aout: add multiple audio tracks playback support

Add multiple audio tracks playback support

Google Summer of Code 2023
Mentee: Ayush Dey
Mentor: Thomas Guillem

Description

Add support for playing multiple audio tracks simultaneously from a media on systems using PulseAudio as the sound server.
Issue: #27738

gsoc-audio-test-2

Work breakdown

• Modify the audio output (aout) module API to support multiple streams by splitting device and stream handling.
• Implement the aout API within the PulseAudio module.
• Adapt the core to conditionally handle device and stream splitting (i.e., stream handling for adapted aout modules and device handling for legacy aout modules).
• Enhance the Player API to provide users with greater control over multiple audio streams.
• Incorporate these modifications into the Qt/VLC-ports to create an optimal GUI for controlling multiple streams.

Current features

• Multiple audio track selection is disabled by default but can be enabled from the GUI, similar to subtitles.
• An infinite number of audio streams can be initiated from a single aout object.
• A single Play/Pause/Mute/Volume button controls all the tracks playing in parallel.
• The core will generate multiple streams from a single aout object for adapted modules and multiple aout objects for legacy modules.

Future work

• Adaptation of the remaining common aout modules (Wasapi, Auhal, AAudio, Pipewire, etc.).
• Individual Play/Pause/Mute/Volume buttons for each track.

Acknowledgement

I would like to thank the team at VideoLAN for their support and especially my mentor Thomas Guillem, who has guided me actively throughout the development of this feature.