http://www.sxlist.com/techref/ubicom/lib/io/dev/modem/index.htm Keeping in mind that the SX is a BIT faster than a PIC, but uses (almost) the same instruction set. -- James. -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Hector Martin Sent: Thursday, November 15, 2007 18:43 To: Microcontroller discussion list - Public. Subject: [PIC]: Implementing FSK in a dsPIC 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 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist