[asterisk-users] Double DTMF digits

[Steve Davies]

On 5/3/07, Ken Leland III <k3leland at monmouth.com> wrote:
> When dtmfmode is set to inband for SIP, and i originate a call from sip
> out to the PSTN, I can hear the DTMF digit twice in the audio stream.
> Once very briefly and once for normal duration.
>
> Our Theory: While Asterisk is parsing the DTMF, for a fraction of a
> second, while the end user generated DTMF is being detected, the DTMF is
> passed inband. Once the DTMF is detected Asterisk silences it and
> regenerates it. Sensitive machines like auto attendants pick up both the
> brief end user generated tone as well as the full length asterisk
> generated tone and ultimately perceive each digit twice.
>
> Is anyone else experiencing this?
>
> I have reproduced this in an environment
>     * with one asterisk server that is both the feature server and the
> media gateway, and is timing off of network T1s
>     * with two servers, one feature server (timing off of ztdummy) and
> one media gateway (timing off of network T1s) using IAX as the inter
> asterisk protocol
>
> It is pretty easy to reproduce:
> -Dial a PSTN number(like your cell) from a sip phone using inband DTMF,
> and configured in asterisk sip.conf with dtmfmode=inband.
> -Answer the PSTN end.
> -Press and hold a digit on the sip phone. On the PSTN phone you will
> hear a very brief, end user generated, tone.
> -Let go of the digit on the sip phone. On the PSTN phone you will hear
> the asterisk generated tone.
>
> Can anyone else hear the brief initial tone?  Any help is greatly
> appreciated!

Yes, we have a similar issue, but do not normally use inband DTMF
because SIP phones very  cleanly generate rfc2833 RTP packets directly
and remove this issue.

On the other hand, asterisk is not alone dealing with this issue in
SIP. The Linksys ATAs have exactly the same issue.

Strangely, I do not have a problem receiving inband DTMF through
Zaptel, which I believe uses the same DSP code for DTMF detection...
Or does it?

Steve