Command Reference Guide

3Com Router 5000 Family and Router 6000 Family dtmf threshold ● 145
Command Reference
Figure 1 Spectrum of Keys
The tone of each telephone digit is composed of two single-frequency tones. For
example, the tone of the digit 1 is compounded by two sine wave signal tones of 697
Hz and 1209 Hz. A valid key tone must last at least 45 milliseconds and have an
inter-digit delay of 23 milliseconds. Refer to ITU-T Q.24 recommendation for full
information.
Figure 2 Spectrum of Key 1
Figure 1-2 illustrates the spectrum of key 1. Compared with other frequency points,
the energy levels at frequency points of 697 Hz and 1209 Hz are relatively greater.
The underlying layer DSP regards the frequencies of 1209 Hz, 1336 Hz, 1477 Hz, and
1633 Hz as column frequency group and the frequencies of 697 Hz, 770 Hz, 852 Hz,
and 941Hz as row frequency group. Each DTMF key tone is composed of one column
frequency and one row frequency. The DSP module determines whether the input
voice signal is a valid DTMF digit by its energy at the above eight frequencies and their
double frequencies. The maximum energy in the row frequency group is ROWMAX,
its double energy is ROW2nd; the maximum energy in the column frequency group is
COLMAX and its double energy is COL2nd.