Post by thread:another post - related to DTMF/tone decoders

On Fri, 18 Dec 1998, Harrison Cooper wrote: > Well, searching thru my saved mail, and URL's, I've found a couple of DTMF > generators using the PIC, but it seems most of the decoders are using > the M8870 DTMF decoders. But I am sure there was some discussion on using a > PIC directly for this. Maybe not. Since I want both DTMF and single tone, > maybe the best route is to have two circuits one for each. > > Or, has anyone used a frequency to voltage converter and then read into a PIC > and used a LUT? I'm trying to get this to a single chip solution if I can. > > comments? John Payson, Mike Keitz, myself and perhaps others have attempted DTMF decoding with a PIC. John and I are unable to give you any software (we did not work together) because of the terms under which we developed our software. However, you can get the gist of the theory from here: http://www.interstice.com/~sdattalo/technical/theory/dtmf.html It describes the technique I used. And if I recall correctly, it's the technique Mike used too. I don't know about Mike or John, but I found that a 'single-chip' solution was not possible. In other words, I had to have a little analog signal processing. However, for your application it sounds like (from an earlier post) that you're willing to accept some errors. If so then the 'analog signal processing' could be just a simple high pass (passive) filter. Scott

Brurr-Brown makes a VFC32 chip which works excelently as either v-f or f-v. ------------------------------------------------------------------------------ A member of the PI-100 Club: 3.1415926535897932384626433832795028841971693993751 058209749445923078164062862089986280348253421170679 On Fri, 18 Dec 1998, Andy Kunz wrote: {Quote hidden}> >Or, has anyone used a frequency to voltage converter and then read into a PIC > >and used a LUT? I'm trying to get this to a single chip solution if I can. > > I'm looking for a decent F-to-V circuit. > > Anybody have one I can copy? > > Andy > > ================================================================== > Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA > ================================================================== >

Hi, theoretically, there is another interesting approach: one can build a Fourier spectrum analyser and then evaluate the power spectra. In the Embedded Controller Handbook (AN's) there is one example for 17C42. Such way, both dual-tone and single-tone can be reliable detected... I did not tried it but maybe I will do it in the future... Imre On Fri, 18 Dec 1998, Harrison Cooper wrote: {Quote hidden}> Well, searching thru my saved mail, and URL's, I've found a couple of DTMF > generators using the PIC, but it seems most of the decoders are using > the M8870 DTMF decoders. But I am sure there was some discussion on using a > PIC directly for this. Maybe not. Since I want both DTMF and single tone, > maybe the best route is to have two circuits one for each. > > Or, has anyone used a frequency to voltage converter and then read into a PIC > and used a LUT? I'm trying to get this to a single chip solution if I can. > > comments? > >

On Sat, 19 Dec 1998, Dr. Imre Bartfai wrote: > Hi, > theoretically, there is another interesting approach: > one can build a Fourier spectrum analyser and then evaluate the power > spectra. In the Embedded Controller Handbook (AN's) there is one example > for 17C42. Such way, both dual-tone and single-tone can be reliable > detected... I did not tried it but maybe I will do it in the future... > Imre In theory there's not much difference between theory and practice; in practice though, there is... :) I would recommend against this method for two reasons: 1) FFT computation in real time for a pic sampling signals at a rate up to 3 or 4kHz is (extremely) difficult. Perhaps the hardware multiplier on the 17C core speeds things up quite a bit. However, I'd be really surprised if it was done (or doable). 2) One of the 'problems' with FFT's is that the frequency bins are evenly spaced. However, for DTMF signals they're logarithmically spaced. Ideally, you'd like the FFT bins to lie directly on top of the DTMF frequencies. At reasonable sample rates it's impossible to achieve this. Additionally, the FFT produces much more information than you need. You're really just interested in the magnitude of the DTMF tones and the total energy in the signal. The FFT decomposes the signal into many, many frequency bins. You may wish to look at Analog Devices' DSP apnotes. They discuss this more thoroughly. Their solution is to compute 16 DFT's: 8 at the DTMF tones and 8 at the 2nd harmonic of DTMF tones. The second set presumably allows one to ascertain whether a 'pure' DTMF tone was present or whether the DTMF energy is a harmonic of some lower frequency present signal. On comp.dsp I have read that people have found this technique to be flawed. But I haven't tried it my self (at least not as described here). Scott

