W3C home > Mailing lists > Public > public-webrtc@w3.org > January 2014

Re: Min DTMF Gap

From: Gunnar Hellstrom <gunnar.hellstrom@omnitor.se>
Date: Sat, 18 Jan 2014 00:01:05 +0100
Message-ID: <52D9B631.2080106@omnitor.se>
To: Roman Shpount <roman@telurix.com>
CC: "Cullen Jennings (fluffy)" <fluffy@cisco.com>, Mike Johns <m.johns@commsalliance.com.au>, "public-webrtc@w3.org" <public-webrtc@w3.org>
On 2014-01-17 22:55, Roman Shpount wrote:
> On Fri, Jan 17, 2014 at 4:39 PM, Gunnar Hellstrom 
> <gunnar.hellstrom@omnitor.se <mailto:gunnar.hellstrom@omnitor.se>> wrote:
>     On 2014-01-17 18:35, Cullen Jennings (fluffy) wrote:
>         We leave the default tone duration at 100 ms.
>     Why this long tone? All columns show minimum 40 ms for duration.
>     If you want to guarantee the minimum total length of tone + gap to
>     be 125 ms as required by Australia, it would make more sense to
>     set the default tone to 55 ms.
>     Then default tone + default gap is 125 ms, and this is also very
>     close to the maximum rate set by Japan and Brazil.
>     Regarding all problems with misbehaving echo cancellers in VoIP
>     gateways, I think it is good to not push these figures to its
>     extremes.
>     So, my proposals for default figures are 55 ms tone and 70 ms gap.
>     And minimums as Cullen's proposal.
> I would recommend to keep default tone length at least at 70 ms. 
> Justification is that it should be long enough to loose a packet of 
> G.711 audio (which normally is no more then 30 ms long) and still be 
> long enough to be recognized.
The transmission must be RFC 4733 coded, that provides good protection 
against packet loss. The tones are sent as RFC 4733 coded event codes, 
with an increasing total duration for each new event code as long as the 
tone lasts. End of tone has a special flag and the end event is sent 
three times for robustness. Gaps by packet loss during the sequence of 
event codes will not generate tone gap and not shortened tone.

But you are right anyway that 70 ms tone is a good choice.
There is an ETSI standard in force for DTMF transmission ES 210 235-2

And it requires transmission of tones 65-75 ms and gap 65-75 ms.
Even the upper limit is said to be needed for some equipment to not 

This requirement seems to give us values that are within all previously 
discussed limits.
Since the detection limit according to Q.24 is 40 ms, it does not make 
sense to allow transmission of as short tones as 40 ms.

I now suggest that we follow the ETSI standard and set 70 tone 70 gap 
fixed with no adjustment possibility.


> This being said, I do not see any reason to reduce default 100 ms tone 
> duration. The limitation in Q.24 is on the max rate. There no harm at 
> sending DTMF digits at a lower rates.
> _____________
> Roman Shpount
Received on Friday, 17 January 2014 23:01:47 UTC

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