RetroArch/audio/test/sinc_test.m

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% MATLAB test case for RetroArch SINC upsampler.
close all;
% 4-tap and 8-tap are Lanczos windowed, but include here for completeness.
phases = 256;
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bw = 0.375;
downsample = round(phases / bw);
cutoffs = bw * [0.65 0.75 0.825 0.90 0.95];
betas = [2.0 3.0 5.5 10.5 14.5];
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sidelobes = round([2 4 8 32 128] / bw);
taps = sidelobes * 2;
freqs = 0.05 : 0.02 : 0.99;
filters = length(taps);
for i = 1 : filters
filter_length = taps(i) * phases;
% Generate SINC.
sinc_indices = 2 * ((0 : (filter_length - 1)) / (filter_length - 1)) - 1;
s = cutoffs(i) * sinc(cutoffs(i) * sinc_indices * sidelobes(i));
win = kaiser(filter_length, betas(i))';
filter = s .* win;
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impulse_response_half = 0.5 * upfirdn(1, filter, phases, downsample);
figure('name', sprintf('Response SINC: %d taps', taps(i)));
freqz(impulse_response_half);
ylim([-200 0]);
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signal = zeros(1, 8001);
for freq = freqs
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signal = signal + sin(pi * freq * (0 : 8000));
end
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resampled = upfirdn(signal, filter, phases, downsample);
figure('name', sprintf('Kaiser SINC: %d taps, w = %.f', taps(i), freq));
freqz(resampled .* kaiser(length(resampled), 40.0)');
ylim([-80 70]);
end