1. Background
The telephone lines were designed to communicate voice signals. Most of the energy of the human voice lies in the 20Hz to about 4kHz. The telephone set (as well as the cell phone) filters out signals outside this range. You will test this filtering by applying tones of different frequencies on a handset and listening to them on the other side of the telephone call.
2. Generating the tones
In order to generate tones of different frequencies, we will sample a sinusoidal signal and save it as an audio (au) file. The au file can then be played and heard on a speaker. We will use a sampling frequency of 11025 Hz. This is a standard voice/music sampling frequency and it is ¼ of the sampling frequency of CD music.
Fs = 11025 ;
t=0: 1/Fs: 4; time samples every 1/Fs seconds and for a duration of 4 seconds.
x1 = 0.9 * cos(2*pi*1000*t); samples of a sinusoid of frequency 1000Hz.
auwrite(x1, Fs, ’tone1kHz.au’); this will create an au file called tone1kHz.au. We multiply by 0.9 to ensure that the samples are in (-1,1).
Generate similar tones but with frequencies 2, 3, 4.5 and 5 kHz. Create au files for each tone and name them appropriately.
3. Testing the Filter
The tone files will be created in the “working folder”. Type: pwd in Matlab to find out the working folder. Now open the working folder from Windows Explorer and look for the au files you created. Play the files and make sure you hear tones from the speakers. Let one student be in a room and another in a different room. Make a phone call to the other student and play the tones one at a time. Ask your colleague whether she/he can hear the tones over the phone. Exchange your roles and repeat the experiment. Write down your observations.
Now talk while you play the tones. Ask your colleague over the phone whether she/he hears your voice and the tones or hears only the tones or only your voice.