[asterisk-users] inband DTMF for g729

[Steve Underwood]

Gary Chen wrote:
> Does anybody know why Asterisk does not support inband DTMF for G.729?
> Our SIP carrier use inband dtmf for G.729. This causes problem for us 
> to use it for our Asterisk IVR system.
>  
> Any suggestion to solve this problem?
I supposed the basic "why" is nobody has done it.

G.729 spoils the quality of DTMF, and detection reliability degrades a 
bit, but not that much. I can put a DTMF test stream through G.729 and 
my decoder gets almost the same results as feeding the test stream 
direct to the decoder. I've never tried this, but it looks like you 
could do reasonable DTMF decoding from the G.729 parameters directly, 
without decoding to linear data at all. That might avoid patent 
licencing for this task.

So, with some effort, and possibly with some patent licencing it could 
kinda work. On the other hand, the industry standard approach is to 
avoid this completely, and use RFC4733 (used to be RFC2833), which 
avoids any degradation in performance, and is simple to implement.

Steve