StemDeck × A³ Motion
At a glance
A normal DJ set reaches the room as one stereo mix. Move it, and the whole track moves — kick, bassline, vocal and all, like a speaker on a trolley.
StemDeck plays each track as four stems — drums, bass, other, vocals — and sends each stem on its own output. In the A³ setup those four outputs land on A³ Core’s four channels, the same four channels A³ Motion moves. So each part of the track gets its own place and its own path through the room:
the drums can hold the floor while
the vocal circles overhead,
the pads and synths breathe from wall to wall, and
the bass stays put, where bass is happiest.
And it all runs in time with the track. StemDeck can be the tempo master of the whole system: its playing deck’s beat goes out on Pro DJ Link, the Beat Analyzer follows it, and A³ Motion follows the Beat Analyzer. The movements are counted in bars off that beat, so a shape that takes four bars lands on the one of the track StemDeck is playing — no tapping, no CDJs needed.
Nothing here is a special mode. It is StemDeck, the Beat Analyzer and A³ Motion doing what each already does, wired together. This page is the wiring, and a first set to try it with.
Why it works so well
One part at a time, not the whole track. A moving full mix smears; a moving vocal over a drum kit that stays put is a gesture. The ear can follow one thing flying about while the rest keeps its footing.
The stems line up with the channels. A set made by StemDeck’s stem creator puts drums on bus 1, bass on 2, other on 3 and vocals on 4 — and bus N arrives on A³ channel N. Every track you split lands the same way round, so a clip on channel 4 is always moving the vocal.
One clock, from the music itself. With StemDeck as master, the beat A³ Motion follows is the beat of the deck that is playing, tempo fader included. Nudge the tempo on StemDeck and the room speeds up with it.
Both decks, one channel per stem. Each bus carries stem N of deck A and deck B together, so channel 1 is “the drums” whichever deck they come from. Blend from one track into the next and the movements carry on: the channel is the part, not the deck.
Which stem goes where
StemDeck’s ten outputs travel to A³ Core over the network as ten audio channels (zita-j2n on the StemDeck side, zita-n2j on the Core). A³ Core’s patchbay wires the ten network channels into REAPER’s inputs 11–20, and the REAPER project spreads them over the four channels:
StemDeck output |
Network channels |
REAPER inputs |
Arrives on |
In a set made by StemDeck |
|---|---|---|---|---|
|
1–2 |
11–12 |
channel 1 |
drums |
|
3–4 |
13–14 |
channel 2 |
bass |
|
5–6 |
15–16 |
channel 3 |
other |
|
7–8 |
17–18 |
channel 4 |
vocals |
|
9–10 |
19–20 |
the Return track, set by the RET pot |
every stem switched to AUX |
From there a stem is just another source on its channel: it goes through that channel’s strip — gain, EQ and fader on the A³ Mixer, 3d, freq and Q on A³ Motion — exactly like a deck plugged into the desk.
A few things follow from that:
A stem shares its channel with the analog input. The network stems arrive beside that channel’s analog input, not instead of it. A CDJ on channel 1 and StemDeck’s drums play through the same strip and move together. Mix accordingly, or keep the analog deck quiet.
A set with its own stem names (
DUB,KICK,PADS,PERC, …) goes on the buses in the order its endings sort — see Stem sets. Which part lands on which channel is whatever that order says.AUX takes a stem off its channel. Switch a stem to AUX in StemDeck’s mixer and it leaves its bus — and so its A³ channel and its movement — and goes to the Return track instead, whose level is the RET pot (on A³ Motion’s MIXER, and the desk’s FX return).
Setting it up
You need StemDeck, A³ Core with the Beat Analyzer, and A³ Motion — the A³ Mixer is welcome but optional, since A³ Motion’s MIXER page has the same controls.
1. The audio: StemDeck into A³ Core
On A³ Core there is nothing to do: the a3-core package runs zita-n2j for ten channels and its patchbay wires them into REAPER inputs 11–20.
On the StemDeck machine, send the ten outputs to the Core over the
network. StemDeck’s repository ships two systemd user services for this,
under .config/systemd/user/:
zita-j2n.servicesends StemDeck’s 10 channels to the Core, UDP port 65100 — the port the a3-core package’s zita-n2j listens on;zita-n2j.servicereceives 2 channels back from the Core on UDP port 55100, where the a3-core package’s zita-j2n sends REAPER’s recording bus.
Both come back by themselves 2 s after they drop out: zita ends on some changes to the audio graph, and the units restart it with no limit on how often.
