September 2008
Intermediate to advanced
550 pages
18h 10m
English
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7.3 DTMF DETECTION
DTMF detection identifies the digit from a two-tone combination. As per specifications given in Section 7.1, DTMF detection has to take care of several constraints in the process of digit detection. The main constraints from the signal parameters are as follows:
- Low-and high-frequency tones are designed as fL and fH and their corresponding amplitudes as AL and AH. The amplitudes levels of AL and AH are the same in an ideal case, but some phones may generate tones with twist or asymmetry, which means that the amplitudes of fL and fH are not the same. If the amplitude of fL is greater than fH, it is called a reverse or negative twist. If the amplitude of fH is greater than fL, it is called a forward twist or positive twist.
Table 7.3. DTMF Tones and Cos and Sin Coefficients Used in Tone Generation
Figure 7.2. Digit-1 tone generation example for 320 samples, 40 ms. (a) Row tone at 697 Hz at unity amplitude. (b) Column tone at 1209 Hz at unity amplitude. (c) Combined tones for digit-1.
- Frequency drift creates different filter responses. Frequency drift along with twist creates additional issues for correct tone detection.
- The DTMF digit minimum duration is greater than 23 ms (typical 50 ms), and the maximum can last for several seconds if the key is held for a long ...
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