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From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: comp.dcom.modems
Subject: Re: Why 300 baud?
Message-ID: <3942@darkstar.ucsc.edu>
Date: 31 May 90 05:33:31 GMT
References: <9686@discus.technion.ac.il>
Sender: usenet@darkstar.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU.UUCP (Jim Haynes)
Organization: University of California, Santa Cruz  CATS
Lines: 37

In article <9686@discus.technion.ac.il> joel%techunix.bitnet@jade.berkeley.edu (Yossi (Joel) Hoffman) writes:
>I was wondering:  why is it that all the common modem speeds are
>multiples of 300 baud?  Is there anything special about that number?
>
I'll post a reply here because it might be of more general interest.
The formula is actually 75 baud time 2^n (so 75 and 150 are also
standard speeds).  This was standardized by somebody, probably the
military, circa 1960.

The origin of 75 is that the standard communication machine of the period
was Baudot Teletype at 100 words per minute, using a 7.42-unit start-stop
code (which today we call asynchronous).  I don't know how 7.42 was arrived
at; but it goes way back in history to speeds slower than 100 wpm.
At 100 wpm, which is 10 char/sec, the Teletype sends at 74.2 baud.
The military, more than anybody else at the time, was interested in
synchronous transmission, apparently because cryptographic equipment
works that way.  So they rounded up to 75 baud from 74.2; and from
there it's reasonable to say that every standard speed should be
double the next lower speed.  Not much point in having a speed increase
if you don't double it.

Now we get 110 baud, the widely-used speed that doesn't fit the
formula, because if you keep the Teletype at 10 char/sec and transmit
ASCII in an 11-unit start-stop code that's what you get.  Eleven
rather than ten because the mechanical equipment needed more than
one bit time to get stopped at 10 char/sec.  There was also a speed
of 1050 baud in use because Teletype made a paper tape punch that
was designed for a top speed of 105 char/sec.  The short-lived
Teletype Model 37 operated at 150 baud, 150 wpm; it was the last
gasp of mechanical ingenuity up against electronics rapidly falling
in price.
haynes@ucscc.ucsc.edu
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