[Asterisk-Users] DTMF Simultaneous Inband and RFC2833 performed by Asterisk => Duplicate tones

[Kevin P. Fleming]

steve at daviesfam.org wrote:

> I have seen the following effect in Asterisk, though:  where it converts 
> an inband DTMF (eg coming off a Zap channel) into an indication, it mutes 
> the audio where that tone is.  But sometimes it leaves a teeny bit of the 
> tone behind.

Yes, that is correct. By the time that DTMF detector has determined that 
the tone exists, some of the tone has already been passed on. This 
occurs because the Asterisk DSP runs in parallel with the audio path, 
not in the middle of it, so it cannot 'delay' the audio to be able to 
mute it retroactively.