Channel Order Confusion

[directmusic]

Hey! I'm having a bit of confusion around the expected order of channels on Linux when recording using the ALSA backend. My audio interface has 4 channels in and 4 channels out.

When using miniaudio.h on Windows or macOS the order in which the callback receives its data is interleaved like this: 1, 2, 3, 4, but on Linux (using ALSA) it receives its data 1, 3, 2, 4. This happens when using any of the "Monitor Of [device name]" loopback devices or when using the inputs on my interface directly.

This is how I am initializing it.

    ma_device_config device_config = ma_device_config_init(ma_device_type_capture);
    device_config.capture.format = ma_format_f32;
    device_config.capture.channelMixMode = ma_channel_mix_mode_simple;
    device_config.dataCallback = miniaudio_callback;
    device_config.pUserData = this;
    device_config.periodSizeInFrames = 256;
    device_config.resampling.algorithm = ma_resample_algorithm_linear;

    ma_result init_result = ma_device_init_ex(nullptr, 0, nullptr, &device_config, &m_device);
    if (init_result != MA_SUCCESS) {
        return;
    }

    auto result = ma_device_start(&m_device);
    while (result != MA_SUCCESS) {
        result = ma_device_start(&m_device);
    }

Audacity and Bitwig seem to receive the audio in the order I expect as well. I tested with the current dev and dev 0.12 branches and they have the same behavior.

Is there something I have missed?

Thank you for your time.

[mackron]

Sorry for the delay. I missed this one.

I believe this will be because you have not specified an explicit channel map which means miniaudio will use the default channel map as reported by the backend. It's a bit awkward because you've also not specified an explicit channel count which means it'll use the "native" channel count. This makes it unpractical to plug in a pre-defined channel map because you won't know the channel count ahead of time. I think your most practical option would be to plug in your own ma_channel_converter.

Thinking about this, I wonder if it might be practical to add an option to the device config to make it so when no channel map is supplied, it always uses a well-defined channel map. I've added an item to my todo list to investigate this for 0.12.

[directmusic]

No worries about the delay! I appreciate you taking the time to answer in the first place. As you pointed out, I am not passing the channel count, that seems to be the issue. If I set capture.channels = 4; for my interface then the order appears to arrive in the callback as I expect.

While I was putting together a test using the PulseAudio API I noticed if I didn't pass a channel map in Pulse it would come in at (1, 3, 2, 4) even if I set the channel count to my interface's channel count explicitly. But once I set a channel map with PA_CHANNEL_POSITION_AUX0 and so forth for each channel it did work as expected.

For my use-case I just need the max channel count so now I am finding the max channel count supported by my device using nativeDataFormats field and passing that to the channel count as I likely should have been doing from the start. Thank you so much for the help.