How to get smaller buffer size in multi-channel audio application with Oboe

Question

I'm using Oboe 1.2 in an audio android application. When I call getFramesPerBurst(), which gives the endpoint buffer size, I get expected results (240 frames) if the number of output channels is set to 2. However when I set 4 output channels, the value returned by getFramesPerBurst() is around 960 (!). Is that normal ? Is that a limitation of the hardware (I tested on 4 different devices though, with different os version) ? A limitation of Oboe ? I notice also that this value is different than the value given by the property PROPERTY_OUTPUT_FRAMES_PER_BUFFER of AudioManager from the AudioService.

oboe::AudioStreamBuilder builder;
if (!oboe::AudioStreamBuilder::isAAudioRecommended()){
    builder.setAudioApi(oboe::AudioApi::OpenSLES);
}
builder.setSharingMode(oboe::SharingMode::Exclusive);
builder.setFormat(oboe::AudioFormat::Float);
builder.setChannelCount(4);
builder.setCallback(&_oboeCallback);
builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);

oboe::Result result = builder.openStream(&_stream);

if (result == oboe::Result::OK) {
    int framePerBurst = _stream->getFramesPerBurst(); // gives value around 960 for 4 channels, 240 for 2 channels
    _stream->setBufferSizeInFrames(2*framePerBurst);
}

score 1 · Accepted Answer · answered Sep 04 '19 at 14:13

Unless you are connecting to an audio device which actually has 4 independent channels (e.g. a USB audio interface or DJ controller like this one) then your 4 channel stream will need to be mixed into an N channel stream where N is the number of channels in your audio device. This could be 2 (stereo) for headphones or 1 (mono) for a built-in speaker.

The mixer introduces latency and larger buffer sizes. This is the difference in buffer sizes you see when you request a channel count of 2 vs 4.

For the lowest latency always leave the channel count unspecified when creating the stream, then do any channel count conversion inside your own app. There's an example of this here.

How to get smaller buffer size in multi-channel audio application with Oboe

1 Answers1