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Re: [Discuss-gnuradio] Fax question

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Fax question
Date: Tue, 21 Jan 2003 22:17:19 -0800
User-agent: Mutt/1.4i

On Wed, Jan 22, 2003 at 12:39:17PM +1100, David wrote:
> I have been looking at GNU Radio and was wondering how feasible it might be
>  to use it for something like a fax protocol analyser.

Definitely.  Most fax modems only sample at 7200 or 8000 samples / second.

> I would be looking at using either baseband analogue POTS or aLaw/uLaw. I
> looked over the source and seem to remember seeing a uLaw implementation.

Assuming you're trying to listen to a real phone line, your quickest
approach is probably to capacitivly couple tip and ring to the sound
card input.  This is a slight simplification, you'll want some kind of
zener's to clamp any high level input (e.g. Ring, 200V P-P), but it's
no big deal.

> What I want to do is be able to detect the different fax tones and then map
> them to the correct parts of the fax protocol (not so concerned about th
> actual fax data itself).
> My question is really two fold:
> 1)    Is this possible with GNU Radio (presumably involving programming).

Definitely.  You might want to look at the linmodem project.  They may
already have it implemented.

> 2)    How feasible is for someone to do it considering I am only currently
> learning C++

There are at least two kinds of challenges:

      (1) the signal processing required to get it to work
      (2) the actual coding

If you're a C programmer, coding for GNU Radio shouldn't be much of
problem.  Pick up a C++ book and look at our existing code.  We'll
eventually have some kind of a scripting interface available that will
allow non-C++ programmers to glue modules together, but it's not ready

That still leaves the signal processing.  Like all of the rest of
life, it all depends on your background and your motivation.

> Thank you for both the project and any suggestions you can make.

You're welcome,


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