Given the following frequencies in DTMF : lfg = [697 770 852 941]; % Low frequency group hfg = [1209 1336 1477]; % High frequency group what is the min. sampling frequency and the number of samples required (using FFT) to detect the keys? I tried Nyquist sampling frequency (2*max freq = 2*1477 ~3000 ) and for the min. freq resolution requirement of 73Hz (from 697-770Hz) the number of samples I took is 45 (given by 3000/73 approx) of course, I am not able to detect all keys, since there will be leakages. Is there any other considerations I must take into account when determining the min sampling freq. of the DTMF decoder? thanks in advance lionel lewis

# min. sampling frequency of DTMF decoder

Started by ●February 11, 2005

Reply by ●February 11, 20052005-02-11

Lionel Lewis wrote:> Given the following frequencies in DTMF : > > > lfg = [697 770 852 941]; % Low frequency group > hfg = [1209 1336 1477]; % High frequency group > > > what is the min. sampling frequency and the number of samples required > (using FFT) to detect the keys? > > > I tried Nyquist sampling frequency (2*max freq = 2*1477 ~3000 ) > and for the min. freq resolution requirement of 73Hz (from 697-770Hz) > the number of samples I took is 45 (given by 3000/73 approx) > > > of course, I am not able to detect all keys, since there will be > leakages. > > > Is there any other considerations I must take into account when > determining the min sampling freq. of the DTMF decoder? > > > thanks in advance > > > lionel lewis >It seems like time-domain techniques might be a better match for DTMF detection since you are only interested in the power values at a handful of frequencies, compared to the power contained within a given time window. Disclaimer: I've never made a DTMF detecter myself.

Reply by ●February 12, 20052005-02-12

Mark Borgerding wrote:> Lionel Lewis wrote: > > Given the following frequencies in DTMF : > > > > > > lfg = [697 770 852 941]; % Low frequency group > > hfg = [1209 1336 1477]; % High frequency group > > > > > > what is the min. sampling frequency and the number of samplesrequired> > (using FFT) to detect the keys? > > > > > > I tried Nyquist sampling frequency (2*max freq = 2*1477 ~3000 ) > > and for the min. freq resolution requirement of 73Hz (from697-770Hz)> > the number of samples I took is 45 (given by 3000/73 approx) > > > > > > of course, I am not able to detect all keys, since there will be > > leakages. > > > > > > Is there any other considerations I must take into account when > > determining the min sampling freq. of the DTMF decoder? > > > > > > thanks in advance > > > > > > lionel lewis > > > > > It seems like time-domain techniques might be a better match for DTMF> detection since you are only interested in the power values at ahandful> of frequencies, compared to the power contained within a given time > window. > > > Disclaimer: I've never made a DTMF detecter myself.DTMF detector algorithms often use Goertzel filters designed for 8 ksps data. There are detailed specifications that a DTMF detector should meet if it is to be connected to the PSTN, and there are test tapes with DTMF on top of speech, unbalanced tones, short tones, noisy tones, etc. Getting the detector to meet spec is not trivial. John

Reply by ●February 12, 20052005-02-12

john wrote:> DTMF detector algorithms often use Goertzel filters designed for 8 ksps > data. There are detailed specifications that a DTMF detector should > meet if it is to be connected to the PSTN, and there are test tapes > with DTMF on top of speech, unbalanced tones, short tones, noisy tones, > etc. Getting the detector to meet spec is not trivial.Unfortunately you can no longer obtain either the Mitel or Bellcore test tapes. Regards, Steve