AN2384 Freescale Semiconductor / Motorola, AN2384 Datasheet - Page 6

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

Two-Component Tone Detector for DTMF
Our design offers the following advantages over traditional detectors based on Fourier transforms:
3
Dual tone multi-frequency (DTMF) is a set of standard signaling tones adopted by telephone companies. A pressed
key on telephone equipment generates a dual-frequency tone composed of a low frequency component and a high
frequency component. The frequencies used for DTMF are not harmonically related, and it is difficult for a voice
signal to imitate the DTMF tone. This choice minimizes the likelihood of spurious signals being accepted as valid
DTMF tones. Frequency separation between consecutive tones inside each group of frequencies is typically 10
percent. DTMF signaling is a popular example of a multi-frequency tone. A DTMF key is defined by two
frequencies, one for the low group (ƒ
to estimate the two frequencies that are present in the signal:
Next, find the closest reference point of a valid key (ƒ
basis. Figure 2 illustrates this concept; each cross represents the location of a reference key.
6
•
•
•
•
•
•
Two-Component Tone Detector for DTMF
Generic Tone Detection Using Teager-Kaiser Energy Operators on the StarCore SC140 Core, Rev. 1
x(n)
Easy to reuse, fine-tune, and maintain.
Efficiently implemented on parallel architectures, such as the SC140 core.
Requires a relatively small look-up table.
Can be used under noisy conditions.
Provides a low false-detection rate—for example, when a voice signal is incorrectly detected as a
signaling tone.
Very good time and frequency resolutions.
H
H
Figure 1. Structure of the Multi-Component Detector
1
N
(m)
(m)
(z)
(z)
L
) and one for the high group (ƒ
m, r
AM-FM Demodulator N
AM-FM Demodulator 1
L
ˆ
, ƒ
L
,
H
ˆ
). This identification process proceeds on a per-sample
H
H
). The principle of the detection algorithm is
Tone Detection
Decision Logic
Mechanism
Self-Tuning
m
, 1
N
,
Freescale Semiconductor
1
N
m
det