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JACK audio routing (Linux alternative to PipeWire graph)

JACK (JACK Audio Connection Kit) is a low-latency audio server with a patchable client graph. It fits when you want deterministic routing between apps, multiple ALSA devices via bridges, or JACK-native clients/plugins. On many desktops PipeWire already covers similar use cases and exposes a JACK-compatible API; see PipeWire multi-channel routing first.

Context: WO-06 Audio playout · T1.3

Install

Ubuntu / Debian (example):

sudo apt update
sudo apt install jackd2 qjackctl

Optional:

Package Role
qjackctl Patchbay, connections, JACK start/stop GUI.
carla Plugin host / advanced routing (optional).
alsa-utils aplay -l for device IDs — see ALSA enumeration.

Start JACK with the ALSA backend (jackd)

Pick one ALSA playback device to be the master clock for JACK (from aplay -l: card N, often device 0).

Example: card 1, 48 kHz, 256 frames/period, 2 periods (adjust for stability vs latency):

jackd -R -d alsa -d hw:1 -r 48000 -p 256 -n 2
Flag Meaning
-R Realtime scheduling (often needs limits.conf / audio group).
-d alsa ALSA backend.
-d hw:1 ALSA card index (not hw:1,3—JACK ALSA driver uses card; use separate tools for subdevices if needed).
-r 48000 Sample rate (match Caspar / broadcast chain).
-p 256 Period size in frames.
-n 2 Periods in the buffer (latency ∝ p * n).

Run in a terminal or via qjackctl → Setup → Parameters → Start. Stop with Ctrl+C or qjackctl.

Exclusive access: while jackd holds hw:N, other apps (including Caspar FFmpeg -f alsa hw:N to the same device) may fail with “device busy”. Plan one owner of the hardware or use bridges below.

Multi-device routing: alsa_in / alsa_out

When you need more than one ALSA card (e.g. USB + onboard), JACK can expose extra devices via:

Typical pattern: JACK master on card A; route mix through JACK to card B with alsa_out:

alsa_out -d hw:2,0 -j mon_out

Flags vary by version; use alsa_out --help. Connect JACK ports to mon_out in qjackctl Graph.

This is how you achieve CasparCG → (ALSA or Pulse) → … → JACK → multiple hardware outputs: Caspar does not speak JACK natively; you either:

  1. Send Caspar FFmpeg to an ALSA device that is not the JACK master, then bridge into JACK with alsa_in, or
  2. Use PipeWire with Pulse compatibility and patch in qpwgraph, or
  3. Use a JACK sink if your FFmpeg build supports -f jack (uncommon in stock Caspar builds—verify ffmpeg -formats | grep jack).

For many CasparCG deployments, direct ALSA hw: or Pulse/PipeWire sinks are simpler than a full JACK graph. Use JACK when you specifically need its graph + multi-card bridges or JACK-only clients; otherwise PipeWire/ALSA is often enough.

qjackctl workflow

  1. Setup: choose Interface (ALSA driver), Sample rate, Frames/period, Periods/buffer.
  2. Start JACK.
  3. Graph (or Connect): wire outputs (playback clients) to inputs (capture / alsa_out ports).
  4. Save a patchbay preset for repeatable shows.

jackd vs PipeWire JACK layer

CasparCG (reminder)

Troubleshooting

Issue What to check
jackd won’t start Wrong hw: card; device in use; try larger -p.
Xruns (red in qjackctl) Increase period; close other apps; CPU governor.
No permission for realtime User in audio group; /etc/security/limits.d/ rtprio / memlock.
Second interface silent alsa_out / graph not wired; wrong -d hw:.

See also

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