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Dear All,

  I've designed a digital DTMF detector in an FPGA, using a LPF, HPF and 8 Goertzel Filters.An
AGC has also been designed which limits the upper thresholds to -3dB and lower to about -30
dB.The DTMF tones from the telephone is exactly getting detected.I am facing a problem when
just air is blown through the telephone receiver a random DTMF tones are continously getting
detected.I tried with adjusting AGC and Energy levels didn't work out.
 Can anyone please guide me to solve this issue.

Thanks in advance,
regards,
Madhu

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Dear Anand,

 Thanks for responding.
My design also contains the average filters and I can find the signal
strengths(energy) over there.As you told I had already tried with the
threshold values for received signals, but the noise(ex : air blown through
the telephone receiver) almost matches with tone's threshold hence a random
number gets detected. I feel like whether the problem with second harmonics,
but not sure. Is there any other solution for this.

thanks & regards,
Madhu
  -----Original Message-----
  From: Anand Kishore [mailto:a...@intersolutions.stpn.soft.net]
  Sent: Friday, July 06, 2007 9:58 AM
  To: m...@signal-networks.com; a...@yahoogroups.com
  Subject: Re: [audiodsp] Noise is getting detected as DTMF Tones.
  Hi Madhu,

     I was also facing the same problem in my project but the spurious DTMF
digit detected was always the digit 2.When I analysed the signal I found
that the signal coming for digit 2 was quite different from the other
digits.If you are getting the same kind of problem use the energy level as
threshold and validate the digits as DTMF if it is greater than that
threshold.

  Thanks and Regards
  Anand
    ----- Original Message -----
    From: m...@signal-networks.com
    To: a...@yahoogroups.com
    Sent: Thursday, July 05, 2007 11:28 AM
    Subject: [audiodsp] Noise is getting detected as DTMF Tones.
    Dear All,

    I've designed a digital DTMF detector in an FPGA, using a LPF, HPF and 8
Goertzel Filters.An AGC has also been designed which limits the upper
thresholds to -3dB and lower to about -30 dB.The DTMF tones from the
telephone is exactly getting detected.I am facing a problem when just air is
blown through the telephone receiver a random DTMF tones are continously
getting detected.I tried with adjusting AGC and Energy levels didn't work
out.
    Can anyone please guide me to solve this issue.

    Thanks in advance,
    regards,
    Madhu

(You need to be a member of audiodsp -- send a blank email to audiodsp-subscribe@yahoogroups.com )