Use of dsp::Convolution in an audio plugin?

1

Before I am investing a lot of time, is dsp::Convolution fast enough to use in an
audio plug-in reverb? My synth plug-in currently consumes about 1-3% of CPU (i5-10600K)
I don’t want this to go up to beyond say 4-6% just because the reverb is active…

2

Simply put, no. It works barely fine with IRs below 200ms, but with longer impulses the CPU usage is too much and not viable to be used in a commercial product.

3

The DSPModulePluginDemo includes a Convolution. You could try replacing the impulse responses with your own files, and check whether the performance is acceptable.

4

Good one reuk. Just tried a 7 second theater IR and the CPU % increase was only about 1%.
Nice!

5

I use dsp::Convolution in my plugin and it works fine. I have had no issues.

6

You could try the NonUniform configuration and give it a headSize of your choosing and see how it affects performance

juce::dsp::Convolution convolutionReverb{juce::dsp::Convolution::NonUniform{ 512 }};

Another idea is to use convolution for the beginning of the reverb and transition to an algorithmic reverb during the tail

8

Could you call convolutionReverb.process() for each audio frame?

9

Yes, just the normal JUCE DSP module way with ProcessContextReplacing:

In your prepare to play:

// Prepare the JUCE DSP module:
juce::dsp::ProcessSpec spec;
spec.sampleRate = sampleRate;
spec.maximumBlockSize = samplesPerBlock;
spec.numChannels = numChannels;

// Set up convolution reverb:
convolutionReverb.prepare(spec);
convolutionReverb.reset();

In your process block:

// Turn JUCE buffer into an AudioBlock
juce::dsp::AudioBlock<float> reverbBlock (buffer);
// Process
convolutionReverb.process(juce::dsp::ProcessContextReplacing<float> (reverbBlock));
10

This is a call for each audio block, not for each audio frame,
So you need to fill one audio block with frames, coming from e.g. a synth,
and then call the convolutionReverb to update the reverbBlock.

11

What do you mean by audio frame in this situation?

12

In my synth plugin, I have following code for nextAudioBlock. Note that it calls nextAudioFrame in the loop, where all the frame-accurate audio is generated. Up to now, all my oscillators, eg’s, lfo’s, filters, etc operate at this audio-frame level. Looks like I need to make a (small) exception for the reverb.

void SynthEngine::nextAudioBlockFloat(int numInputChannels, int numOutputChannels, juce::AudioSampleBuffer buffer)
{
// populates the next audio block, only 2 audio output channels are supported.

AudioFrame next{ 0.0, 0.0 };

if (numOutputChannels == 2)
{
    auto* channelDataL = buffer.getWritePointer(0);
    auto* channelDataR = buffer.getWritePointer(1);
    int numSamples = buffer.getNumSamples();
    for (auto sample = 0; sample < numSamples; ++sample)
    {
        next = nextAudioFrame();
        channelDataL[sample] = (float)next.left;
        channelDataR[sample] = (float)next.right;
    }
}
else
{
    // silence:
    for (int channel = 0; channel < numOutputChannels; ++channel)
    {
        auto* channelData = buffer.getWritePointer(channel);
        for (auto sample = 0; sample < buffer.getNumSamples(); ++sample)
        {
            channelData[sample] = 0.0;
        }
    }
}

}

13

Done. Looks like an 8s reverb adds about 1% to my CPU load, very good.

14

Ahh, per sample processing. Yeah there’s no processSample() function in the convolution but I reckon you could make a 1 sample buffer and wrap it in an AudioBlock to process