
28 Practical Biomedical Signal Analysis Using MATLAB
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only frequencies below the Nyquist frequency of the resultant downsampled signal.
This is usually achieved by lowpass filtering the original signal before downsam-
pling. If the original signal s
0
is sampled at frequency F
s
0
and the desired frequency
is F
s
1
= F
s
0
/q for an integer q, then the frequency band of the signal s
0
needs to be
reduced to the range (0,
1
2q
F
s
0
). In practice, one needs to set the cut-off frequency of
the lowpass filter smaller then
1
2q
F
s
0
), because the filter roll-off is finite. In MATLAB
Signal Processing Toolbox the downsampling with anti-aliasing filtering is imple-
mented in function ...