TELECOM Digest OnLine - Sorted: Re: Help Needed With WAV File

Re: Help Needed With WAV File

Thu, 11 May 2006 21:01:35 -0400 wrote:

> I would like to recover the bits of the digital data from a
> conversation between two modems.

> Why do you think this is virtually impossible for a standard modem
> connection with an analog-side speed above 2400 BPS?

Modems these days and for a long time have a limit of (I think) 2400
baud. This means they can only change the signal 2400 times a second
on a voice grade line. So to get the higher bit rates they have to
encode multiple things 2400 times a second. One way is the multiple
frequencies. The other is that the tones on each frequency are
shifted, twisted, etc ... in a way such that each frequency can
represent multiple bits at any one time. For all of this to work two
modems first go through a process where they test the line at each
frequency to determine to determine the attenuation and distortion
imposed on by the circuit at each frequency so that these issues can
be factored into the real signal. Plus some frequencies will not be
used if the signal at that frequency doesn't meet some minimal

So to reconstruct the bit patterns you'd have to know the result of
the "training" plus have a high enough quality of recording to
reconstruct not just the frequencies but the actually phase
distortions and skews and run the result back through a DSP
process. If you have lots of time, signal processing experience, and a
very fast computer (or computers) you might do it. But the first issue
is the initial training in effect results in an encryption key that
you have to break before you can even start.

Now I'm sure that US Robotics and Rockwell have some programs that
might do this and maybe the DIA, CIA, and NSA but past that ???

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