Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: nyu notesfiles V1.1 4/1/84; site rocksvax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!mit-eddie!godot!harvard!seismo!rochester!rocksanne!rocksvax!dave From: dave@rocksvax.UUCP Newsgroups: net.music.synth Subject: Re: MIDI and RS232 Message-ID: <26500003@rocksvax.UUCP> Date: Thu, 14-Mar-85 18:42:00 EST Article-I.D.: rocksvax.26500003 Posted: Thu Mar 14 18:42:00 1985 Date-Received: Sun, 17-Mar-85 01:41:56 EST References: <426@x.UUCP> Lines: 33 Nf-ID: #R:x:-42600:rocksvax:26500003:000:1738 Nf-From: rocksvax!dave Mar 14 18:42:00 1985 /***** rocksvax:net.music.synth / x!wjr / 1:34 am Mar 14, 1985*/ > (called a dibit) into each signal change (Baud). 9600 Baud modems are The dibit thing is the key. 1200 baud modems work by using a PSK (Phase shift Keying) modulator. This modulator takes 2 serial NRZ bits and assigns a phase shift based on the 4 combinations you get of the 2 bits. The tone is then shifted some number of degrees, say 360/4 degrees. When the receiver gets this signal it looks at the phase shift it saw and spits out 2 bits based on the shift it saw. This is slightly more simplified than what really goes on but fundamentally what happens. Each baud is one of these phase shift times, each phase shift represents 2 bits. The higher bit rate modems still use the fundemental 600 baud signalling rate, only they divide the shift up into smaller units, say 360/16 or 360/32 to encode 4 or 5 bits respectively for each baud. The fundemental thing to remember is that each baud can be encoded to represent multiple bits and that bauds != bits. Because the receiver has to recreate the senders clock which is running at 0 degrees phase shift, the 1200 baud standard scrambles the bits to assure that you won't have long stretches of data where the phase shift stays at one position. This randomization allows a PLL (Phase locked loop) to average out the transitions and recreate the sender's clock. Also on the 212 modems, a circuit buffers the asynchronous data and synchronizes them to the modems synchronous data and the reverse process occurs at the receiving end. Hopefully that will answer some questions and rest this bits and bauds discussion!! Dave arpa: Sewhuk.HENR@Xerox.ARPA uucp: {allegra,rochester,amd,sunybcs}!rocksvax!dave