MATLAB Answers

dsp.channelizer acts unexpectedly with oversamplingRatio=4

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Harold Alexander
Harold Alexander on 13 May 2021
Answered: Khodour Al Kadry on 14 Jul 2021
Signal to be channelized:
Channelizer filter has normalized bandwidth of .16. Channelizer has eight channels. So without oversampling, the fringes of each .16-wide channel signal get aliased into the .125-wide channel bandwidth:
Oversampling by two clears this up nicely:
I would expect oversampling by four to do "even better", with the eight spectra getting skinnier, but no...:
Here's the code:
d=randn(250000, 1);
d=d+randn(250000, 1)*1i;
d=d+.05*exp(1i*2*pi*.10*[1:250000].');
d=d+.08*exp(1i*2*pi*.11*[1:250000].');
d=d+.11*exp(1i*2*pi*.12*[1:250000].');
figure
plot(abs(fftshift(fft(d))))
f = designfilt('lowpassfir', ...
'PassbandFrequency', .16, ...
'StopbandFrequency', .18, ...
'PassbandRipple', 1, ...
'StopbandAttenuation', 60);
for oversampling=[1 2 4]
channelizer=dsp.Channelizer('NumFrequencyBands', 8, ...
'OversamplingRatio', oversampling, ...
'Specification', 'Coefficients', ...
'LowpassCoefficients', f.Coefficients);
channels=channelizer(d);
figure
for channel=1:8
subplot(2,4,channel)
plot(abs(fftshift(fft(channels(:,channel)))))
title(sprintf("Channel %d", channel))
end
sgtitle(sprintf("Oversampling=%d", oversampling))
end

Answers (1)

Khodour Al Kadry
Khodour Al Kadry on 14 Jul 2021
Hi Harold
This is a known issue with dsp.Channelizer, check the following bug report.
The issue is fixed for R2021a, R2020b (Update 3) and R2020a (Update 6).
Please update your MATLAB based on the release you are using. You can follow the steps outlined in the following documentation page to update your MATLAB

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