Android / Oboe sink and source

[gavv]

Last revised: Oct 2023

Currently our Android support lacks audio I/O support. For now this is good because C library does not do I/O yet, and CLI tools, which do I/O, are not needed on Android. However later we'll need to add I/O capabilities to the C library as well.

The recommended way for doing audio I/O in C++ on Android is Oboe library, which under the hood uses either AAudio or OpenSL ES.

We'll need to add target_oboe target directory to roc_sndio with three new classes:

• OboeSink
• OboeSource
• OboeBackend

(OboeSink and OboeSource will probably be similar; in this case we can extract a helper or a base class).

And then we should register OboeBackend in sndio::BackendDispatcher.

The overall structure will be similar to roc_sndio/target_pulseaudio.

Some notes:

• we likely should use Oboe callback mode, which provides a lower latency;
• we should also implement onErrorAfterClose callback and restart stream if it's disconnected;
• we should use oboe::PerformanceMode::LowLatency and oboe::InputPreset::Unprocessed modes;
• we should ensure to perform resampling only once: either in Oboe or in Roc; so if resampling is enabled in Roc, it should be disabled in Oboe, and vice versa.

As a first step we can test the new sink and source using roc-send and roc-recv command-line tools running under Termux. That is still not convenient for the user, but is easier to start with.

Further info:

• #222 describes how to build Roc in Termux
• #231 is similar to this issue, but for ALSA backend
• #251 provides an overview of roc_sndio concepts
• #246 provides instructions for adding new audio backend

[gavv]

This issue will also allow to decrease latency in roc-droid, if we move I/O completely into roc.