Switchable dsp::Oversampling Factor

Audio Plugins
1

Hi, I am trying to make my plugin always run at 176.4k or 192k sample rate, regardless of the actual sample rate of the source audio. I figured something like this would work:

// In Prepare to Play
if (sampleRate <= 44100.0 || sampleRate == 48000.0) {
    oversampler.factorOversampling = 2;
} else if (sampleRate == (44100.0 * 2.0) || sampleRate == (48000.0 * 2.0)) {
    oversampler.factorOversampling = 1;
} else if (sampleRate == (44100.0 * 4.0) || sampleRate >= (48000.0 * 4.0)){
    oversampler.factorOversampling = 0;
} else {
    oversampler.factorOversampling = 0;
}

This seems to change the value of oversampler.getOversamplingFactor(), but not of the actual number of samples generated in oversampler.processSamplesUp(). Changing the initialization in

oversampler(2 /* channels */, 1 /* factor oversampling */, juce::dsp::Oversampling<float>::FilterType::filterHalfBandPolyphaseIIR, true, false)

does not change the number of samples created in oversampler.processSamplesUp() either.

Do you all have some suggestions of what I could change to make this idea work?

Thank you!

2

(Removed)

3

Your branches assume only multiples of 44.1khz, what about 48khz?

Can you post the code you are using to process samples up?

4

Sure thing!

// In PluginProcessor.h
juce::dsp::Oversampling<float> oversampler;

// In Initalizer List
oversampler(2, 2, juce::dsp::Oversampling<float>::FilterType::filterHalfBandPolyphaseIIR, true, false)

// In Prepare to Play
oversampler.setUsingIntegerLatency(false);
oversampler.clearOversamplingStages();
oversampler.numChannels = getTotalNumInputChannels();
 
if (sampleRate <= 44100.0 || sampleRate == 48000.0) {
    oversampler.factorOversampling = 2;
} else if (sampleRate == (44100.0 * 2.0) || sampleRate == (48000.0 * 2.0)) {
    oversampler.factorOversampling = 1;
} else if (sampleRate == (44100.0 * 4.0) || sampleRate >= (48000.0 * 4.0)){
    oversampler.factorOversampling = 0;
} else {
    oversampler.factorOversampling = 0;
}
 
oversampler.addOversamplingStage(juce::dsp::Oversampling<float>::FilterType::filterHalfBandPolyphaseIIR, 0.05f, -90.0f, 0.05f, -90.0f);
 
oversampler.reset();
oversampler.initProcessing(samplesPerBlock);

// In Process Block
juce::dsp::AudioBlock<float> block (buffer);
auto returnBlock = oversampler.processSamplesUp(block);
/* use returnBlock in processing */
oversampler.processSamplesDown(block);

Regardless of what I put for oversampling factor anywhere in the chain (initializer or prepareToPlay), it always comes back x2 oversampled at the returnBlock.

5

I think that’s because in prepare to play you are clearing all the stages then adding a single stage back, which means it oversamples once no matter what. The way this class works is in stages of 2x. For 4x oversampling you need 2 stages, etc.

A solution could be to make the instance a unique ptr, then call reset on it in prepare to play and construct a new instance with the right arguments.

Or add stages in a for loop, eg for i < numStages

1 Like
6

Great! Thank you!

1 Like