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From: paul@csnz.co.nz (Paul Gillingwater)
Newsgroups: comp.dcom.modems
Subject: Re: Modems for Minitel ( 1200/75bps )
Message-ID: <48@csnz.co.nz>
Date: 14 Jun 89 21:32:08 GMT
References: <762@sbsvax.UUCP> <874@maths.tcd.ie>
Reply-To: paul@csnz.co.nz (Paul Gillingwater)
Organization: Computer Sciences of NZ Limited
Lines: 34

In article <874@maths.tcd.ie> ch@maths.tcd.ie (Charles Bryant) writes:
+In article <762@sbsvax.UUCP> emma@sbsvax.UUCP (Martin Emmerich) writes:
+}In article <692@flatline.UUCP>, erict@flatline.UUCP (J. Eric Townsend) writes:
+}> I'm looking for leads/pointers/sources of modems that will work
+}> with the French Minitel (U.S. Videotel in the United States).  These
+}> modems use a wierd 75bps one way/1200bps the other way protocol.
+}
+}Rumors say it would be possible to use a normal V.22 modem for split-speed. You
+}only had to lengthen every bit into 16 bits ( = 1200 / 75 ) by software. But I
+}never tried it. It is said that someone had done it sucessfully.
+
+It is definitely not possible. The 1200/75 standard is V.23 (which is quite
+popular in Europe). This uses a Frequency Shift Keyed modulation scheme. The
+V.22 standard uses Phase shift modulation (same as Bell 212A).

V.23 is a CCITT standard that was developed by British Telecom as a low-cost
solution for videotext applications.  It uses FSK as opposed to DPSK
as correctly pointed out by Charles.  V23 also allows for a 600 bps fall-back
on the receive channel.  The problem with most split-speed modems is that
a normal PC does not have a split-speed serial interface.  However, a
clever programmer here in New Zealand has written software that SIMULATES
the 75 baud on the DTE by playing with DTR.  And it works!  However, if
you don't want to dial into a NZ BBS to obtain that, then you'll need to
obtain a modem which does speed conversion on the TX channel.
Note that V23 is a half-duplex protocol - that means that it can be turned
around to give 1200 bps TX and 75 bps RX, e.g. for uploading to a BBS.

I still get people who try to upload at 75 bps though...  :-(


-- 
Paul Gillingwater, Computer Sciences of New Zealand Limited
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