Re: DTMF ENERGY LEVELS

thanks to all,
but can anyone guess the significance of part

if ( r[max_index] > 1.0e9 )
=A0 =A0 =A0 t =3D r[max_index] * 0.158;
=A0 =A0 else
=A0 =A0 =A0 t =3D r[max_index] * 0.010;

in above code?

That entire signal to noise test is bogus. The spec says the detector
must work for an SNR as poor as 16dB. It doesn't insist on not
detecting when things are worse. What you really need to achieve is a
low false detect rate with heavy voice testing - e.g. by testing
against the Bellcore/Telcordia test tapes.

You asked abut 2nd harmonic assessment. A lot of DTMF decoder app
notes perform second harmonic tests, but you can achieve superior
results without doing that. Applied intelligently, "the sum of the
energies in the two dominant spectral lines must be a big percentage
of the the total" is a very effective strategy. However you applied
these heuristics, you have to allow for a) the decoder is used on a
line where dial tone spillback will pollute the first digit or b) the
decoder is trying to detect in the presence of an outgoing voice
prompt spilling back. In case a) a dial tone filter can do the trick.
In case b) you will only get great results if you echo cancel away the
voice.

I have never found an app note design for DTMF decoding that will get
through any reasonable set of performance tests. You have to work at
that.

Regards,
Steve

Thanks steve,
can anyone suggest any c code fulfilling all energy level
requirments? whats your opinion on tone_detect.c from astrisk spandsp
library ?
Regards,
Rahul.
.

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