dsp.channelizer acts unexpectedly with oversamplingRatio=4

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

Answer 1

[Khodour Al Kadry]

Hi Harold

This is a known issue with dsp.Channelizer, check the following bug report.

https://www.mathworks.com/support/bugreports/details/2290885

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

https://www.mathworks.com/help/matlab/matlab_env/check-for-software-updates.html