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From: makela@tukki.jyu.fi (Otto J. Makela)
Newsgroups: sci.electronics
Subject: Re: MODEM SPEED INCREASE
Summary: There IS 1200 b/s FSK...
Keywords: 300 to 1200+
Message-ID: <1166@tukki.jyu.fi>
Date: 18 Aug 89 23:08:14 GMT
References: <1436@esunix.UUCP> <3194@scolex.sco.COM>
Reply-To: makela@tukki.jyu.fi (Otto J. Makela)
Organization: Grand Hall of Justice, Mega-City One
Lines: 45

In article <3194@scolex.sco.COM>, deanr@sco.COM (Dean Reece) says:
>(bps = bits per second & ber = bit error rate)

>bps:   Encoding:   Baud:   Bits/Baud:   Orig Carrier:   Ans Carrier:
>0-600  FSK (FM)    0-600   1            1050/1250? hz   2025/2225? hz
>1200   PSK         600     2            1200 hz         2400 hz
>2400   PSK         600     4            1200 hz         2400 hz

>The most important difference between 300 bps modems and all others is that
>the slow ones use FSK (frequency shift keying).   This means that the modem
>shifts frequences depending on the bit you are sending (I think 0=1050 hz &
>1=1250 hz, but its been a while).  This allows TOTALLY ASYNCHRONUS data to
>be sent, meaning ANY bps rate can be used up to the limit of the channel.

Well, there exists a thing called CCITT V.23, which is 1200/75 b/s split-
speed FSK.  This is basically the same thing as the 300 b/s FSK's, but with
different frequencies, and the 75 b/s "back channel" really squeezed up so
it doesn't get in the way of the 1200 b/s channel; sorry, it's such a long
time that I don't remember the frequencies.  This actually made a lot of
sense in the bad old days when modems were built from discretes and lotsa
gates, as very few users constantly maintain a typing speed >7.5 chars
per second (@ start bit, 7 bits data, even parity, one stop bit).
This was used a lot in the UK, on the British Telecom Prestel(tm) system
(and on many clones all around the world).  There was also a method for
signalling channel turn-around, making bidirectional data transfers possible.

>PSK encoding (phase shift keying) encodes the data by keeping the frequency
>constant and shifting the phase forward or backward by so many degrees for
>each bit PAIR (or more) being transmitted.  (a bit pair is called a di-bit)

I've also seen the CCITT V.22bis (that's 2400 b/s for all americanos out
there) called "Quadrature Amplitude Modulation", is this just techno-talk
hype for PSK with the 4-bit codings ?

>You really can't convert the modem itself without totally replacing the
>encoding/decoding & filtering circuits (and that is most of what a modem
>is).

Agreed.  Converting the 300 b/s things to anything else would most probably
be more trouble than it was worth.  Might be educational, though...
-- 
* Otto J. Makela (makela@jyu.fi, MAKELA_OTTO_@FINJYU.BITNET)               *
* Phone: +358 41 613 847, BBS: +358 41 211 562 (CCITT, Bell 2400/1200/300) *
* Mail: Kauppakatu 1 B 18, SF-40100 Jyvaskyla, Finland, EUROPE             *
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