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Hardware puzzle: Recording modem sounds

Posted on 2003-11-12
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Last Modified: 2012-06-21
I'll start off by stating that this may be in the wrong forum -- it seems that it belongs somewhere between here and the puzzle area....   Thus it has been posted in both.  For simplicity purposes, I would prefer it if you post your suggestions at http://www.experts-exchange.com/Miscellaneous/Puzzles_Riddles/Q_20795133.html.  If however, you post here, that is fine.

Anyway, when files are sent across phone lines via a modem, they are first converted into sounds by the modem.  I would like to capture the sounds onto an audio tape.  In essence what I am wishing to do is connect a tape recorder to my modem and have the modem send the file to the tape recorder.  I would also like to be able to do the reverse process.

So far, I have spliced telephone cable with an RJ-11 to a cable with an audio in connector.  The problem I am now running into is getting the modem to send a file with no dial tone or handshaking occurring.  I've looked at some of the AT commands that can be used in hyperterminal, but so far I have not had any luck.

Anyone ever played with this before?  

Thanks.
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Question by:Zadel
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jhance earned 100 total points
ID: 9734912
While what you are suggesting is possible for older modems it's not possible to do so for current ones.

Older modems were very primitive in their encoding and used a scheme called FSK or Frequency Shift Keying.  Essentially this is using two different tones, one for a "1" and one for a "0".  Pretty straightforward and easy to understand.  The problem with this scheme is that you get only 1 bit per baud. A "baud" is a line transition.  Older 300 baud up to 1200 baud modems worked this way.

Above 1200 baud you need more speed and so more advanced encoding schemes were used.  In the range of 2400-9600 bps modems you generally have a combination of PSK.  PSK is Phase Shift Keying.  The nice thing about PSK is that you can send more than one bit per baud by varying the amount of phase shift you send.  It goes like this, 0 degree shift = "00", 90 degree shift = "01", 180 degree shift = "10", 270 degree shift = "11".  So with this scheme you can send 2 bits per baud and get 2X the data rate.  By sending finer increments of phase shift you can get more and more bits per baud until you hit the noise level of the media (i.e. the phone line).

This scheme gets you to about 19.2K bits/sec.  Above that you need another scheme that is less sensitive to the types of noise characteristic to a POTS line. (POTS = plain old telephone system)

There are several methods of packing more data into a baud but the most commonly used is called QAM/PSK.  This is a combination of TWO schemes, QAM - quadrature amplitude modulation and the PSK we had before.  So now you end up with an arrary of possible values where the phase and the amplitude are modulated at the same time.

OK, back to your question....  An audio recorder is not very good at capturing PHASE changes.  The human ear is not sensitive to phase so a large amount of phase distortion is typical with audio equipment.  If you record a 56K modem sound you will not be able to play it back and have the modem understand it even if you could get the modem to "listen".

Another problem is that 56K modems are "handshaking" each other all the time to correct for changes in the line over time.  If the modem doesn't get a regular handshake from the other end, it will assume the connection has died and hang up.

Sorry to dump cold water on your hot idea but it's just not possible to do what you are considering.
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by:public
ID: 9735034
>While what you are suggesting is possible for older modems it's not possible to do so >for current ones.
Huh?
just connect a phone cable to two pc's with a modem, tap into the line and record the sound. You can start recording after the training completes, and start the reconstruction the same way. You will need a pretty good recorder with at least 40dB SNR and good phase linearity.
Would it not be simpler just to record the data on a cd at about 10c ea?
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by:jhance
ID: 9736356
>>>Huh?

I stand by my comments!  It's NOT possible to do this regardless of the recording device.

It would probably help if you read the ENTIRE question as well as my explaination.

Doh!
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by:kiranghag
ID: 9737186
well, i support jhance..

but this question reminds me of the older commodores and other such computers where you could use tapes as your storage media.
but those times had a different cassate socket to connect the tape recorder thrugh line-in.
data transfer rates as well as capacity was not so great. even the original ibm pcs had the same thing.

most of the pcs had basic code embedded in the bios which kicked off if the pcs is not booted with any bootable disk...you can still see the placeholder by calling int 18 on any modern pc thru dos-debug

now coming back to your problem, as jhance said, u cannot do exactly what u want.
but if u still wish to store data on tape, i have one idea....
you can connect the tape recorder thrusound card's line-in socket and then write a program to output the dataformat  thru it which will be recorded.
and another piece of code will then recover back the data from the tape when connected in reverse way...

you may need to do some hard thinking and experimenting on this...but i think this can be done...

waiting for others to comment...
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by:Luc Franken
ID: 9737904
Can't be done, so I support jhance and kiranghag.
You won't be able to connect the two because the cassette won't even make it trough the handshake (wich negotiates maximum speed etc)

btw. "Anyone ever played with this before?"
yep, just before I found out that it couldn't be done!! Then I stopped.

LucF
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by:public
ID: 9741766
>Can't be done, so I support jhance and kiranghag.
>You won't be able to connect the two because the cassette won't even make it trough >the handshake (wich negotiates maximum speed etc)

you set up two pc's with a line in between to handle the connection protocol. Once established, record the sound with a line tap. For reconstruction reverse the process.
you can even set the connection speed with at commands if the recorder is not good enough quality.
At leas one hacker used this technique to capture login info.
Not very practical, but not impossible.
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by:Luc Franken
ID: 9741847
public, please read the question:
>I would also like to be able to do the reverse process.<
Now, how would you like to do that, even if the recording is possible.

>you can even set the connection speed with at commands if the recorder is not good enough quality.
And how would that hacker do that without anyone notice it (like sitting behind the computer) I think he would have chosen a key-logger instead, easier and more effective.

>At leas one hacker used this technique to capture login info.
Not with a tape recorder, I can assure you that!
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by:jhance
ID: 9741872
>At leas one hacker used this technique to capture login info.

You're talking about DIFFERENT problems here.  

The hacker has the leisure of being able to do a STATIC ANALYSIS of the data that is captured.  While I agree that a tape recorder is not suitable for this work, a digitized recording of the phone line could be analyzed after the fact and the data reconstructed.  Actually, this is not that difficult.

The problem posed in this question is a DYNAMIC converstation between a "real" modem and a recording device.  That is what is being stated as not possible.

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by:public
ID: 9745129
>The problem posed in this question is a DYNAMIC converstation between a "real" >modem and a recording device.

does not have to be. One can setup two modems and a line tap recording device for both the recording and the reconstruction.

>At leas one hacker used this technique to capture login info.
>Not with a tape recorder, I can assure you that!
he did precisely that!!

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by:Luc Franken
ID: 12260658
I'd say points to jhance. This is a clear case of "No you can't do that" IMO

LucF
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