highascg

ALSA device enumeration (Linux)

This guide explains how to list and interpret ALSA playback devices on Ubuntu (and similar distributions). Use it when wiring CasparCG FFmpeg or system-audio consumers to specific hardware (HDMI, USB interfaces, onboard jack) for multi-channel audio playout (WO-06).

Package: alsa-utils (aplay, arecord). Install if needed: sudo apt install alsa-utils.

List playback hardware: `aplay -l`

Prints hardware PCM devices: one block per card, with device indices.

aplay -l

Example (illustrative):

**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC892 Analog [ALC892 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: USB [Scarlett 18i20 USB], device 0: USB Audio [USB Audio]
  Subdevices: 0/1
  ...

How to read it

Field	Meaning
card N	ALSA card index (0, 1, 2, …).
device M	Device index on that card (0, 1, …).
Bracket name	Human-readable name (chip, HDMI port, USB product).

ALSA PCM hardware ID: hw:N,M = card N, device M.

Examples:

Onboard analog: hw:0,0
NVIDIA HDMI on card 1, device 3: hw:1,3
USB interface on card 2, device 0: hw:2,0

Use these IDs in CasparCG FFmpeg paths such as -f alsa hw:1,3 (see config-generator / WO-06 Phase 2).

List PCM names (including plugins): `aplay -L`

Prints all PCM device names ALSA knows, including soft aliases and plug devices (plughw:, default, pulse, pipewire, etc.).

aplay -L

Typical patterns:

Name	Role
`default`	Often PipeWire/PulseAudio default sink (session-dependent).
`sysdefault:CARD=...`	Card-specific default.
`hw:CARD=...,DEV=...`	Named access to hardware (see `aplay -l` for indices).
`plughw:N,M`	Hardware with automatic format conversion.

Rule: For low-latency, fixed-format routing from CasparCG, FFmpeg → ALSA usually targets hw:N,M or a plughw:N,M if sample format must be converted. For desktop default speaker, default or PipeWire may be easier but less deterministic on servers.

Identify device types

Clue in `aplay -l`	Likely device
`HDA Intel`, `Realtek`, `ALC`, `Analog`	Onboard 3.5 mm / internal codec.
`NVidia`, `HDMI`, `DP`, `DisplayPort`	GPU HDMI/DP audio (often up to 8ch on one HDMI).
`USB`, vendor name (Focusrite, RME, MOTU, …)	USB audio interface (channel count varies).
`HD-Audio Generic`	Could be onboard or secondary HDA; check `device` name.

NVIDIA often exposes multiple HDMI/DP devices as separate device numbers on one card—pick the connector you use.

That same HDMI/DP output is often used as the main multi-channel program feed into a video switcher that de-embeds audio and bridges to Dante or other networks—see Caspar outputs: NVIDIA, stereo, USB §1.6.

Channel capabilities

aplay -l does not always show max channel count. To probe:

Vendor specs for USB interfaces (e.g. 8-out, 16-out).
Test playback with a known multi-channel WAV and FFmpeg/ALSA (e.g. -ac 8), or use speaker-test if installed:
```
speaker-test -D hw:2,0 -c 8 -t wav
```
Inspect card details (if available):
```
cat /proc/asound/cards
```
For PipeWire, use pw-cli, pw-dump, or wpctl status to see node channel counts (see WO-06 Phase 1 T1.2 PipeWire section).

Caspar: match channel layout (stereo, 8ch, custom live-8ch, etc.) to consumer ac / device capability.

Quick reference: card index vs `hw`

Goal	Command
Playback devices	`aplay -l`
All PCM names	`aplay -L`
Card summary	`cat /proc/asound/cards`
Record devices (same cards, capture side)	`arecord -l`

Mapping checklist for CasparCG / HighAsCG

Run aplay -l on the same machine that runs CasparCG.
Note card and device for each output you need.
Build FFmpeg ALSA path: -f alsa hw:N,M (or plughw:N,M if required).
Align sample rate (-ar) and channel count (-ac) with the Caspar channel video-mode cadence and channel-layout.
Document the chosen hw:N,M in your station runbook (firewall and WO-06 T1.4 NVIDIA notes apply separately).

highascg

ALSA device enumeration (Linux)

List playback hardware: aplay -l