AN2384 Freescale Semiconductor / Motorola, AN2384 Datasheet - Page 16

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

DTMF Detector on StarCore
The second experiment tests the influence of the signal power and the noise (see Figure 9). The other
characteristics (twist, power of the tone, and so on) are the same as in the previous experiment, but the signal power
is now reduced to –12 dBm0, SNR = 10dB. The average of each part is:
These results are approximately the same for all keys, and if there is less twist, the transitions are usually faster.
These experiments demonstrate the sensitivity of duration as a function of other factors, such as noise, low power
level, twist, and so on. Nevertheless, theoretical and practical duration thresholds also differ because of C
implementation effects, particularly 16-bit rounding. This is why previously calculated duration thresholds must be
further adjusted by looking at the detected duration of valid and non-valid digits with the final C program.
Figure 10 shows the duration of keys detected in Mitel test 6 [7]. This test consists of pulses with a duration that
starts at 49 ms and gradually reduces to 10 ms. The measured duration (vertical axis) is calculated according to the
number of times a digit is detected, whereas the real duration (in ms) of the sound is 49 – key index / 10. This test
shows that a significant variance in duration must be accounted for in the C implementation. The duration,
represented by the number of samples at 4 KHz in Figure 7 (floating-point version of the detector), is still correct
on the average. For instance, at the beginning, 200 samples per key are reported (a 49 ms pulse corresponds to 49
ms
even for keys that must be detected.
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4 KHz = 196 samples). However, the variation (peak-to-peak deviation of the line) often reaches 20 samples,
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Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core, Rev. 1
Detected key: 121 samples
Pause: 119 samples
Transitions: 14 samples
Detected Key: 89 Samples
Pause: 85 Samples
Transitions: 13 Samples
Duration of the Transitions
Time Duration: 23 ms
Figure 9. Influence of Signal Power and Noise
Duration of the Transitions
Detected Key: 153 Samples
Pause: 153 Samples
Transitions: 15 Samples
Time Duration: 40 ms
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