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group and a high-frequency group (see Table 5.2). The DTMF signal
is an algebraic sum of two tone frequencies, one tone from each
frequency group (see Figs. 5.13 through 5.15). If we do the math,
we see that there are 4 4 16 possible combinations.
Table 5.2
Pin# Row# or column# Frequency, Hz

Low-frequency group
R1 Row 0 697
R2 Row 1 770
R3 Row 2 852
R4 Row 3 941
High-frequency group

C1 Column 0 1209
C2 Column 1 1336
C3 Column 2 1477
C4 Column 3 1633

71

5.13 Low-frequency tone waveform

5.14 High-frequency tone waveform

5.15 Algebraic sum of low and
high frequencies (DTMF)

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