Android: Initial audio processing method call takes a long time

Issue

I'm getting a very peculiar issue with my audio callbacks in my Android app (that's using NDK/OpenSL ES). I'm streaming audio output at 44.1 kHz and 512 frames (which gives me a callback time of 11.6 ms). In the callback, I'm synthesizing a couple of waveforms, filters, etc (like a synthesizer). Due to optimization I never reach over 5 ms of the callback time. However, when I turn on a specific effect (digital delay line), it starts to take a radically longer time in the callback. The digital delay line will jump from 7.5 ms (after all voices/filters have been processed) and jump up to 100 to 350 ms.

This is the most confusing part; after maybe 1 or 2 seconds, the digital delay execution time will jump from the extremely high time to 0.2 ms completion time per callback.

Why would the Android app take a long time to complete my digital delay processing code the first few callbacks and then die down to a very short and audio-happy time? I'm kind of at a loss right now and not sure how to fix this. To confirm, this only happens with the delay processing method. It's just a standard digital delay line (you can find some on github) and I feel like the algorithm isn't the problem here...

Kind of a pseudocode/rough sketch of what my audio callback code looks like:

static bool myAudioCallback(void *userData, short int *audIO, int numSamples, int srate) {
     AudioData *data = (AudioData *)userData;
     // Resets pointer array values to 0
     for (int i = 0; i < numSamples; i++) data->buffer[i] = 0;
     // Voice Generation Block
     for (int voice = 0; voice < data->numVoices; voice++) {
          // Reset voice buffers:
          for (int i = 0; i < numSamples; i++) data->voiceBuffer[i] = 0;
          // Generate Voice
          data->voiceManager[voice]->generateVoiceBlock(data->voiceBuffer, numSamples);
          // Sum voices
          for (int i = 0; i < numSamples; i++) data->buffer[i] += data->voiceBuffer[i]];
     }

     // When app first starts, delayEnabled = false so user must click on a  
     // button on the UI to enable it.
     // Trouble is that when we enable processDelay(double *buffer, in frames) the 
     // first time, we get a long execution time.
     if (data->delayEnabled) {
          data->delay->processDelay(data->buffer, numSamples);
     }

    // Conversion loop
    for (int i = 0; i < numSamples; i++) {
        double sample = clipOutput(data->buffer[i]);
        audIO[2*i] = audIO[(2*i)+1] = CONV_FLT_TO_16BIT(sample * data->volume);
    }
}

Thanks!

Solution

Not a great answer to the solution but this is what I did:

Before the user is able to do anything on the app, I turned on the delay and let it run its course for like 2 seconds before switching it off. This allows the callback to do its weird long 300 ms execution time while not destroying the audio.

Obviously this is not a great answer and if anyone can figure out a more logical explanation I would be more than happy to mark that as the answer.

Answered By - yun