[asterisk-users] Clarification on DAHDI Fax Detection

[Steve Underwood]

On 01/05/2011 02:39 AM, Tom Rymes wrote:
> On 01/04/2011 8:55 AM, Kevin P. Fleming wrote:
>> On 01/03/2011 06:47 PM, Thomas Rymes wrote:
>>> On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote:
>>>> On 01/03/2011 11:26 AM, Tom Rymes wrote:
>
> [snip]
>
>>> OK. Either way, though, the changes to echo cancellation are not
>>> affected by the faxdetect setting, right?
>>
>> That is correct; the faxdetect setting and the echo canceller behavior
>> are completely unrelated.
>
> Excellent.
>
> [snip]
>
>>> Is there a time limit to when DAHDI listens for faxes (say the first
>>> 10 seconds of a call?), or might it detect one in the middle of a ten
>>> minute call?
>>
>> I haven't double-checked, but I believe the software DSP will be in
>> place on the call until it sees a CNG tone, regardless of when that
>> happens during the call.
>
> Wouldn't it make sense to be able to specify a time period after which 
> chan_dahdi disables fax detection? Only calls that begin with a voice 
> call and end with a fax would benefit from detection after the initial 
> ~8 seconds of a call, unless I am overlooking something.
>
> If the DSP keeps listening and detects a spurious fax tone (I know I 
> have seen the human voice incorrectly identified as CNG), it will send 
> the call off to the fax extension if one exists in the same context. 
> In fact, we ran into some issues with exactly that happening.
It is very normal for many people to chat and then start their FAX 
machines, especially domestic FAX users with a FAX machine attached to 
their home land line. If you don't care about those your proposal is OK, 
otherwise.....

There is no excuse for false detection of FAX tone. It takes a very poor 
detector to mistake voice for FAX, unless the person is specifically 
trying to whistle the right tones (which some people are quite good at).
>
> [snip]
>
>>> Thanks for the clarification, there's a lot of conflicting info out
>>> there.
>>
>> Feel free to comment on wiki.asterisk.org if any of the information
>> there led you astray; we'd like to get that to be the most accurate
>> place for people to find this sort of information.
>
> I'll give it a look. I had not specifically looked at the asterisk 
> wiki, but Google searches brought up lots of messages confusing the 
> fax operation of the echo canceler with the faxdetect= setting for 
> DAHDI/Zaptel.

Steve