TELECOM Digest OnLine - Sorted: Re: Packet8 DTMF Tones Sound "Clipped"

Re: Packet8 DTMF Tones Sound "Clipped"

David Clayton (dcstar@myrealbox.com)
Wed, 25 May 2005 12:02:13 +1000

On Mon, 23 May 2005 17:58:50 -0600, Clark W. Griswold, Jr. wrote:

> slippymississippi@yahoo.com wrote:

>> I hooked up a line analyzer and listened to the call, and all I'm
>> hearing on my end is a "blip blip blip" as he repeatedly presses the key
>> to save the message. The DTMF tone basically sounds like it was clipped
>> at just a few milliseconds

> I seem to recall that the old Bell System spec for tone duration was 20ms,
> or 1/5th of a second. Some switches allowed you to go as fast as 10ms,
> which put the speed in speedial. :)

The biggest problem with "short" DTMF tones - along with short gaps
between tones - was determining when two identical digits were in fact
two separate ones and not a single tone burst split by some line
interruption etc.

I once worked on a voice mail system with DTMF in-band signalling to
the PBX for Message Waiting indication, and there were intermittent
complaints from some users that they had messages but were not
always notified.

After many, many months (actually a couple of years) it was finally
determined that the gap between the DTMF tones coming out of the Voice
Mail wasn't long enough for the PBX to reliably detect two identical
digits following one another.

The gap was fine for different digits, but when the combination of
Message Waiting prefix and extension number had a pair of digits
somewhere, and the PBX CPU just happened to be busy at that moment (we
thought), the Message Waiting wasn't received correctly.

So to sum up, fast DTMF can cause issues ...

Regards

David Clayton, e-mail: dcstar@XYZ.myrealbox.com
Melbourne, Victoria, Australia.
(Remove the "XYZ." to reply)

Knowledge is a measure of how many answers you have,
intelligence is a measure of how many questions you have.

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