Path: utzoo!attcan!uunet!sco!deanr
From: deanr@sco.COM (Dean Reece)
Newsgroups: sci.electronics
Subject: Re: MODEM SPEED INCREASE
Keywords: 300 to 1200+
Message-ID: <3194@scolex.sco.COM>
Date: 17 Aug 89 19:16:14 GMT
References: <1436@esunix.UUCP>
Reply-To: deanr@sco.COM (Dean Reece)
Organization: The Santa Cruz Operation, Inc.
Lines: 48

In article <1436@esunix.UUCP> hcooper@esunix.UUCP (Harrison Cooper) writes:
>I have a couple of 300 baud modems, one is a Radio Shack external, and
>the other is an old acoustic coupler type (I don't have either one in
>front of me right now, so no part numbers available).
>
>The question is can they be upgraded to 1200 or more?  I have posted
>this before I have opened either one up, so I don't know what devices
>they are using.  I don't have a schematic for the acoustic type (paid
>$3.00 for it), so the RS unit is probably a better unit to play with.
>I don't care if it can be programmed by serial, just hardwired for the
>baud rate.  E-mail or post.

No, can't be done.  You can PROBABLY use the RS modem (a DC1?) up to
600 bps as is, but you need something on the other end expecting it.
I doubt the acustic couple modem will run above 300 bps without the ber
skyrocketing.  This may also happen with the direct connect modem;  I've
never tried it, but the standard allows for it: (I think)

(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.

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)

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).

A technical note: Calling a 2400 bps modem a 2400 baud modem is incorrect.
They are actually 600 baud (each baud contains 4 bits).  For conversation,
either is acceptable, but its worth knowing anyway. (the same goes for a
1200 bps modem as well)

Hope it helps.
--
Dean Reece  (deanr@sco.COM)                     (408)458-1422
The Santa Cruz Operation,  P.O.Box 1900, Santa Cruz, CA 95061