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