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Hi all,

I've been thinking about DTMF detection in software using a PIC.
Has anyone tried to implement this using one bit samples and a DFT
calculation?

I've implemented a little program in C to simulate the calculations
that could be performed by a PIC. The DFT routine is very simple to
implement because you don't need any multiplication when the samples
are only one bit.

The DFT calculation is a bit simplified since it does not perform a
square and squareroot in the last addition. I use absolute values and
skip the root. This should work fine though.

The results are pretty good. I was surprised to see that the DFT
calculated amplitude was about 5-10 times higher than other frequency
components with a noise amplitude that's half the DTMF tone
amplitude.

This is the output of my C program which is attached:

Crystal frequency:                  4000000.00
Instruction cycles between samples: 278
Sampling frequency:                 3597.12
Entries in sine table:              256

                 T0    T1    T2    T3    T4    T5    T6    T7
Tones 0 and 4: 14450  1371   810  1201 14981  1055   405   822
Tones 0 and 5: 13212  1627  1946   981   243 16503   715   516
Tones 0 and 6: 12650  2461  1156   351   957  5781 16911  1284
Tones 0 and 7: 12496   659  1574   743   177   867  1159 13150
Tones 1 and 4:   576 15003  2316  2349 13713  1143  1055  1128
Tones 1 and 5:   558 13071  1714  1093   507 19749   573   820
Tones 1 and 6:  1738 13829  1410  1041   769  1315 13499  1040
Tones 1 and 7:  2372 12375   934   529  1527   931   359 14726
Tones 2 and 4:  2764   961 17674   855 15585  1799  1109   488
Tones 2 and 5:  1904   467 15996  2085  2287 19371  1703  2152
Tones 2 and 6:   774  1135 18868  1667  2293   881 13693  1210
Tones 2 and 7:  1350  2643 16788  1467   841  1551   869 13496
Tones 3 and 4:  1318   413  1018 18973 15057  1365  4839   860
Tones 3 and 5:   246  2059  2242 18121  1213 17371  1491   872
Tones 3 and 6:  1054   663  1198 16785   937   645 14039  1444
Tones 3 and 7:  1340  2007  1568 17247  1639   963  1043 13312

Comments? Any problems with this approach? Do you think it will work
in practise?

Regards,
Danjel McGougan
Student at Linkoping Institute of Technology, Sweden.

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     File:  dft3.cpp
     Date:  25 Mar 1998, 20:31
     Size:  3345 bytes.
     Type:  Program-source

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