Re: About spectrum of speech and fax transmission

VSP wrote:
hi all,
Someone please tell me why the spectrum of speech rolls off
towards the higher frequencies ?????? I am a newbie to DSP so, simple terms please as far as possible.

I am hoping to use this fact to distinguish it from other
data transmissions such as fax. (this was suggested to me by Mr.Vladimir Vassilevsky on Comp.DSP only)

Yes Mr. John I am thankful to u also for ur reply. I am trying to read about ITU-T T.30 protocol but could not find a good enough source online.

Mr.John had suggested using the two tone (frequency) handshake between two
fax machines as a way of identifying fax transmissions.

All ideas are welcome and appreciated.

You seem to be making this really hard by not explaining what you are really trying to achieve.

If you hear the whole call, you can distinguish FAX by the pulsing of 1100Hz from the caller (almost always present), or by a burst of 2100Hz from the answerer (often suppressed).

Do you need to distinguish quickly at the start of the call, so you can select how the call is handled? If so, the above tones are pretty much the only thing you can use.

If you want to distinguish by a burst of audio somewhere in the middle of a call, how much audio do you have to work with? If its a fairly short burst, looking for a stationary average energy level over, say, a couple of hundred milliseconds will tell you it isn't voice. It won't confirm that it is FAX, though. It could be something like busy tone

Do you need to distinguish between FAX and other modem signals. If you don't care about other modem signals and you have a long tract of audio, you can try decoding the V.21 HDLC bursts. If you can decode one between pages its fax.

Hunting for the T.30 spec won't get you very far. It specifies the message protocol, and not the modem signals. The modem signals are V.21, V.27ter, V.29, V.17 and maybe V.34 for the newer FAX machines. All these specs (including T.30) can be obtained from www.itu.int. You can download a few (3, I think) for free.

Regards,
Steve
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