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From: wb8foz@mthvax.cs.miami.edu (David Lesher)
Newsgroups: comp.dcom.modems
Subject: Re: Recording of Modem output
Message-ID: <1990Oct6.141909.14024@mthvax.cs.miami.edu>
Date: 6 Oct 90 14:19:09 GMT
References: <1990Oct6.114313.9227@wubios.wustl.edu>
Reply-To: wb8foz@mthvax.cs.miami.edu (David Lesher)
Distribution: na
Organization: NRK Clinic for habitual NetNews Abusers
Lines: 37

In <1990Oct6.114313.9227@wubios.wustl.edu> phil@wubios.wustl.edu (J. Philip Miller) writes:


>I would like to know if it is possible to record the output of a modem and
>later play it back into the modem and recover the original digital signal?
>If so what black boxes (if any) do I need.

This will work with async modems, but not those that run in sync. Note
that I am NOT talking about the *ports* being {a}sync, but the way the
modems talk.

300 bps FSK modems do use this method, 1200 and up do not. In general
the reason is that to squeeze faster rates out of a finite phone line,
the faster modems use not just frequency changes, but phase changes,
too. While it is reasonably easy to build a modem that can tell the
difference between, say 1800 hz and 1900 hz, based on a factory
calibration, the same does not apply to phase differences. You have to
let the modems talk and agree to a baseline phase to differ from.
That's part of what all the noise at the beginning of a call is -- the
rest is the modems gossiping about their owners ;_]

One exception to the delta phase scheme is PEP, the Trailblazer's
multi-carrier method. But since a blazer has to regularly converse with
another to decide which frequencies to use, and which to avoid, that
won't work either.

You will not need any DC phone line power or such. You will need some
scheme to keep the originating modem thinking it is talking to an
answer modem, or it will get lonely and hang up. Maybe some modem chips
are strappable to talk blindly, but I don't recall.

Good Luck!
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