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From: cmoore@amdimage.UUCP (chris moore)
Newsgroups: net.music.synth
Subject: Re: MIDI and RS232
Message-ID: <42@amdimage.UUCP>
Date: Thu, 14-Mar-85 14:52:56 EST
Article-I.D.: amdimage.42
Posted: Thu Mar 14 14:52:56 1985
Date-Received: Sun, 17-Mar-85 01:07:53 EST
References: <1854@pucc-h> <65@daisy.UUCP> <426@x.UUCP>
Organization: AMDIMAGE, Sunnyvale, CA
Lines: 27

> This doen't make any sense. The simplest scheme I know of for encoding
> bits into signal changes (NRZ encoding, used by most RS-232 devices)
> assigns one signal state to mean 1 and the other to mean 0, giving one
> signal change per bit so that bit rate == Baud rate. All other schemes
> I know of (FM, MFM, etc.) use more than one signal change to encode
> each bit so the bit rate for these encodings is always less that the
> Baud rate. I belive that it is theoreticaly impossible to encode more
> than one bit into one signal change.

The Bell 212A modems use DQPSK (Differential Quadrature Phase Shift
Keying).  By shifting the PHASE of the signal (0, 90, 180,or 270 
degrees) you can encode four values (2 bits) in each change.

The V.22bis standard for 2400 bps uses QAM (Quadrature Amplitude
Modulation) which uses four different AMPLITUDES in addition to
the four different PHASES.  This gives you 16 values or 4 bits per
signal change.

Both of these modem standards run at 600 baud.

-- 

"My system is so slow we don't use 'who' anymore - it's faster to
 walk around the building and count the users."

 Chris Moore (408) 749-4692
 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!amdimage!cmoore