Decimation — decrease sampling rate
y =
decimate(x,r)
y =
decimate(x,r,n)
y =
decimate(x,r,'fir')
y =
decimate(x,r,n,'fir')
Decimation reduces the original
sampling rate of a sequence to a lower rate. It is the opposite of
interpolation. decimate
lowpass
filters the input to guard against aliasing and downsamples the result.
Note:
For better results when |
decimate
uses decimation algorithms 8.2 and
8.3 from [1].
decimate
creates a
lowpass filter. The default is a Chebyshev Type I filter designed
using cheby1
. This filter has
normalized cutoff frequency 0.8/r
and passband
ripple 0.05 dB. Sometimes, the specified filter order
produces passband distortion due to roundoff errors accumulated from
the convolutions needed to create the transfer function. The filter
order is automatically reduced when distortion causes the magnitude
response at the cutoff frequency to differ from the ripple by more
than 10^{–6}.
When the 'fir'
option is chosen, decimate
uses fir1
to design a lowpass FIR filter with
cutoff frequency 1/r
.
When using the FIR filter, decimate
filters
the input sequence in only one direction. This conserves memory and
is useful for working with long sequences. In the IIR case, decimate
applies
the filter in forward and reverse directions using filtfilt
to remove phase distortion.
This in effect doubles the filter order. In both cases, the function
minimizes transient effects at both ends of the signal by matching
endpoint conditions.
Finally, decimate
resamples
the data by selecting every r
th point from the
interior of the filtered signal. The resampled sequence is such that y(end)
matches x(end)
when
the IIR filter is used and y(1)
matches x(1)
in
the FIR case.
[1] Digital Signal Processing Committee of the IEEE Acoustics, Speech, and Signal Processing Society, eds. Programs for Digital Signal Processing. New York: IEEE Press, 1979, chap. 8.