[asterisk-dev] DTMF detection and generation code

[Andreas Sikkema]

On Sun, 2006-04-23 at 23:50 +0800, Steve Underwood wrote:
> Many many things in the telephone network - cellular systems, key 
> systems, many large PBXes with digital telephones - change the length of 
> DTMF so what is sent is completely unrelated to what it pressed. 
> Anything which cares about the length of a DTMF digit, other than 
> meeting the requred minimum, is badly broken.

I agree with you, but there's one spot where length of a DTMF tone is
really important. 

Consider this:

Phone --- ATA ------- Asterisk ------ GW ------ PSTN ------ IVR
               RFC2833         RFC2833    ISDN        ISDN

ATA converts a DTMF tone received from a phone to RFC2833 events with a
certain length. These are relayed through asterisk which sends a couple
of RFC2833 events to the gateway that converts the events back to a DTMF
tone.

When the events generated by Asterisk result in a DTMF tone that is
shorter than the one initially created by the phone you might get into
problems. A lot of ATA's are not very good at filtering out all of the
DTMF tone, so the end result might actually sound like two DTMF tones to
the IVR. 

If the DTMF events were created of the correct length the problem might
go away.

-- 
Andreas