Reference no: EM133382189

Design a DTMF signal decoder that can take a segment of audio (sound) samples containing a single key touch tone and correctly determine the key that was pressed to produce the sound; even though the DTMF tones were produced by an inexpensive telephone handset (so the signals have harmonic-producing amplitude clipping) and have picked up a considerable amount of noise in the process of transmission. Your design should be able to function with different time durations allowed for the individual touch tone signals in a dial sequence (with each single tone allowed to vary in length from 0.2-3.0 seconds). You are to use DFT/FFT signal processing techniques for your tone decoder, and some other logic to decide what key was pressed based on the tone frequencies present; all implemented in Matlab. You should not, however, use signal correlation for this project, as that will be the subject of next week's project. Here, you should analyze the frequency content of the signal, and determine the row and column frequencies present in the signal by observing the signal's spectral content.
Your Matlab program (a function m-file) should accept an audio signal sample array name as an input parameter, and return a text variable with the key value [ as in return_value = '2'; ]. Your program should display the spectrum of the signal you are decoding (perhaps using your plot_DFT_mag() function. Using the text () Matlab function, your program should also display the decoded key character somewhere on the same DFT spectrum plot.
The audio signals for both uncorrupted and noisy signals are available in the touchtones.mat file posted on Canvas. These signals are all 0.5 seconds in length and sampled at 8 KHz (the standard sampling rate for telephone voice signals). You should verify that your DTMF decoder works for the audio files for all of the individual keys; and that it is robust enough to operate properly even with the noisy signals. You should experiment with shorter and longer versions of these test signals, to be sure that your decoder works for any of the allowed touchtone signal durations (0.2 3.0 seconds).