[Freeswitch-users] Better results from mod_vmd

[Eric des Courtis]

Matt,

I think the only way to know for sure is to try it. I would try to get
the value as high as possible while still detecting that 738Hz sine
(with a small margin of error). Lowering the value increases false
positives rapidly.

Eric des Courtis

On Thu, Aug 20, 2009 at 8:36 PM, Matthew Fong<mattdfong at gmail.com> wrote:
> I changed
>
> /*! Minimum time for a beep. */
> #define MIN_TIME 8000
> to 6500 and it seemed to work, but I'm not sure how many false positives I
> will get in a real-world environment. at 4000 it fired the event like 5
> times in a session, but 6500 only once. Do you think I should expect a lot
> of false positives after changing this value?
>
> --matt
> http://www.hellohunter.com
>
> On Thu, Aug 20, 2009 at 4:54 PM, Eric des Courtis
> <eric.des.courtis at gmail.com> wrote:
>>
>> Matt,
>>
>> As is mod_vmd will not detect tones shorter then 138ms. However I
>> could get that value down to ~30ms at best by making a few
>> modifications to the algorithm.
>>
>> Cheers.
>>
>> Eric des Courtis
>>
>>
>> On Thu, Aug 20, 2009 at 7:51 PM, Eric des
>> Courtis<eric.des.courtis at gmail.com> wrote:
>> > Matt,
>> >
>> > For your information the tones you gave me are exactly 738Hz. If you
>> > want to try that tone detection thing.
>> >
>> > Cheers.
>> >
>> > Eric des Courtis
>> >
>> > On Thu, Aug 20, 2009 at 2:20 PM, Michael Collins<msc at freeswitch.org>
>> > wrote:
>> >>
>> >>
>> >> On Thu, Aug 20, 2009 at 11:06 AM, Steve Underwood <steveu at coppice.org>
>> >> wrote:
>> >>>
>> >>> 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
>> >>
>> >> Thanks for the lesson uncle Steve! I'm guessing that the OP will need a
>> >> new
>> >> strategy. Possibly waiting for silence? Not sure what's the best way to
>> >> go
>> >> but I'm interested in hearing if someone has a solution.
>> >>
>> >> -MC
>> >>
>> >>
>> >>
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