AN2384 Freescale Semiconductor / Motorola, AN2384 Datasheet - Page 17

• Manufacturer Part Number: AN2384
• Description: Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core
• Manufacturer: Freescale Semiconductor / Motorola

Considering that a shorter signal duration threshold may decrease the performance in talk-off tests, the final value
is calculated to keep the right margin below the upper duration limit (40 ms, 160 samples) and above the lower one
(23 ms, 92 samples). The interruption duration is adjusted in the same way. However, the pause duration can be
more tolerant since its reduction does not have undesirable influence on other parts of the detector (see Table 5).
4.2.5 Digit Processing Unit
The ITU-T Q.24 recommendations for signal and pause duration are as follows:
To implement these requirements, the following variables are used:
Freescale Semiconductor
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Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core, Rev. 1
Number of Samples (4 KHz)
The signal must be at least 40 ms long to be detected as a key.
Between two detected keys, there must be a non-valid signal that lasts at least 40 ms (that is, a pause).
Signals with a total of 10 ms or less interrupt duration (silence, for example) should be detected.
new_digit. The digit just found by the sample processing unit.
previous_digit. Previous loop digit.
digit. Digit recorded before previous_digit and having a significant duration.
i_min. Index corresponding to the beginning of the digit.
i_edge. Index corresponding to the end of the digit, the beginning of previous_digit, and the last
detected edge.
Theoretical
Practical
Table 5. Theoretical and Practical Durations
Figure 10. Duration of Detected Keys
Pause_duration
160
100
Signal_duration
160
110
Interruption_duration
DTMF Detector on StarCore
40
50
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