Page 69 - Basics of MATLAB and Beyond

audiotool. On a PC the volume can be controlled from the “properties”
panel of the sound recorder.
You can invoke a sound demo GUI by typing xpsound. This GUI
includes these bird chirps plus a few other sounds, three different display
types, a volume slider, and a play button.

20 Time-Frequency Analysis

Signals, such as the sound data of the previous section, often consist of
time series data with a time-varying frequency content. The specgram
function allows you to analyse this kind of time-frequency data. As
an example we generate a frequency modulated carrier and analyse its
frequency variation with time. The modulate and vco function can be
used to produce signals with many different modulation types. 7 We
begin with a linear frequency sweep from 0 to 500 Hz sampled at 1 kHz.
First, you must prepare a frequency control vector, which is normalised
between −1 and 1, where −1 corresponds to the minimum frequency and
1 corresponds to the maximum frequency. Here we use a linear frequency
control and 8192 points:

x = linspace(-1,1,8192);
Now use the vco function (in the Signal Processing Toolbox) to convert
this to a frequency modulated signal:

Fs = 1000;
y = vco(x,[0 500],Fs);
The input vector [0 500] says that our frequency sweep will go from
0 Hz to 500 Hz and the sampling frequency is Fs = 1000 Hz. The first
thousand points of this signal reveal the steady increase in frequency:
7 In fact what we are doing here could also be done with the m-file chirp.m (not
to be confused with the data file chirp.mat).

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