[Asterisk-Dev] has somebody ever tested faxdetection ?

[Steve Underwood]

The EC disable tone is the 2100Hz tone that data modems answer with, 
which seems to stutter every 450ms. It doesn't really stutter, it just 
jumps in phase. FAX machines never send this tone.

Steve

Thomas Häger wrote:

> Hi Steve,
>
> ah! I understand. Is it so that faxmachines aren't configurde for 
> sending ec-disable tone ?
> Is this a special setting at the faxmachine, or will it be sent 
> everytime normally ?
>
>
> Greetings,
>
> Thomas.
>
> Steve Underwood schrieb:
>
>> Thomas Häger wrote:
>>
>>> Hi all,
>>>
>>> lately we had a little bug with misdn echo cancel. If a fax was 
>>> detected we got a kernel freeze.
>>> This was caused by NULL pointer access inside of our echocanceler 
>>> code. In this context we wanted to test this issue by calling a fax 
>>> machine to effect the kernel freeze. But nothing happend. So we 
>>> debugged a little bit, and we saw that the tone of 2100 Hz was never 
>>> detected ... respectively the amount of 3 cyles were never reached.
>>>
>>> In debugging we saw several values for "det->tone_cycle_duration", 
>>> but no value was between   (425*8) 3400 and (475*8) 3800.
>>> This must happens more than twice, so that ec will be disabled.
>>> Has someone an idea what exactly is the meaning of these values 
>>> (3400-3800) ?
>>> You can find the function in "zaptel/ecdis.h".
>>
>>
>>
>> The detector you are looking at does not detect continuous 2100Hz 
>> tone. It detects a 2100Hz tone with phase reversals every 450ms. This 
>> is the echo canceller disable tone specified in G.164/G.165. It is 
>> not the FAX answer tone. The numbers you quote allow a suitable 
>> tolerance for the 450ms.
>>
>>> We have tested it as follows:
>>>
>>> SIP --> Asterisk --> chan_msidn (TE) ---> PSTN / ISDN(PtP) ---> 
>>> faxmachine other side.
>>> How customers reported us, they got a kernel freeze in this case. 
>>> But we didn't get a kernel freeze, because the 2100Hz was never 
>>> detected, and EC was enabled all the time.
>>> We tested also fax detection with playing back a reference audio 
>>> file of 2100 Hz (5 sec.), but nothing happened.
>>>
>>> Has someone enough experiences with fax detection and can give us a 
>>> hint why the fax detection didn't work for our configuration ?
>>
>>
>>
>> The FAX answer tone is detected in *, not in the driver.
>>
>> Regards,
>> Steve
>