At 09:12 12/19/98 -0800, Scott Dattalo wrote: >You may wish to look at Analog Devices' DSP apnotes. They discuss this >more thoroughly. Their solution is to compute 16 DFT's: 8 at the DTMF >tones and 8 at the 2nd harmonic of DTMF tones. The second set presumably >allows one to ascertain whether a 'pure' DTMF tone was present or whether >the DTMF energy is a harmonic of some lower frequency present signal. On >comp.dsp I have read that people have found this technique to be flawed. >But I haven't tried it my self (at least not as described here). if the goal is to find out whether a certain filtered band is a base frequency of a tone or a harmonic of some other, the second harmonic is not very suitable, because it would be part of both, whether the filtered frequency is a base frequency or a power of two harmonic of some other frequency -- unless you can make sure the DTMF tones are pure sine. i'd guess that is rarely the case. ge

> Hi, > theoretically, there is another interesting approach: > one can build a Fourier spectrum analyser and then evaluate the power > spectra. In the Embedded Controller Handbook (AN's) there is one example > for 17C42. Such way, both dual-tone and single-tone can be reliable > detected... I did not tried it but maybe I will do it in the future... > Imre |I would recommend against this method for two reasons: |1) FFT computation in real time for a pic sampling signals at a rate up to |3 or 4kHz is (extremely) difficult. |2) One of the 'problems' with FFT's is that the frequency bins are evenly |spaced. However, for DTMF signals they're logarithmically spaced. Ideally, |you'd like the FFT bins to lie directly on top of the DTMF frequencies. Using a pair of DFT's (discrete Fourier transform) it is possible to look for a the existence of any single frequency in a signal which may also contain other stuff. Performing /N/ DFT's will take /N/ times as long as performing one, but if you only need a few, that's not a problem. An FFT is primarily useful in cases where you want to decompose a "sample" [e.g. 256 A/D readings taken at an 8192Hz rate] of a signal into "all" its constituent frequencies; the result of the FFT will be equivalent to the result of running a pair of DFT's on every frequency which divides into the sample length. The FFT will be much faster than running all the DFT's individually, but will take much longer than running a small handful of DFT's. Note that when running DFT computations, you may (and almost certainly should) perform the multiply-accumulate-sum operations on the fly. Beyond that, there are some gotchas to be aware of: [1] Multiplies on anything short of a 17Cxx part are going to be expensive. Rather than using a sinewave as the integrating waveform, you may use a simpler step wave. My favorite wave- form for this purpose is 1/3 period at +1, 1/6 at zero, 1/3 at -1, and 1/6 at zero. This waveform has no harmonic content at multiples of 2 or 3, and its harmonic content drops prop- ortional to frequency (the first non-zero harmonic is the fifth harmonic at 1/5 strength). [2] Another way to simplify your life is to use a zero-crossing detector rather than an A/D. I've done a DTMF defector using just the 16C622's comparator and it worked decently. Using an ADC input is better, but it's more expensive and requires more math. [3] Rather than running DFT's on chunks of, e.g. 1/32 of a second and having them independent, it's better to use sliding-window filtering and averaging techniques to combine them. This is especially important if the frequency is not a multiple of the sample-group length. |You may wish to look at Analog Devices' DSP apnotes. They discuss this |more thoroughly. Their solution is to compute 16 DFT's: 8 at the DTMF |tones and 8 at the 2nd harmonic of DTMF tones. The second set presumably |allows one to ascertain whether a 'pure' DTMF tone was present or whether |the DTMF energy is a harmonic of some lower frequency present signal. On |comp.dsp I have read that people have found this technique to be flawed. |But I haven't tried it my self (at least not as described here). A better approach (which I've done on a DSP--it'd be beyond the abilities of a PIC) is to have two detectors at each frequency, one of which is more discriminating than the other. Then require that the more discriminating detector pickup be at least a certain fraction of the less detecting one. This results in negative sen- sitivity near the frequency of interest, but a very narrow spike in sensitivity at the desired frequency. Since more discriminating filters have slower response times, it's necessary to slow down the response of the less discriminating filter to match. Using these techniques, though, can produce outstanding DTMF detection and false-tone rejection.