EECS20N: Signals and Systems

Filtering for Tone Detection

Recapping, we are interested in detecting whether a 2100 Hz answer tone is present. We have calculated an impulse response that seems promising, and can apply it to the answer tone using the Matlab code below: 

cd d:\eal\eecs20\sounds
x = auread('modemAnswerTone');
sound(x);
n = [0:1/8000:1/80];
h = cos(n*2*pi*2100/8000)/50;
y = conv(x,h);
sound(y);

The result before and after filtering can be heard below:

If you had a java-enabled browser, you would see an applet here.

If the answering device is not a modem, then the result will be very different:

If you had a java-enabled browser, you would see an applet here.