LAB 10:Encoding and Decoding Touch-Tone Signals

EE224 section C JINGXUAN SUN

4/15/2013

Introduction: In this lab, we will introduce a practical application where

sinusoidal signals are used to transmit information: a touch-one dialer. then, we

will design and implement a bandpass FIR filer in matlab in order to do the

decoding automatically.

3.1 DTMF Dial Function

a) In this part, we are going to write a function to implement DTMF dialer based

on the frequency table defined in fig1.The code is as follows:

3.2 Simple Bandpass Filter Design

In this part we are going to generate a bandpass filter with the center frequency of

wc =0.2π also the length of the filter should be equal to 51 Figure out the value of β

so that the maximum valu of the frequency response magnitude will be one. So the

code looks like the following:

The frequency response looks as follows:

Then, we can find out that the region which got passed on y axis is larger than

0.707, then, we use the matlab function to locate the frequency, the code looks as

follows:

And the result BOM = 0.068π.

4.1 Simple Bandpass Filter Design: dtmfdesign.m

In this part, we are going to find out the coefficient factors that equals to 1. So we can find out the max

value first and using 1 divide by the max value and multiply it with the max value to get factor of 1. The

code looks as follows:

c) In this part, we should figure out how to choose L, which is the length of the

filters. We need to be able to find the min value of L. According to the frequency

table, it must be between the 697Hz and 770Hz and if L we found within this

region then, it will automatically fit for another frequencies as well.

d and e) in this part we will generate the eight(scaled) bandpass filters with L = 40

and fs = 8000 and we are going to find out what’s the differences between L = 40

and L = 80, the code looks as follows:

And the output looks as follows:

4.2 A Scoring Function: dtmfscore.m

The final objective is decoding—a process that requires a binary decision on the

presence or absence of the individual tones. So in this part, we need to finish he

dtmfscore function based on the skeleton given in the instruction. The code looks

as follows:

4.3 DTMF Decode Function: dtmfrun.m

The DTMF decoding function, dtmfrun must use information from dtmfscore to

determine which

key was pressed based on an input DTMF tone. The finished code looks as

follows:

\

4.4 Telephone Numbers

The functions dtmfdial.m and dtmfrun.m can be used to test the entire DTMF

system as shown in the instruction, so testing the DTMF system code looks as

follows:

Code for dtmdail:

Code for dtmcut:

Then, we dial the phone number:ABCD*#0123456789

The code looks as follows:

The spectrogram of the signal looks as follows:

The result between Magnitude and N

The sound of the dial can be heard clearly as we dial the number

The code of the demonstration number looks as follows:

The spectrogram of the signal looks as follows:

The sound of the dial can be heard clearly, and the result shows as

ans=407*89132#BAD C

Conclusion:

In this lab, we learned how to filter out different parts of a signal and how to

acutally apply that in to a real application like a dual tone dialer to use bandpass

filter to filter out frequencies, the most important thing in this lab is how to control

the width of the central frequency—the passband.