On Thu, Apr 14, 2011 at 1:58 PM, Steve Underwood
<steveu@coppice.org> wrote:
On 04/15/2011 01:32 AM, Christopher Rienzo wrote:
> 2100 Hz is the ANS tone from the receiving side of the fax call.
> That's what I detect on my outbound calls.
An answering machine sends 2100Hz, but its very unreliable to rely on
hearing it. In spandsp there is a module which does a much more thorough
job of detecting a FAX machine. It will accept the V.21 preamble, which
normally follows the 2100Hz tone, as a substitute for the 2100Hz itself.
That way a call which starts passing audio a bit late (which they often
do) still results in proper FAX detection. We need to expose that
functionality in FreeSwitch, but have not done so yet.
Steve