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Somewhat off-topic post: DTMF transfer with wireless codecs

Started by James Calivar November 14, 2003
I am researching the capability of various wireless speech codecs in
handling DTMF digits.  Since these codecs are typically rather low rate, one
would expect that their ability to reliably transmit pure tones, music
and/or multitone digits would be compromised.  I see in the SMV codec
specification that they specifically have implemented a method of supporting
both music and digits; what about other wireless codecs?  I don't see
anywhere in the spec's for EVRC, or GSM-FR, GSM-EFR or the AMR codec where
digits are handled.  I'm specifically interested in how these digits would
be handlend over an IP network - just using generic RFC 2833 support?

Thanks in advance,

J



James Calivar wrote:
> > I am researching the capability of various wireless speech codecs in > handling DTMF digits. Since these codecs are typically rather low rate, one > would expect that their ability to reliably transmit pure tones, music > and/or multitone digits would be compromised.
You are right. Low rate codecs are distorting the DTMF, especially for the high speed dialing.
> I see in the SMV codec > specification that they specifically have implemented a method of supporting > both music and digits; what about other wireless codecs? I don't see > anywhere in the spec's for EVRC, or GSM-FR, GSM-EFR or the AMR codec where > digits are handled.
Although some codecs like GSM 6.10 or G.728 can handle the DTMF with the reasonable amount of distortion you should not rely on that. The standard practice is to have separate DTMF decoder/encoder chain which bypasses the voice codec.
> I'm specifically interested in how these digits would > be handlend over an IP network - just using generic RFC 2833 support?
That makes things worse. One dropped or late packet is no problem for voice however it is unacceptable for the DTMF speed dialing. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
"Vladimir Vassilevsky" <vlv@abvolt.com> wrote in message
news:3FB50F22.CAB6A43D@abvolt.com...
> > > James Calivar wrote: > > > > I am researching the capability of various wireless speech codecs in > > handling DTMF digits. Since these codecs are typically rather low rate,
one
> > would expect that their ability to reliably transmit pure tones, music > > and/or multitone digits would be compromised. > > You are right. Low rate codecs are distorting the DTMF, especially for > the high speed dialing. > > > I see in the SMV codec > > specification that they specifically have implemented a method of
supporting
> > both music and digits; what about other wireless codecs? I don't see > > anywhere in the spec's for EVRC, or GSM-FR, GSM-EFR or the AMR codec
where
> > digits are handled. > > Although some codecs like GSM 6.10 or G.728 can handle the DTMF with the > reasonable amount of distortion you should not rely on that. The > standard practice is to have separate DTMF decoder/encoder chain which > bypasses the voice codec. > > > I'm specifically interested in how these digits would > > be handlend over an IP network - just using generic RFC 2833 support? > > That makes things worse. One dropped or late packet is no problem for > voice however it is unacceptable for the DTMF speed dialing. > > Vladimir Vassilevsky > > DSP and Mixed Signal Design Consultant > > http://www.abvolt.com
I agree that packet loss can be devastating to non-voice type traffic on an IP network (esp. for modem and fax), but these effects can be mitigated with redundancy or other FEC mechanisms. For DTMF digits, I don't believe that packet loss and other network degradation are as debilitating as for fax/modem traffic. The purpose of RFC2833 (as far as I can gather) is to send DTMF and other digit indications "relayed" or "out of band" (i.e. not through the voice codec itself), both to save packet network resources and to bypass the degrading effects of the voice codec on non-speech signals. Proper protection of these special digit indication packets is assumed. To re-phrase my question: how to perform digit relay over IP networks, with wireless codecs that do not themselves specify a method of handling non-speech traffic? Thanks J