highascg

PipeWire multi-channel routing (Linux)

PipeWire is the default audio server on Ubuntu 24.04+ (with WirePlumber as the session/policy manager). Use it to route multi-channel CasparCG output (FFmpeg → Pulse/PipeWire, or system default sink) to HDMI, USB, or other devices with flexible graph routing.

Companion doc: ALSA device enumeration · WO-06

Packages

Typical desktop/session install:

sudo apt update
sudo apt install pipewire pipewire-pulse wireplumber qpwgraph

Useful extras:

Package	Purpose
`qpwgraph`	Visual patchbay (nodes ↔ links).
`pipewire-audio-client-libraries`	Dev headers (optional).

Verify the user session is running PipeWire (common on desktop; headless may differ):

systemctl --user status pipewire pipewire-pulse wireplumber

Sample-rate and buffer size (drop-in config)

CasparCG channel modes are usually 48 kHz-aligned. You can set default clock and quantum (buffer in frames) via a drop-in so apps share a stable time base.

Create e.g. /etc/pipewire/pipewire.conf.d/99-caspar-clock.conf (path may be ~/.config/pipewire/pipewire.conf.d/ for per-user overrides):

context.properties = {
  default.clock.rate = 48000
  default.clock.quantum = 256
  default.clock.min-quantum = 64
}

Property	Role
`default.clock.rate`	Global sample rate (Hz); 48000 matches typical Caspar/FFmpeg consumer args.
`default.clock.quantum`	Default buffer size in frames at the default rate (latency vs stability tradeoff).
`default.clock.min-quantum`	Lower bound for dynamic quantum (lower = lower latency, higher CPU).

Restart the session services after editing system-wide files:

systemctl --user restart wireplumber pipewire pipewire-pulse

Note: Exact merge behaviour depends on PipeWire version; if a property is ignored, check journalctl --user -u pipewire for warnings.

Virtual sinks for CasparCG routing

Caspar can send audio to a named sink (e.g. FFmpeg -f pulse <sink_name> or system default). Creating a dedicated sink lets you:

Patch Caspar → mixer → hardware in qpwgraph
Isolate PGM vs monitor buses without renumbering ALSA hw:N,M in XML

PulseAudio compatibility (PipeWire implements this)

With pipewire-pulse, pactl often controls PipeWire’s Pulse-compatible sinks.

Example: null sink with a friendly name and multiple channels (adjust channels= to your layout):

pactl load-module module-null-sink sink_name=caspar_pgm \
  sink_properties=device.description=CasparPGM channels=8

List sinks:

pactl list short sinks

Use the sink name in Caspar FFmpeg consumer paths that target Pulse, e.g. -f pulse caspar_pgm (see WO-06 Phase 2).

Unload (note module id from pactl list modules short):

pactl unload-module <module-id>

For persistent null sinks across reboots, prefer a WirePlumber snippet or a small user systemd unit that runs the pactl load-module line at login—station-specific.

Tools

`qpwgraph`

Launches a graph of PipeWire nodes (apps, sinks, sources).
Drag connections between outputs and inputs to route Caspar (or pw-play) into the right hardware or virtual sink.
Ideal for validating multi-channel paths before baking paths into casparcg.config.

`pw-cli` and `pw-dump`

Inspect objects, properties, and links:
```
pw-cli ls Node
pw-dump | less
```
Use to confirm channel counts and port names on a device node.

`pw-top`

Live CPU and quantum / driver timing view (similar to top).
Useful when chasing xruns or high load while running Caspar + FFmpeg consumers.

CasparCG integration notes

Path	Behaviour
FFmpeg `-f alsa hw:N,M`	Talks ALSA directly; bypasses PipeWire unless ALSA is plumbed to PipeWire.
FFmpeg `-f pulse <sink>`	Goes through PipeWire’s Pulse layer; virtual sinks and `qpwgraph` routing apply.
`<system-audio/>` consumer	Usually follows default desktop sink (often PipeWire).

For a single fixed device, ALSA hw:N,M is often simpler. For flexible routing between multiple outputs, Pulse-compatible sinks + PipeWire graph are appropriate.

Troubleshooting

Issue	What to check
No nodes for Caspar	Consumer uses ALSA `hw:` only → no PipeWire node; use `pulse` or test `pw-play` into a sink.
Crackling / dropouts	Quantum too low vs CPU load; try raising `default.clock.quantum`; close other heavy clients.
Wrong rate	Force 48000 in Caspar channel mode and FFmpeg `-ar 48000`; match `default.clock.rate`.
`pactl` fails	Ensure `pipewire-pulse` socket is active; user session vs system PipeWire.