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DFT taps for DTMF

Discussion in 'Electronic Design' started by Roger, Jun 9, 2006.

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  1. Roger

    Roger Guest

    What size of DFT whould be appropriate for DTMF?

    I know the bands of the tones are quite large to allow generation
    without crystals, which suggests to me that you should not be too
  2. Genome

    Genome Guest

    Hmm, no idea mate..... we still have to sort out the top versus bottom
    posting debate around here.

    Anyway, why are you talking about taps?

    Whoops, my cake is burning.


  3. Joerg

    Joerg Guest

    Hello Roger,

    You could match the BW to the old standard. Don't remember where that
    was published, could have been Bell. Nowadays it'll almost always be
    more precise in frequency but that depends on the application, whether
    or not there is a chance that signals could come from an old phone.
    Also, there are regs about how fast you must be able to detect each tone
    event (20msec?) so you can't go too low in BW.
  4. There's a very clever DTMF DFT at
    Luhan posts here so maybe could add background.
  5. Bob

    Bob Guest

    If you search for "Goertzel algorithm" and "dtmf" you'll find what you're
    looking for. I've used this algorithm several times with good success. There
    is additional processing needed to meet certain requirements, but without
    knowing your exact needs it's tough to recommend a complete solution.

  6. Luhan

    Luhan Guest

    I found using 16 samples was a good compromise. This gives filter
    ranges with little overlap but still decodes each quickly. The method
    decodes all 8 frequencies at the same time by scheduling 'convenient'
    times to take each sample.

  7. Nico Coesel

    Nico Coesel Guest

    The frequencies not being critical is one of the problems for
    detecting DTMF. You'll need to process the signal in a way you know
    the frequency of the incoming signal precisely. There are extensions
    to DFT which use a sliding window and the phase information to extract
    the dominant frequency in an piece of the frequency spectrum.
  8. Roger

    Roger Guest

    QDFT? That is brilliant, and not just for DTMF. There was a project I
    did with sub-tone signals some years ago that could have been done with
    cheap micro rather than the DSP I used.....had I thought about this
    technique. I suppose it would work anywhere were you can control your
  9. Roger

    Roger Guest

    BTW, you only need to look for seven frequencies because the 4th column
    is never actually used on phones.
  10. Bob

    Bob Guest

    If there is a need to have the DTMF detector enabled while there's a
    possibility of having voice energy present then you should look at 14
    frequencies -- the 7 row and column frequencies plus each of their second
    harmonics. The idea is to make sure that there is no second harmonic energy
    if the fundamental is detected. This technique works pretty well, to not
    detect possible voice energy as DTMF, because voice is rich is harmonics.
    For example, if the detector senses 697Hz and also sees 1394Hz then it is
    most likely voice and not DTMF. Search for the term "talk off" and "DTMF" to
    see what this is all about.

    A properly-functioning DTMF decoder is much more complex than merely doing
    some sort of digital filtering to detect row and column frequencies. It all
    depends on what the application is whether or not you need to make it more
    or less rigourous.

    For example, we once used a voicemail system that while speaking its own
    menu would detect this outgoing voice as an incoming DTMF digit (depending
    on the selected trunk's 4 wire transhybrid return loss). It turned out that
    the dectected digit meant "delete message". It was pretty comical, and yet
    very annoying if you didn't want that particular message deleted.

  11. Roger

    Roger Guest

    Just realised, you are the "inventor". Congratulations :)
  12. joseph2k

    joseph2k Guest

    Yes, the well known issue of simplifying just a little too much; and a
    surprise gotcha.
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