AN2384 Freescale Semiconductor / Motorola, AN2384 Datasheet - Page 12

• 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
4.2.2 Frequency Tolerance
According to the Q.24 standard, a frequency tone must be accepted if it deviates less than 1.5 percent from the
nominal frequency, and it must be rejected if it deviates more than 3.5 percent. These constraints bound the area
around the ideal values within which the frequency should be interpreted as valid, represented by a cross in Figure
2. Table 2 shows the required acceptance and rejection bands for the various frequencies.
As described in Section 1, Tone Detection Basics, the frequencies are implicitly defined through
where the sampling rate is 4 KHz (that is, half of the original sampling rate because every other sample from the 8
KHz input vector is processed). The reference values of
distance between the tone frequency and the accepted one, and
tone frequency and the rejected one.
The decisive values of the potential keys must be taken into the interval [
key is declared valid if its distance to the closest point for the low frequency and for the high frequency is smaller
than their corresponding decisive values.
12
Tone Frequency (Hz)
0.20993041992188
0.12493896484375
0.05307006835938
0.00857543945313
0.10397338867188
0.25363159179688
0.46389770507813
0.70288085937500
Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core, Rev. 1
1209
1336
1477
1633
697
770
852
941
Bandwidth (Hz)
0.2235;0.1967
0.1372;0.1131
0.0625;0.0444
0.0873;0.1219
0.2268;0.2814
0.4292;0.4987
0.6672;0.7374
0.0131;0.005
Acceptance
Acceptance
Bandwidth
Table 2. Required Acceptance and Rejection Bands
(< 1.5%)
±10.5
±11.6
±12.8
±14.1
±18.1
±20.0
±22.2
±24.5
Table 3. Reference Values of
[1190.9;1227.1]
[1454.8;1499.2]
[1608.5;1957.5]
Bandwidth (Hz)
[686.5;707.5]
[758.4;781.6]
[839.2;864.8]
[926.9;955.1]
[1316;1356]
Acceptance
0.0136
0.0123
0.0094
0.0045
0.0179
0.0278
0.0348
0.0357
acc
are tabulated in Table 3.
rej
represents the minimum distance between the
Bandwidth (Hz)
0.1543;0.0983
0.0207;0.0017
0.6179;0.7813
W
0.242;0.1796
0.076;0.0341
0.067;0.1476
0.3837;0.545
acc
0.1926;0.32
Bandwidth
Rejection
Rejection
(> 3.5%)
,
±24.4
±27.0
±29.8
±32.9
±42.3
±46.8
±51.7
±57.2
rej
], which is reported in Table 4. A
acc
represents the maximum
Freescale Semiconductor
Rejection Bandwidth
< 1166.7; > 1251.3
< 1289.2; > 1382.8
< 1425.3; > 1528.7
< 1575.8; > 1690.2
< 672.6; > 721.4
< 822.2; > 881.8
< 908.1; > 973.9
< 743; > 797
0.0303
0.0266
0.0069
0.0802
0.0784
0.019
0.037
0.061
(Hz)
= cos
rej
2
( ),