DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 344

TABLE 10.1 DTMF Encoding

Frequencies 1209 Hz 1336 Hz 1477 Hz

697 Hz 1 2 3

770 Hz 4 5 6

852 Hz 7 8 9

941 Hz * 0 #

on the C6416 DSK (can be transported readily to the C6713 DSK). The complete
executable files for all four subprojects are included on the CD.
A DTMF signal consists of two sinusoidal signals: one from a group (row) of four
low frequencies and the other from a group (column) of three high frequencies. This
is illustrated in Table 10.1. When a key is pressed from a telephone, a DTMF signal
is generated. For example, pressing button 6 generates a tone consisting of the sum-
mation of the two tones with frequencies of 770 and 1477 Hz, as shown in Table 10.1.
For easier detection, these frequencies are chosen so that the sum or difference of
any two frequencies does not equal that of any of the other frequencies.
Various schemes can be used to decode DTMF signals:

1. A correlation scheme, as described in this first mini-project. An RTDX option
in the second mini-project provides a PC-DSK interface displaying the dialed
(received) numbers on the PC screen.
2. The FFT (or the DFT) to detect the signals corresponding to the DTMF tones.
The FFT is used in the third mini-project to estimate the weights associated
with the seven frequencies.
3. Use of a bank of FIR filters so that each filter passes only one of the fre-
quencies. The average power at the output of two of these filters should be
larger than that at the other outputs, yielding the corresponding DTMF tone
(not used in this project).
4. Use of Goertzel's algorithm [2,22,28,29] in lieu of the FFT or DFT since only
two frequencies need be detected/selected. This method (see Appendix F) can
be more efficient than the FFT when a “small” number of spectrum points are
required rather than the entire spectrum (implemented in Appendix H with
the C6416 DSK and can be readily transported to the C6713 DSK).

Each DTMF signal can be represented as

() = ( ( ) + ( ) )

ut A sin wj t + sin wj t +

1 1 2 2

j 2
are unknown phases. Frequency f 1 is one of the following frequencies: 697, 770, 852,
or 941 Hz; and frequency f 2 is one of the following frequencies: 1209, 1336, or 1477
Hz [30,31].

where w 1 and w 2 are the two frequencies that need to be determined, and j 1 and