AN2384 Freescale Semiconductor / Motorola, AN2384 Datasheet - Page 13

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

4.2.3 Power Level Difference Between Frequencies
The ITU-T Q.24 standard recommends that the high group frequency power level may be up to 4 dB larger or 8 dB
smaller than the low group frequency power level. These conditions can be represented as follows:
or in a single inequality:
P
DTMF key with power ratios (also referred to as twist) within that range must be detected.
In the algorithm based on energy operators, the envelope estimate of the signal is calculated for the low and for the
high frequencies. They correspond to the variables dlf2 and dhf2, respectively, in the code. The ratio dhf 2/ dlf2, if
dhf2 < dlf2, (dlf2/dhf2, if dlf2 < dhf2) is compared to a proper threshold so that the Q.24 recommendation is
respected. To take into account noise, limited precision, and polynomial approximation error, the actual thresholds
used in the code are determined experimentally, yielding the following:
The plots shown in Figure 6 illustrate some of the test results. Beneath the first graph, the two extracted curves are
shown with a more precise scale to find those two bounds. However, the Q.24 standard does not specify a rule for
twists outside the recommended range, which allows for different implementations. Q.24 specifies only that any
DTMF signal with a twist inside that range must be detected. Therefore, the twist test is disabled in the optimized
implementation of the DTMF detection.
Freescale Semiconductor
H
is the high group frequency power level and P
Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core, Rev. 1
Tone Frequency (Hz)
P
P
H
H
(dB) < P
(dB) > P
1209
1336
1477
1633
697
770
852
941
L
L
(dB) + 4dB
(dB) – 8dB
dlf2
dhf2
dhf2
dlf2
Table 4. Interval [
0.158 <
> 0.398101806640625
> 0.158477783203125
L
is the low group frequency power level. In other words, any
Decisive Value
P
P
P
P
H
H
L
0x2A9
0x22A
H
0x327
0x400
0x600
0x800
0x900
0xE0
< P
> P
L
L
< 2.512
acc
2.512
0.158
,
rej
]
(0x1449
(0x32F5
0.024627685546875
0.020782470703125
0.01690673828125
0.0068359375
0.03125
0.046875
0.0625
0.0703125
P
P
P
P
Decimal Value
H
L
L
H
DTMF Detector on StarCore
> 0.398
> 0.158
13