AES67 streams

Every audio stream the rig sends on the audio network, and where a receiver patches it in. The table is rendered from a3-osc.json (truth); the receiver section below is written by hand. The audio network itself (second socket, PTP, EEE) is on the ports page.

What is sent

All streams: 44.1 kHz, 24 bit, multicast, port 5004.

Stream	Group:port	Packet time (ms)	Channels
AES67 {host} out	239.69.150.243:5004	0.5	12
AES67 deck {deck}	239.69.150.244 + slot:5004	0.25	41
AES67 Core MAIN	239.69.200.1:5004	0.5	n + 1
AES67 Core BOOTH	239.69.200.2:5004	0.5	n + 1
AES67 Core REC	239.69.200.3:5004	0.5	2
AES67 Core PHONES	239.69.200.4:5004	0.5	2

• {host} is the sending machine’s hostname, {deck} the deck letter. A deck’s group is the base address plus one per deck slot (A = .244, B = .245, and so on).

• A zone stream (AES67 Core MAIN, AES67 Core BOOTH) carries the zone’s n loudspeakers and its sub, if it has one: n + 1 channels with a sub (sub first: SUB, T1..Tn), n without (T1..Tn). The packaged default layout is 4.1, so 5 channels. REC and PHONES carry two channels, L R.

• A deck stream has 41 channels: MAIN1-16, AUX1-16, CUE1-9.

• A packet must fit an MTU of 1500: at most 22 channels at 0.5 ms, 44 at 0.25 ms.

Finding a stream

Streams are announced with SAP/SDP on 239.255.255.255, port 9875. PipeWire’s module-rtp-sap both announces and discovers them. The clock is PTP on the audio network.

Which announced streams a receiver turns into sources depends on its module-rtp-sap config:

• No explicit stream.rules: PipeWire creates a source for every announced stream.

• Explicit rules: only the sessions they match. The A3 rig’s own configs list rules (they match ~^AES67 deck and ~ out$), so they ignore the AES67 Core streams. A host that should receive them needs one more rule in its stream.rules:

{ matches = [ { rtp.session = "~^AES67 Core " } ]
  actions = { create-stream = { node.virtual = false media.class = Audio/Source device.api = aes67 } } }

Where a receiver patches it

The Core receives streams as input ports of the engine. They are not linked for you: patch them by hand in qjackctl or Helvum.

Stream received	Engine input ports
deck X B-format	deckX_1 … deckX_16
deck X return B-format	deckX_aux_1 … deckX_aux_16 (these feed the delay)
analog 1-2, analog 3-4, …	analog_N, analog_N+1 for a pair N, N+1

Warning

The analog_N ports are shared with the sound card’s capture ports. A stream patched onto them adds to whatever the card delivers on the same channel; it does not replace it.

A deck in stereo mode

A deck set to Output: Stereo (StemDeck settings) does not send the 41-channel stream. It sends one stereo master pair as a 2-channel stream, named like the full one (AES67 deck <X>), with its channels called MASTER_L and MASTER_R. A receiver sees them as receive_MASTER_L and receive_MASTER_R.

Every deck slot of the Core has its own pair of inputs for it, deckX_stereo_L and deckX_stereo_R, beside its B-format inputs. The link map links them from the slot’s source, the same source as its B-format: the local StemDeck’s master_L/master_R, or the stream’s receive_MASTER_L/receive_MASTER_R.

Deck	Left	Right	Core input
A (local StemDeck)	StemDeck-A:master_L	StemDeck-A:master_R	deckA_stereo_L, deckA_stereo_R
B (AES67 stream)	AES67 deck B:receive_MASTER_L	AES67 deck B:receive_MASTER_R	deckB_stereo_L, deckB_stereo_R
C, D, …	the slot’s source, as for A or B		deckX_stereo_L, deckX_stereo_R

No patching is needed. The links are made as soon as the deck is in stereo mode and skipped while its ports do not exist. A link map saved before these inputs existed gets them when it is loaded, from each slot’s source. The pair does not touch analog_N, so it never adds to the sound card’s capture.

How Core tells the modes apart: by the links, not by the signal. A slot whose stereo pair is linked and whose B-format inputs are not is in stereo mode. A slot with both linked, or neither, is treated as B-format.

On the desk the deck keeps its letter on the channels’ input selectors, as in B-format mode, and is picked the same way. On a channel, its pair runs through that channel’s strip like an analog input: TRIM, the 3-band EQ, the DJ filter, the 3D band and VOLUME, encoded at the channel’s position. CUE and AUX-Send act on it as on any analog input. A change of source, or of the deck’s mode while it plays, is smoothed over 10 ms. The deck’s bar shows the middle of its pair.

With A³ Motion a channel playing a stereo deck is an analog channel. The engine tells Core each slot’s mode (/engine/deck/{deck}/stereo), and Core tells the desk and A³ Motion (/stemdeck/{deck}/stereo). A³ Motion still shows DECK A (or B, C, D) on the channel, and moves it as it moves ST: the position, the clips and their trajectories, actions, 3d, FREQ and Q. The position, 3D, FREQ and Q belong to the channel, not to its source. The channel keeps them when it changes between its analog input, a stereo deck and a B-format deck. While a B-format deck plays, StemDeck places it and the channel’s values only wait. When the channel turns analog again, by a pick or by the deck changing to stereo while it plays, Core gives the engine the channel’s current values first.