Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!mailrus!ncar!noao!arizona!naucse!rwi From: rwi@naucse.UUCP (Robert Wier) Newsgroups: comp.dcom.modems Subject: Re: Autobaud Matching Summary: autobaud matching Message-ID: <939@naucse.UUCP> Date: 29 Sep 88 03:46:10 GMT References: <56.015163@adam.DG.COM> <9262@cup.portal.com> <1435@netmbx.UUCP> Organization: Northern Arizona University, Flagstaff, AZ Lines: 34 I once wrote a program for a 6800 a number of years ago (running on a Southwest Tech - anybody remember them?) which was to be used in an amateur radio situation. Radioteletype (or RTTY) can come in over the airwaves from anywhere in the world and you are likely to run into some pretty strange formats (although there are some recognized "standards"). Since I was particulary interested in reading the new service wires (Brazilian soccer scores, etc.), my micro would sit there and sample the output from the TU (terminal unit ... basically an decode from off the air modem). Since the micro could sample tens of times at the baud rates involved, I implemented an autobaud function by sampling some of the bits going by, and then taking what appeared to be the shortest mark time that appeared consistantly (to get rid of noist spikes, etc). From this I could then calculate what the baud rate should be. This generally worked pretty well, except on exceptionally hight noise signals (QRM). It was less easy to automatically decide if I was listening to Baudot or ASCII signals, although I had some success by sampling in the 4th, 5th, and 6th bit positions. I could determine that by waiting for an extended mark condition, and then taking the first bit that came through as the start bit, having already determined the baud rate. If the 6th bit (or was it 7th?) bit was consistantly a mark (or was it space ? - things are a bit fuzzy now) then it had to be baudot since I was seeing the stop bit. Otherwise it was probably ASCII. Once I got it going, I saw all sorts of strange conbinations, like 450 baud with mark-space inverted. Course if they were encrypted or multiplexing several data streams onto the same channel, it blew me out of the water. Interesting project, though. -- Bob Wier (WB5KXH) - Flagstaff, Az. ...!arizona!naucse!rwi