libobs, win-wasapi: Enable channel routing for Audio Input Capture

[pkviet]

Description

On libobs side, this adds the capability for the audio resampler (relying on swresampler lib) to
set a matrix for routing of channels.
On win-wasapi side, this applies the new capability to enable channel selection.
This brings parity to wasapi input capture with coreaudio input capture on macOS.

Screenshots

Motivation and Context

One motivation for that feature are devices which separate stereo inputs.
So if a mike is connected to , say, Input 1 while nothing is connected to Input 2, wasapi usually treats them as
stereo pairs and when obs audio is set to Stereo, the captured Right channel will be silent.
With channel routing capability, it is now possible to duplicate the left channel to the right and have a consistent level.
The issue was brought to my attention by @Fenrirthviti

How Has This Been Tested?

Tested on windows 11 pro 25H2 w/ a logitech webcam + USB sound board.
Disabling the downmix cehckbox enables channel routing.
Swapping between devices resets the channel selections.
Changing the channel selections updates the captured channel.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[PatTheMav]

Looks fine to me overall, also tested this with an virtual 6-ch audio device and allowed me to map them as expected.

Haven't done in-depth checks whether the audio is correctly routed though.

[psyirius]

Just wondering,
Why don't we have speaker-layout selection like obs-asio has?

[psyirius]

Just built this pr from the current master.
Can't able to select channels in the channel comboboxes.

[pkviet]

Just built this pr from the current master. Can't able to select channels in the channel comboboxes.

yes there was a regression but it is fixed with last push

[pkviet]

Just wondering, Why don't we have speaker-layout selection like obs-asio has?

Because I want to avoid an uncontrolled upmix or downmix.
In obs-asio , if the input audio format differs from obs format (set in Settings > Audio), there is automatically an swresampler called in the audio pipeline which will make the conversion. The channel weights are sane only for the most common conversions (xxx ==> stereo, stereo ===> xxx ) as stated explicitly by the swresampler API. There can be surprising defaults with odd channel volumes otherwise.
There is no conversion though if the input speaker format is the same that is set in obs settings.
Channel routing is imo an advanced feature for people who want fine grained audio control; so the uncontrolled upmixer/downmixer insertion defeats that purpose and that is why I have avoided that here, very deliberately.

[RytoEX]

cc @Warchamp7 on brace changes. If needed, we can just update all of these in the clang-format PR, but we will soon be requiring and enforcing braces for all control statements in C++.

[Warchamp7]

A couple bits of feedback and some questions.

Can we invert the checkbox?
i.e. Unchecked by default, and the label is instead "Enable custom channel routing"

---

When Device is set to Default the channel selection dropdowns are missing a device name and simply say channel 1, etc.

Ideally the actual device name should be used in the device channel dropdowns and not just Default.

---

At time of review, wasapi output devices don't appear to be working:

17:18:30.101: [win-wasapi: 'Audio Output Capture'] update settings:
17:18:30.101: 	device id: {0.0.0.00000000}.{825bd69f-078a-481c-831c-eda29a7002bb}
17:18:30.101: 	use device timing: 1
17:18:30.108: Device 'Speakers (Scarlett 2i2 4th Gen)' invalidated.  Retrying (source: Audio Output Capture)
17:18:30.118: warning: Input channel layout "" is invalid or unsupported.
17:18:30.118: avresample_open failed: error code -22
17:18:30.118: WASAPI: Device 'Speakers (Realtek USB2.0 Audio)' [48000 Hz] initialized (source: Audio Output Capture)```

[pkviet]

A couple bits of feedback and some questions.

Can we invert the checkbox? i.e. Unchecked by default, and the label is instead "Enable custom channel routing"

This can be done, but this means wasapi would not be aligned with core audio where the feature was first implemented.

When Device is set to Default the channel selection dropdowns are missing a device name and simply say channel 1, etc.
Ideally the actual device name should be used in the device channel dropdowns and not just Default.

Ok I will add the logic for that.

At time of review, wasapi output devices don't appear to be working:

Indeed. My scope was to bring parity to wasapi with coreaudio.
I didn't consider Audio Output Capture.
I'll investigate the extent of changes that would be required for that and either add that or defer to another PR.

[Warchamp7]

A couple bits of feedback and some questions.
Can we invert the checkbox? i.e. Unchecked by default, and the label is instead "Enable custom channel routing"

This can be done, but this means wasapi would not be aligned with core audio where the feature was first implemented.

I'd like to update core audio to match that as well then. It can be a separate PR though.

[pkviet]

Update
Addressed @Warchamp7 comments.

1. Changed UI so that the checkbox now enables channel routing ; checkbox unchecked:

checkbox checked:

2. Default device now displays the device name in channels list:

3. Fixed the issue where Audio Output Capture wouldn't capture anymore.