highascg

CasparCG audio outputs on Linux: NVIDIA, 3.5 mm stereo, USB

Practical guide for the three most common local playout paths on Ubuntu:

NVIDIA GPU — HDMI / DisplayPort embedded audio (often up to 8 channels on one link). On many installs this is the main multi-channel program output, not only a local monitor feed—see §1.6.
3.5 mm jack — onboard stereo analog (headphones / line out).
USB audio interface — class-compliant multi-channel (2–32+ outputs depending on hardware).

Prerequisite: know how to read aplay -l and map hw:N,M — see ALSA device enumeration. For PipeWire routing, see PipeWire multi-channel routing.

Context: WO-06 Audio playout

Quick pick

Goal	Typical ALSA clue (`aplay -l`)	Caspar channel layout	FFmpeg consumer hint
Monitor / TV over HDMI	`NVidia`, `HDMI`, `DP`	`stereo` … `8ch` (HDMI 2.0 often 8ch capable)	`-f alsa hw:N,M` + `-ar 48000` `-ac 2` … `8`
Switch / router (HDMI/DP → de-embed / Dante / buses)	Same — cable to switcher input	`stereo` … `8ch` (match switcher embed path)	Same `hw:N,M` path; downstream device handles network audio and routing
Simple stereo room / cue	`Analog`, `ALC`, onboard HDA	`stereo`	`-ac 2`
Multi-bus / stems	USB vendor name, many channels	custom `8ch` / `16ch` / `live-8ch`	`-ac` matches layout

Always set sample rate (-ar, usually 48000) and channel count (-ac) to match the Caspar channel video-mode cadence and channel-layout in casparcg.config.

1. NVIDIA HDMI / DisplayPort audio

1.1 Verify kernel audio stack

NVIDIA HDMI audio typically uses Intel HDA binding to the GPU:

lsmod | grep snd_hda_intel

You should see snd_hda_intel (and often snd_hda_codec_hdmi). If the module is missing, fix GPU / driver install first (proprietary nvidia driver vs nouveau affects which codec nodes appear).

1.2 Find the ALSA device

aplay -l | grep -i -E 'nvidia|hdmi|display'

Note card and device (e.g. card 1, device 3 → hw:1,3). Many GPUs expose one HDMI port per device number—use the connector that matches your display cable.

1.3 Test playback

aplay -D hw:1,3 /usr/share/sounds/alsa/Front_Center.wav

(Replace path with any stereo WAV; use a multi-channel test file to validate >2ch if needed.)

1.4 Optional ALSA alias (`/etc/asound.conf` or `~/.asoundrc`)

pcm.nvidia_hdmi_main {
  type hw
  card 1
  device 3
}

Then FFmpeg can use -f alsa nvidia_hdmi_main instead of raw hw:1,3.

1.5 CasparCG notes

Multi-channel: HDMI often supports up to 8ch; confirm with speaker-test -D hw:N,M -c 8 and your display/sink capabilities.
Multiple displays: you may have several device entries—one per output; pick the active monitor chain.
Example consumer args (8-channel): -f alsa hw:1,3 -codec:a pcm_s24le -ar 48000 -ac 8 (match your channel-layout and codec choices to Caspar’s expectations).

1.6 HDMI / DisplayPort into a video switcher (embedded audio as “main” output)

Treat the NVIDIA HDMI or DisplayPort link as a normal multi-channel digital audio output, not only as “sound to a monitor.” Many video switchers and production routers accept HDMI/DP with embedded PCM on their inputs, de-embed that audio internally, and expose it on Dante, MADI, analog buses, or other network / facility routing—so the GPU path can be your primary playout into house audio, IFB, or a Dante domain, with the switcher acting as bridge between Caspar’s embedded stream and the rest of the plant.

Design checks: match channel count and sample rate to what the switcher documents for that input; confirm on the switcher that the correct input port and audio mapping (per-channel vs downmix) match your Caspar channel-layout. If the switcher is also the monitor path, you still use the same ALSA hw:N,M for Caspar—the difference is system role (program vs confidence), not a different Linux device type.

2. 3.5 mm stereo (onboard analog)

2.1 Identify the device

In aplay -l, look for Analog, Front, Headphones, or codec names (ALC892, Realtek, etc.) on the motherboard audio card (often card 0).

Example mapping: card 0, device 0 → hw:0,0.

2.2 Caspar configuration

Use channel layout stereo (or default stereo mapping) on that channel.
FFmpeg consumer: e.g. -f alsa hw:0,0 -codec:a pcm_s16le -ar 48000 -ac 2
System-audio consumer: sends to the default desktop sink (PipeWire/Pulse); good for quick tests on a workstation, less deterministic on a headless server than explicit hw:.

2.3 PipeWire / default sink

If Caspar uses Pulse (-f pulse <sink>) or <system-audio/>, the default output may be the 3.5 mm jack when that’s the selected OS output—use qpwgraph or OS sound settings to confirm which physical port is active.

3. USB audio interface

3.1 Class-compliant devices

Most USB interfaces appear automatically as an ALSA card:

aplay -l

Look for USB and the vendor/model. Note hw:N,0 (device index is often 0).

3.2 Channel count and rate

Match Caspar channel layout to the interface: e.g. 8 outputs → layout 8ch (or custom live-8ch in generated config) and -ac 8 in FFmpeg.
Sample rate must match the channel clock (typically 48000 Hz in broadcast Caspar setups). Mismatch causes pitch/speed errors or device refusal.

Verify channels (example: card 2, device 0):

speaker-test -D hw:2,0 -c 8 -t wav

3.3 Recommended multi-channel gear (Linux-friendly)

Device class	Notes
RME Digiface USB	Very high channel count; class-compliant on Linux.
MOTU UltraLite	Many channels; verify UAC2 mode if needed.
Focusrite Scarlett	Often needs firmware / class mode for full channel count—check vendor Linux notes.

3.4 CasparCG FFmpeg example (8 outputs)

<ffmpeg-consumer>
  <path>-f alsa hw:2,0</path>
  <args>-codec:a pcm_s24le -ar 48000 -ac 8</args>
</ffmpeg-consumer>

Adjust hw:2,0, codec, and -ac to your card and <channel-layout>.

Troubleshooting (all three)

Symptom	What to check
No sound	Wrong `hw:N,M`; another app holds the device exclusively; mute in `alsamixer`.
NVIDIA silent	Cable on correct HDMI/DP port; monitor audio input selected; try another `device` index.
USB only 2 ch	Interface in class-1 mode; driver limitation—vendor firmware / UAC2.
Crackling	Buffer size; try `plughw:`; raise PipeWire quantum (see PipeWire doc).