CH10 DSP System Design,Dual ToneMulti-Frequency (DTMF) Signalin

CH10 DSP System Design,Dual ToneMulti-Frequency (DTMF) Signalin

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C H A P T E R 10 DSP System Design: Dual Tone Multi-Frequency (DTMF) Signaling In this and the next two chapters, three DSP system project examples are discussed and built using the LabVIEW hybrid programming approach. These examples show how relatively complex DSP systems can be devised in a relatively short amount of time by deploying hybrid programming. In the order of complexity, these examples consist of dual tone multi-frequency signaling, software-defined radio, and cochlear implant simulator. Dual tone multi-frequency (DTMF) signaling is extensively used in voice commu- nication applications such as voice mail and telephone banking. A DTMF signal is made up of two tones selected from a low and a high tone group. Each pair of tones contains one frequency from the low group (697 Hz, 770 Hz, 852 Hz, 941 Hz) and one frequency from the high group (1209 Hz, 1336 Hz, 1477 Hz). Figure 10-1 shows the frequencies allocated to the telephone pad push buttons. 1209 Hz 1336 Hz 1477 Hz 697 Hz 770 Hz 852 Hz 941 Hz 2 3 6 5 4 7 * 8 0 # 9 1 Figure 10-1: Keypad and allocated frequencies. 265
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The implementation of the DTMF receiver system is normally done by using the Goertzel algorithm [ 1 ]. This algorithm is more efficient than the FFT algorithm for DTMF detection both in terms of the number of operations and amount of memory usage. Furthermore, unlike FFT, it does not require access to the entire data frame, leading to faster execution. As indicated in Figure 10-2 , seven Goertzel filters are used here in parallel to form a DTMF detection system. Each Goertzel filter is designed to detect a DTMF tone. The output from each filter is squared and fed into a threshold detector, where the strongest signals from the low and high frequency groups are selected to identify a pressed digit on the keypad. The difference equations of a second-order Goertzel filter, as illustrated in Figure 10-3 , are given by v k ½ n ¼ 2 cos 2 p k N ! v k ½ n ± 1 ± v k ½ n ± 2 þ x ½ n ; n ¼ 0 ; 1 ; . . . ; N (10.1) y k ½ n ¼ v k ½ n ± W k N v k ½ n ± 1 (10.2) Digitized Signal Input Detector for 697 Hz Goertzel Filter ( . ) 2 Threshold Detector Detector for 770 Hz Detector for 852 Hz Detector for 941 Hz Detector for 1209 Hz Detector for 1336 Hz Detector for 1477 Hz Decision Logic Figure 10-2: DTMF system using Goertzel algorithm. 266 Chapter 10
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where x n ½ denotes input, y k n ½ denotes output, v k n ½ denotes intermediate output, the subscript k indicates frequency bin, N is the DFT size, and W k N ¼ exp ± j 2 p k N ± ² . The initial conditions are assumed to be zero, i.e., v k ± 1 ½ ¼ v k ± 2 ½ ¼ 0. Considering that only the magnitude of the signal is required for the DTMF tone detection, the following equation is used to generate magnitude squared outputs: y k N ½ j j 2 ¼ v 2 k N ½ þ v 2 k N ± 1 ½ ± 2 cos 2 p k N ± ² v k N ½ v k N
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