[asterisk-tech] DTMF Frequencies

Damjan Jovanovic damjan.jov at gmail.com
Fri Jun 6 10:45:40 SAST 2008

On Fri, Jun 6, 2008 at 10:31 AM, Conrad Moolman <cmoolman at gmail.com> wrote:
> Hi,
> I'm trying to set up a PBX to interact with a paging system, and the guys
> are asking me about DTMF frequencies and the
> differences between all the DTMF options. What is the real difference
> between, say info,inband and RFC2833? Is there any
> change in the frequency between these settings and if so, where can i get
> the frequencies each one operates on? I've
> been googling a bit, but nothing came up.

The differences are in representation and transport.

Inband carries DTMF in the RTP stream for voice, which obviously takes
the media path. If this stream uses a compressing codec at any point,
it can distort the DTMF, possibly beyond recognition.

RFC2833 carries DTMF in another RTP stream which also takes the media
path, and is supposed to use redundant packets to avoid one DTMF tone
breaking up into several due to packet loss or jitter.

INFO demodulates the DTMF and carries the digit as text in the SIP
signalling path, which is useful since it lets a server not in the
media path react to DTMF. INFO could be more bandwidth efficient since
it's just 1 packet per digit and thus there can be no breakup. But,
INFO has no redundancy, so if it is lost the digit disappears, and it
can also be reordered. In addition, the length of the DTMF is only
known when you release the button on the phone, so if you hold it down
for 2 seconds then let go, the other side only then starts hearing the
2 second long DTMF tone.

DTMF is DTMF - the frequencies at the end are exactly the same.

> Thanks
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