I have designed a 1200bps modem using the code from the link below 
and it works very well. Running on an ATMEGA8 at 6.144MHz. Used fixed 
point 16bit math.

	http://www.baycom.org/~tom/ham/da95/d_dspmod.pdf

	Mark Jordan - PY3SS


On 15 Nov 2007 at 21:43, Hector Martin wrote:

> I'm looking for information on how to implement a Bell 202 FSK decoder
> in software (DSP).
> 
> I'm creating a control module that will communicate with a base station
> via FM radio with audio-FSK modulation in the Bell 202 standard (this
> seems to be really popular with hams, especially for APRS, etc). I
> figured that, since we're using either a PIC24 or a dsPIC for the cpu,
> we might as well do the modem in software. I'm a DSP newbie though.
> 
> 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.
> 
> Any pointers on the subject? How complicated is this? Can I expect
> performance comparable to or superior to that of a standard hardware
> decoder chip?
> 
> On the same subject, does anyone know of a source for Bell 202 modem
> chips, preferably in SMT package, at a reasonable price in single
> quantities?
> 
> -- 
> Hector Martin (hector@marcansoft.com)
> Public Key: http://www.marcansoft.com/marcan.asc
> 
> -- 
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