[Freeswitch-users] Better results from mod_vmd

[Steve Underwood]

On 08/20/2009 05:22 AM, Michael Collins wrote:
>
>     There is no noise on those 3 beeps. In fact, for something that's been
>     through ulaw/alaw compression those beeps are very clean. They are
>     quite
>     short, though.
>
>
> Heck yeah they're short! Steve, in your experience is there a 
> practical way to detect a beep that short without chewing up system 
> resources or having lots of false positives?
> -MC
>
The tone samples I just looked at are about 130ms long. The problem is 
the detector is trying to be a very open ended detector of anything 
narrowband enough to be a single tone, and of any duration beyond some 
small minimum. Its difficult to make such a thing voice immune unless 
you can also count on a very large signal to noise ratio. With a digital 
trunk you can probably rely on a large SNR, but what happens when people 
use analogue lines? There is a reason why DTMF detectors try hard to 
work down to about 10dB SNR. :-)

Steve