Check the Core’s address in
zita-j2n.service(theExecStartline) and correct it if your Core sits elsewhere.Install and start both units, from the StemDeck checkout:
cp .config/systemd/user/zita-*.service ~/.config/systemd/user/ systemctl --user daemon-reload systemctl --user enable --now zita-j2n zita-n2j
Run the same lines again to update them after a StemDeck update.
Patch StemDeck’s outputs into
zita-j2n’s inputs in order:deck1_L→ 1,deck1_R→ 2, …aux_R→ 10. StemDeck never connects anything by itself, and neither do the zita units, so this is on you — once, in qjackctl or any patchbay, and saved. The two channels arriving atzita-n2jare patched by hand the same way, wherever you want them.Settle the StemDeck machine’s sample rate before StemDeck starts. StemDeck takes the rate it finds and asks for nothing, on purpose: changing a running graph throws zita out — it comes back after 2 s, but the stems drop out meanwhile.
StemDeck on the Core machine itself? Then skip the network: patch its ten
outputs straight to REAPER’s in11 … in20, in the same order.
2. The clock: StemDeck as master
In StemDeck, load a set on a deck and wait for the BPM readout. A deck whose tempo is still being analysed sends no beats.
Press MASTER on that deck — or just press PLAY: with no master chosen, the only playing deck becomes master by itself. The top bar reads
PIO master: A(or B).The Beat Analyzer has to be in clock mode 2 (pioneer). The normal way to get there is the next step.
On A³ Motion, tap the clock key (top left) until it reads PIO. The BPM beside it now follows StemDeck’s master deck.
StemDeck sends its beat as a broadcast into the network of its own network interface. The Beat Analyzer on A³ Core only hears it if the Core is in that same network. On one machine it always is; the whole clock chain is described on the Beat Analyzer page under Playing without CDJs.
3. The room: 3d up
On A³ Motion, turn 3d up on the channels you want to move — the pot above each channel’s pads, or 3D in its field in the channel row. At 3d = 0 a channel is plain stereo, however busy its blob looks on the screen.
Your first set
Ten minutes, one track, four stems in four places.
Load a track in StemDeck. Pick one you know well, split by StemDeck (drums, bass, other, vocals). Check the meters in StemDeck’s mixer: all four buses move. On A³ Motion the four channel meters move with them.
Make StemDeck the clock as above: press PLAY, check
PIO master: A, set A³ Motion to PIO. The BPM on A³ Motion matches StemDeck’s deck.Load a set on A³ Motion. FILES › SETS › Warmup › Load, then close FILES. Four calm clips, one per channel — so one per stem.
Bring up the room one part at a time. Turn 3d up on channel 4 first: the vocal leaves the speakers’ stereo image and starts to travel. Press channel 4’s Play|Pause; the clip starts on the next downbeat. Add channel 3 (the other parts) next, then channel 1 (drums).
Leave the bass alone — or at least low and slow. Low frequencies are hard to place anyway, and a bassline on a roller coaster mostly makes the room feel seasick.
Play the stems with the actions. Hold A1 on the vocal channel for a gentle lift, let go, and it comes back. In every shipped set the left pads push the energy up, the right pads take it down.
Mix the next track in on StemDeck’s other deck. Its drums join channel 1, its vocal channel 4 — already on the move. When you hand MASTER to the new deck, A³ Motion follows its tempo.
Walk the night on. A6 on a channel cues that channel into the next phase (Groove after Warmup); load the next set in FILES when you want all four at once.
Something to try once it runs: mute a stem in StemDeck (M, or keys
1–4 for deck A) and watch its channel meter drop on A³ Motion while the
clip keeps going. Bring it back on the one and it re-enters already in
motion. It’s a small thing; the room will think you rehearsed it.
Troubleshooting
Symptom |
What to do |
|---|---|
A³ Motion’s channel meters stay still while StemDeck plays |
The stems don’t reach the Core. Check StemDeck’s outputs are patched into zita-j2n, and that zita-j2n reaches the Core |
Drums show up on the vocal channel |
The outputs are patched out of order on the StemDeck side. |
A stem plays but doesn’t move |
Its channel’s 3d is down, or its clip isn’t playing. Or the stem is on AUX: then it is on the Return track, not on a channel |
A³ Motion doesn’t follow StemDeck’s tempo |
Wait for the deck’s BPM; is the deck playing and the top bar saying |
The movements start one beat off the bar |
StemDeck counts the bar from the first beat of the track’s beat grid. Set the downbeat with the jog |
The stems stop after a change in the audio settings |
Changing the sample rate or buffer of a running graph throws zita-j2n out; its unit restarts it within about 2 s. If the stems stay away, check |