User`s guide

FIR Decimation
5-193
5FIR Decimation
Purpose Filter and downsample an input signal.
Library Filtering / Multirate Filters
Description The FIR Decimation block resamples the discrete-time input at a rate K times
slower than the input sample rate, where the integer K is specified by the
Decimation factor parameter. This process consists of two steps:
•The block filters the input data using a direct-form II transpose FIR filter.
•The block downsamples the filtered data to a lower rate by discarding K-1
consecutive samples following every sample retained.
The FIR Decimation block implements the above FIR filtering and
downsampling steps together using a polyphase filter structure, which is more
efficient than straightforward filter-then-decimate algorithms. The output of
the decimator is the first phase of the polyphase filter.
The
FIR filter coefficients parameter specifies the numerator coefficients of
the FIR filter transfer function H(z).
The length-m coefficient vector,
[b(1) b(2) ... b(m)], can be generated by
one of the filter design functions in the Signal Processing Toolbox, such as the
fir1 function used in the example below. The filter should be lowpass with
normalized cutoff frequency no greater than 1/K. All filter states are internally
initialized to zero.
Sample-Based Operation
An M-by-N sample-based matrix input is treated as M∗N independent
channels, and the block decimates each channel over time. The output sample
period is K times longer than the input sample period (T
so
=KT
si
), and the
input and output sizes are identical.
Frame-Based Operation
An M
i
-by-N frame-based matrix input is treated as N independent channels,
and the block decimates each channel over time. The
Framing parameter
determines how the block adjusts the rate at the output to accommodate the
reduced number of samples. There are two available options:
Hz() Bz() b
1
b
2
z
1–
… b
m
z
m 1–()–
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