Is it possible to use specific device channels for input/output?

[bjorn-nesby]

I'm currently evaluating miniaud.io for a project, and trying to figure out if it's possible to route audio to specific a specific set of device channels (output).

From what I could quickly gather, API mentions specific features that are handy for full-duplex. For example, capturing audio from a mono source (microphone) but outputting a stereo signal. And for this purpose, configuration options such as config.channelsIn are obviously handy.

But my use case is slightly different: ideally, I would like to access an audio device with an arbitrary number of channels and freely choose which channels to use for output.
As a concrete example, imagine that you have an audio device with a total of six outputs, with channels 1+2 hooked up to speakers while channels 3+4 are connected to some headphones.

With such a device, what would be the best way to route audio from my application to the headphones?
For example, I could imagine that it would possible to send audio to all six channels, but only include an audible signal for the desired channels (i.e. let the other channels remain silent)?
This approach seems a bit wasteful perhaps, but I have no idea what would be considered best practice :-)

[mackron]

In the past I've suggested doing as you mentioned - set the channels to 0 in the device config which will result in the native channel count being used, and then just don't use the unused channels in the data callback.

There's also the channel mapping system, but the thing is, it's been designed around the concept of spatial audio channels in that you specify channels like FRONT_LEFT, BACK_RIGHT, etc. It's doesn't really map too well to your scenario. In any case, even if you were to use a channel map, it would still be processing audio for those unused channels under the hood anyway.

Sorry I can't be of much help with this. There's probably no real reason I couldn't update the channel map system to allow you to specify the exact channels you want and to ignore the others. But right now, if you were wanting to go with miniaudio my preferred way is to just ignore the unused channels.

[bjorn-nesby]

Thanks for the quick reply!

That was very helpful - I simply didn't know if such a feature existed or not.
I was simply thinking in terms of ASIO, where I believe that you are able to access/initialize certain channels only.
In any case, I think outputting silence is a perfectly acceptable solution :-)

[erikjalevik]

Are there any useful helper functions in miniaudio for taking 2-channel audio from a decoder, and then splicing it into two arbitrary channels in void* pOutput parameter given to the audio callback from a multi-channel device?

[mackron]

It's just data. You can do whatever you want with it. The decoder and device are entirely decoupled. Just read from your decoder, do whatever processing you need to do, and then reconstruct the data as necessary when outputting it to the output buffer. The data is always interleaved with miniaudio so keep that in mind.