75
> 2671.875
> 4546.875
> 5718.75
>
> Input:
>
> 100010001100010011100100001100001011000101101101111111110111001000100100111100000010000100001100
>
> Decoded:
>
> 000100010010000100101001000010000001100001010110011010101001100001000101001010000000010001000010
>
> The 4FSK frequency peaks didn't do too well, so I plotted the output using
> audacity:
>
>
> ​
> I don't know where it got that 5700 Hz number from, ha. But you can see
> that this random 96-bit sequence didn't really produce a 4FSK spectrum.
>
> If I send it "01" or "10", all "1" or all "0" it can very well be a CW
> signal.
>
> Anyway, I was thinking of a way to make sure that each of the 4
> frequencies were hit about 25% of the time. Maybe I'm just playing too
> hard...
>
>
>
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