Hector Martin wrote:
> I'm looking for information on how to implement a Bell 202 FSK decoder
> in software (DSP).
[...]
> Bell 202 is an audio FSK modulation at 2200Hz for mark and 1200Hz for
> space. A standard UART stream rides on top of that, at 1200baud.
> I figure encoding is pretty easy (create tables of the sines at the
> right frequencies, and optionally tables for the inverses if you want to
> do continuous-phase FSK). However, I'm pretty clueless about the
> decoding. I've heard about all sorts of different schemes, from counting
> zero crossings (yuck) to fourier transforms, but I haven't been able to
> find a clear explanation that doesn't assume I have DSP knowledge. I
> also need to figure out what the best way for syncing with the UART
> stream is. This is a long-range link (cubesat satellite), so I don't
> expect a perfect radio link.

Hi Hector, I've been using a schematic/firmware for 18F2550 for CallerID 
on tlephone lines.
I found it somewhere on a site (maybe microchipc.com or alike, don't 
remember) and adapted it to 16F628 assembler.
It works good enough: it is based on zero-crossings and interrupts/timer.

I can post the link or the code in case you're interested.

-- 
Ciao, Dario
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