Audio output overview

Pick a PCM sink. Switch any time over the API.

Rockbox writes decoded PCM into a single active sink at a time. The sink is selected by the audio_output key in settings.toml, but it can also be changed at runtime — e.g. by clicking a discovered Chromecast in the device picker.

Sink	audio_output	Use it for
Built-in CPAL	builtin	Local speakers / headphones
HLS + DASH (CMAF)	cmaf	Browser playback, ffplay/vlc/mpv, CDN origin
FIFO / pipe	fifo	Snapcast (pipe://), ffplay, any pipe consumer
Snapcast TCP	snapcast_tcp	Snapserver tcp:// source with auto-discovery
AirPlay (RAOP)	airplay	Apple TV, HomePod, Airport Express, shairport-sync
Squeezelite	squeezelite	Logitech-style multi-room with squeezelite clients
Chromecast	chromecast	Google Home, Chromecast Audio, Nest Hub
UPnP / DLNA	upnp	Kodi, VLC, BubbleUPnP, any UPnP MediaRenderer

Stream format

Every sink receives the same byte stream: S16LE stereo PCM at 44 100 Hz. The Rockbox DSP pipeline dithers and downmixes higher-bit-depth decoder output before it reaches the sink.

Fan-out

A few sinks support sending the same stream to multiple receivers simultaneously:

• AirPlay — list multiple [[airplay_receivers]] and they all share the same initial_rtptime, keeping playback within ~8 ms across the LAN.
• Squeezelite — any number of squeezelite clients can attach to one rockboxd; a sync packet aligns their clocks once per second.
• Snapcast — fan-out is handled by snapserver, not Rockbox.

For Chromecast, AirPlay and UPnP the LAN is also scanned with mDNS / SSDP at startup; discovered devices show up in the web UI and desktop picker without any config-file edits.

Switching sinks at runtime

Through the GraphQL API:

mutation Connect {
  connectDevice(id: "chromecast-living-room")
}

…or through the desktop / web device picker. The active sink is stopped, the new sink’s init and start hooks run, and audio resumes within a frame.

Architecture

Each sink implements the pcm_sink_ops vtable in firmware/export/pcm_sink.h. The full list of sinks with file paths and implementation notes lives in Architecture › PCM sinks.