Can miniaudio obtain the OS-specified channel configuration?

[bjorn-nesby]

In most OSes, it's possible to configure the speaker channels - e.g. via "Speaker Setup" (Windows) or "Audio MIDI Setup" (macOS).

I'm trying to determine how to obtain this configuration - essentially, a channel map - so that I can automatically apply it in my application. Ideally, I would like to use the OS-specified settings as the default.

For example, when I configure my audio interface (8 output channels) using Audio MIDI Setup, I can specify that a stereo file should be played through channels 3+4. And, for the most part, the setting is applied when playing audio in applications - both ones developed by Apple, and others. It's a bit hit-and-miss though - for example, (otherwise great) foobar2000 doesn't seem to be compliant.

So, my question is if it's possible to obtain the OS-specified channel via one of the miniaudio APIs?
It doesn't seem so, but I wouldn't be completely surprised if I overlooked something.

[mackron]

In your device config, if you leave the channel count set to the 0 and the channel map empty, it'll tell the miniaudio to use the native channel configuration as reported by the backend. To access it, you query it from the ma_device object after initialization.

device.playback.channels;   // <-- The channel count as reported by the backend.
device.playback.channelMap; // <-- The channel map as reported by the backend.

You can use #define MA_DEBUG_OUTPUT to output to stdout the channel count and channel map so you can easily review what it's given back to you in case you wanted to verify the results.

[bjorn-nesby]

Thanks for the answer! I'm still having some trouble, unfortunately.

First, I launched the Audio MIDI Setup utility and configured my audio so that stereo files would be played on channels 3+4, that 5.1 surround files would have a slightly different layout. A quick test (using the Apple Music app and Audacity) confirmed that the settings got applied.

I then took the simple playback example and modified it so that the device config specifies a channel count of 0. As for the channelMap (pChannelMap) property, I simply didn't define it - but then, perhaps you meant something else by leaving the channel map "empty"? Also tried setting it to NULL - same result.

In any case, this is the debug output when using a device config with 0 channels

INFO:   FCA610 (Playback)
INFO:     Format:      32-bit IEEE Floating Point -> 32-bit IEEE Floating Point
INFO:     Channels:    10 -> 10
INFO:     Sample Rate: 48000 -> 48000
INFO:     Buffer Size: 480*3 (1440)
INFO:     Conversion:
INFO:       Pre Format Conversion:  NO
INFO:       Post Format Conversion: NO
INFO:       Channel Routing:        NO
INFO:       Resampling:             NO
INFO:       Passthrough:            YES
INFO:       Channel Map In:         {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT CHANNEL_FRONT_CENTER CHANNEL_LFE CHANNEL_BACK_LEFT CHANNEL_BACK_RIGHT CHANNEL_SIDE_LEFT CHANNEL_SIDE_RIGHT CHANNEL_AUX_0 CHANNEL_AUX_1}
INFO:       Channel Map Out:        {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT CHANNEL_FRONT_CENTER CHANNEL_LFE CHANNEL_BACK_LEFT CHANNEL_BACK_RIGHT CHANNEL_SIDE_LEFT CHANNEL_SIDE_RIGHT CHANNEL_AUX_0 CHANNEL_AUX_1}

From what I can gather, miniaudio determines the channel map to be the maximum available size (10 channels). But there are no rearranged channels at all.

I guess what I was hoping that it would enforce my "stereo profile" when playing back 2 channels of audio, my "5.1 surround profile" when playing back 6 channels, and so on. But perhaps this is indeed the problem - miniaudio obtains a full 10-channel configuration no matter what - and the macOS Audio MIDI Setup utility doesn't even have the capability to configure so many channels (8 is the maximum). So I can't actually verify if this is the problem.

[mackron]

Yeah when I say leave the channel map empty, I mean just leave it unset. ma_device_config_init() will default it to NULL. In this case, the in and out channel maps will be the same.

So when you set the channel count to 2 or 6, do you get back the channel map you are after? When you specify an explicit channel count, miniaudio will tell that to the backend so it can attempt to give you back an appropriate configuration. miniaudio will initialize the device and then use IAudioClient::GetMixFormat() to retrieve the actual configuration (https://learn.microsoft.com/en-us/windows/win32/api/audioclient/nf-audioclient-iaudioclient-getmixformat). I'm not sure what Audacity would be doing.

[bjorn-nesby]

So when you set the channel count to 2 or 6, do you get back the channel map you are after?

No, when I define 2 channels, I get this output:

INFO:       Channel Map In:         {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT}
INFO:       Channel Map Out:        {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT CHANNEL_FRONT_CENTER CHANNEL_LFE CHANNEL_BACK_LEFT CHANNEL_BACK_RIGHT CHANNEL_SIDE_LEFT CHANNEL_SIDE_RIGHT CHANNEL_AUX_0 CHANNEL_AUX_1}

And if I define 6 channels, it looks like this:

INFO:       Channel Map In:         {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT CHANNEL_FRONT_CENTER CHANNEL_LFE CHANNEL_SIDE_LEFT CHANNEL_SIDE_RIGHT}
INFO:       Channel Map Out:        {CHANNEL_FRONT_LEFT CHANNEL_FRONT_RIGHT CHANNEL_FRONT_CENTER CHANNEL_LFE CHANNEL_BACK_LEFT CHANNEL_BACK_RIGHT CHANNEL_SIDE_LEFT CHANNEL_SIDE_RIGHT CHANNEL_AUX_0 CHANNEL_AUX_1}

Both look pretty vanilla to me. I tried debugging simple_playback.cpp, and indeed, those are default channel maps created by ma_channel_map_init_standard_channel_microsoft.

miniaudio will initialize the device and then use IAudioClient::GetMixFormat() to retrieve the actual configuration

Interesting! Is there an equivalent function being invoked on macOS? Because, that's where I have been doing all my testing so far.

[mackron]

I couldn't tell you off the top of my head what Core Audio uses specifically, but the concept is the same for all backends - you initialize a handle to the device or stream or whatever concept the backend uses, and then they'll have some way for you query the specific data configuration required for playback.

I don't have any real experience with macOS, but on Windows, wouldn't your speaker set up be abstracted away such that it Just Works when you play a sound? For example, if you have set up your front-left and front-right channels to output from channels 3 and 4, wouldn't that just mean that when you output to channels 0 and 1 from your program, which is what would be happening with miniaudio, Windows would just route it through to channels 3 and 4 for you automatically?

[bjorn-nesby]

For example, if you have set up your front-left and front-right channels to output from channels 3 and 4

Hm, I'm actually not even sure that Windows allows channel-by-channel configuration. From the look of things, you're stuck with some overall speaker configuration profiles (stereo, 5.1 and 7.1) which you can tweak a bit - but apparently, nothing as detailed as macOS.

But, indeed, when I take my 7.1 capable audio interface and choose the stereo profile on Windows, and specify over "0" as the channel count then miniaudio indeed will just see two channels (2 -> 2). But here I guess Windows is essentially just presenting the device as not having all those extra channels.

I'm starting to think that allowing people to configure the channels inside my miniaudio implementation (a sample browser) isn't such a bad an idea, Because, realistically only very few people will need to do this, and they might even find it easier than having to dive through various obscure OS panels that may not even provide the ability to rearrange channels (as seems the case on Windows).