Lab3_w2010 - CARLETON UNIVERSITY Department of Systems and...

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CARLETON UNIVERSITY Department of Systems and Computer Engineering SYSC 4405 Lab #3: DTMF Decoding – Part I: Bandpass Filter Design 1 Introduction This lab introduces a practical application where sinusoidal signals are used to transmit information: a touchtone dialer. One approach to decode touch-tone sounds is to use bandpass filters to extract the infor- mation encoded in the waveforms. The goal of this lab is to design and implement bandpass filters in M AT- LAB , and then in the next lab to use these designed filters and implement them on the c6713 DSK to perform the touch-tone decoding. Be sure to keep your filter designs and M ATLAB code from this lab for the next lab. In the experiments of this lab, you will use filter() to implement filters and freqz() to obtain the filter’s frequency response. As a result, you should learn how to characterize a filter by knowing how it reacts to different frequency components in the input. The lab itself starts with background information in Sec. 2 and deals with synthesizing and decoding dual tone multi-frequency signals used in dialing a touch tone telephone. You should read through the entire lab BEFORE your scheduled lab time so that you know what to expect and can move quickly through the lab manual. If you cannot complete all of the lab during the scheduled time, you should show your results to the TAs at the beginning of your next lab session. 2 Lab Background 2.1 Telephone Touch Tone 1 Dialing Telephone touch pads generate dual tone multi-frequency (DTMF) signals to dial a telephone. When any key is pressed, the tones of the corresponding column and row (in Fig. 1) are generated and summed, hence the name dual tone. As an example, pressing the 5 key generates a signal containing the sum of two tones at 770 Hz and 1336 Hz. The frequencies in Fig. 1 were chosen (by the design engineers) to avoid harmonics. No frequency is an integer multiple of another, the difference between any two frequencies does not equal any of the frequen- cies, and the sum of any two frequencies does not equal any of the frequencies. This makes it easier to detect exactly which tones are present in the dial signal in the presence of non-linear line distortions. 2.2 DTMF Decoding There are several steps to decoding a DTMF signal: 1. Divide the signal into shorter time segments representing individual key presses. 2. Filter the individual segments to extract the possible frequency components. Bandpass filters can be used to isolate sinusoidal components. 3. Determine which two frequency components are present in each time segment by measuring the size of the output signal from all of the bandpass filters. 1. 1 Touch Tone is a registered trademark
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4. Determine which key was pressed, 0–9 , * , # , or A–D for extended DTMF, by converting frequency pairs back into key names according to Fig. 1.
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Lab3_w2010 - CARLETON UNIVERSITY Department of Systems and...

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