Help! DTMF Software Decoding

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2 total messages Started by Southern Mon, 23 Nov 1992 23:12

#1828

Author: Southern
Date: Mon, 23 Nov 1992 23:12

6 lines
744 bytes

Hello there.. I am working on a project that requires DTMF decoding through
software on an IBMPC. I am using a SB card to sample the tones through a
microphone and I could sample anywhere from 300-13000hz. My question is
what methods are there to decode the sample? Are there any algorythms
allready out there that I could learn from? Even knowledge on basic
frequency detection would be most helpful. Thanks!

Re: Help! DTMF Software Decoding

#1831

Author: Stephen L Moshier
Date: Tue, 24 Nov 1992 03:44

13 lines
1145 bytes

  Perhaps the simplest way is to provide two bandpass filters,
one for the low frequency tone group and the other for the
high frequency group, and count zero crossings to determine
the frequency of the tone in each passband.  The tone frequencies
are (if I remember right) 697, 770, 852, 941 for the low group
and 1209, 1336, 1477, 1634 for the high group.
  A much better way is to supply a +/- 1.5% bandpass filter
for each tone frequency.  The filter outputs (of the two tones
or of the high and low band) should be within 8 or 10 db of
each other, else it is a reject.  Also it is a good idea to
compare with the band from 350 to 620 Hz to differentiate
from progress tones and speech.  An amplitude threshold at
about -35 dbm will reject false alarms from background noises.

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