phil@wubios.wustl.edu (J. Philip Miller) (10/06/90)
[I posted this earlier, but don't think it left here -jpm]
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.
Here is what I want to do:
I am recording a number of chanels of analog data and I would like to add a
chanel of annotation about events occurring. My idea was to connect a
computer to a modem and record its output. Later, when playing the tape back
and doing A/D conversion I would listed on the com port of the computer to the
modem (now getting the output from the original recording) and recover the
information.
My recollection of the technology would suggest that it would be easiest to do
with the FSK of 300 baud modems (which would be plenty fast enough for my
application) technology.
Has anyone done this? If so what did you have to do to connect the modem to a
recorder? Since a phone line normally has a voltage, does one have to be
supplied?
thanks for any help with this.
-phil
--
J. Philip Miller, Professor, Division of Biostatistics, Box 8067
Washington University Medical School, St. Louis MO 63110
phil@wubios.WUstl.edu - Internet (314) 362-3617
uunet!wuarchive!wubios!phil - UUCP (314)362-2693(FAX) C90562JM@WUVMD - bitnetwb8foz@mthvax.cs.miami.edu (David Lesher) (10/06/90)
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! -- A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu & no one will talk to a host that's close............(305) 255-RTFM Unless the host (that isn't close)......................pob 570-335 is busy, hung or dead....................................33257-0335
bill@bilver.UUCP (Bill Vermillion) (10/07/90)
In article <1990Oct6.114313.9227@wubios.wustl.edu> phil@wubios.wustl.edu (J. Philip Miller) writes: >[I posted this earlier, but don't think it left here -jpm] > >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. > That used to be quite common, and one of the modems I had (many years ago) had jacks to be able to connect to a cassette machine. The idea was that it was much cheaper to put a big program on a cassette and mail it, particularly when going between different computers, eg CPM to APPLE, or similar applications. You should be able to do the same with any device that will let you connect to the phone line, as the modem signals are just audio at that point. You are right in wanting to do it at 300bps however, as the FSK is much easier to record and playback reliably than the PSK. >My recollection of the technology would suggest that it would be easiest to do >with the FSK of 300 baud modems (which would be plenty fast enough for my >application) technology. > >Has anyone done this? If so what did you have to do to connect the modem to a >recorder? Since a phone line normally has a voltage, does one have to be >supplied? And I can't remember the name of the company that made the modems. It was in the early days, before Hayes made such inroads. I do remember that I had that modem before IBM introduced their PC in August of 1981. -- Bill Vermillion - UUCP: uunet!tarpit!bilver!bill : bill@bilver.UUCP
tnixon@hayes.uucp (Toby Nixon) (10/08/90)
In article <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? > ... > My recollection of the technology would suggest that it would be easiest to do > with the FSK of 300 baud modems (which would be plenty fast enough for my > application) technology. You're on the right track. You should be able to record and play back an FSK signal. PSK, QPSK, or QAM (the modulation schemes used in higher-speed modems) probably couldn't be done that way, primarily due to the more complex initial training that must take place between the modems (the timing of which would be virtually impossible to get right on a recording). > Has anyone done this? If so what did you have to do to connect the modem to a > recorder? Since a phone line normally has a voltage, does one have to be > supplied? I believe Radio Shack sells devices that allow you to connect a recorder to a phone line. You don't have to worry about the line voltage; a modem supplies it's own (unless you use one of the new line-powered modems, but you don't want to do that for this application). -- Toby ----------------------------------------------------------------------------- Toby Nixon, Principal Engineer Fax: +1-404-441-1213 AT&T: !tnixon Hayes Microcomputer Products Inc. Voice: +1-404-449-8791 CIS: 70271,404 Norcross, Georgia, USA BBS: +1-404-446-6336 MCI: TNIXON UUCP: ...!uunet!hayes!tnixon Internet: hayes!tnixon@uunet.uu.net