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Re: Min DTMF Gap

From: Gunnar Hellstrom <gunnar.hellstrom@omnitor.se>
Date: Fri, 17 Jan 2014 08:11:49 +0100
Message-ID: <52D8D7B5.3010908@omnitor.se>
To: public-webrtc@w3.org
On 2014-01-17 01:43, Roman Shpount wrote:
> I was the person who asked for this change.
>
> Based on http://www.itu.int/rec/T-REC-Q.24-198811-I/en Annex A, valid 
> tone duration is 40 ms and up. Valid gap duration is 30 ms (minimal 
> for Japan) and up to 70 ms minimum in Australia. So, my suggestion was 
> to keep defaults at their current values but allow to set minimal 
> values to minimal possible legal values (40 ms tone and 30 ms gap). My 
> justification is that DTMF is a legacy interop feature and it should 
> be able reproduce any legal DTMF string which can occur in the wild by 
> modifying the JavaScript parameters.
The same table in Q.24 has a value for signal velocity, that is the 
minimal sum of a tone and a gap. Figures are between 93 and 125 ms, with 
93 for USA, 100 ms for Europe, 120 for Japan and Brazil and 125 for 
Australia.
That would require for example 50 tone and 50 pause to cover USA and 
Europe, and 50 tone and 75 pause to cover all.

Since RFC 4733 should be used for the transmission and detection of 
DTMF, one could expect to rely on RFC 4733 for the timing. In section 
3.1 it refers to Q.24 and points out 40/40 but a limit of 8 to 10 digits 
per second.  That would be accomplished for example by 50 tone and 70 
pause.

It would be interesting to know if there are any international 
experience from setting parameters for RFC 4733 usage that we could use.

We should also remember that Q.24 is talking about timing for detection 
at the receiving end. So, some tolerance should be given at the 
generating end.

So, it seems that 50 tone and 50 pause would be good timing for 
transmission except for Australia, Brazil and Japan ( if the Q.24 limits 
are still valid in these countries ).

Gunnar


>
> _____________
> Roman Shpount
>
>
> On Thu, Jan 16, 2014 at 7:19 PM, Cullen Jennings (fluffy) 
> <fluffy@cisco.com <mailto:fluffy@cisco.com>> wrote:
>
>
>     This has been sitting on the editors todo list for a long time and
>     I wanted to try and sort it out 
>
>     The gap between DTMF digits is currently specified at 50ms. Long
>     ago someone requested we change this to 40 ms.
>
>     Does anyone remember why people wanted to make this change?
>     Thought on if it should be 40 or 50?
>
>     Thanks, Cullen
>
>
>
>
Received on Friday, 17 January 2014 07:12:26 UTC

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