Crossfader Setup and Deck-Level Modulation

How your DJ app's crossfader is configured, and which controls you reach for to modulate deck levels during a mix, both shape how a loaded fusor behaves at runtime. The interaction applies in both Manual Run Mode and Auto Run Mode: anything the fusor outputs to deck-level controls reaches the same destination whether progress is being driven by the crossfader or by a timer.

What follows is the three common setups, what each one implies for a running fusor, and the authoring pattern that lets a single fusor work cleanly across all three.

Three setups

There are three common ways DJs modulate deck levels during a transition. Each one is a real, working workflow.

Setup 1 — Crossfader not bypassed, crossfader controls deck levels

The crossfader is the primary deck-level control. As you move it from outgoing to incoming, your DJ app's crossfader-curve setting (constant power, linear, sharp, etc.) shapes the actual deck volumes. The deck-level faders stay at full.

Setup 2 — Crossfader not bypassed, deck-level faders control deck levels

The crossfader is left near center; the user mixes by riding the two deck-level faders. The crossfader still contributes to the audio (it isn't bypassed), so on a constant-power crossfader curve a deck playing solo doesn't reach full volume. That's the cost of leaving the crossfader near center on a non-bypassed setup. A DJ might still choose this setup if they want to alternate between or combine crossfader and fader workflows without having to dive into their DJ app and flip the bypass setting mid-set.

Setup 3 — Crossfader bypassed, deck-level faders control deck levels

The crossfader is decoupled from the audio entirely. Deck levels are driven exclusively by the deck-level faders. Many DJs prefer this setup because it gives the widest range of fine deck-level control without crossfader-curve quirks getting in the way.

Setup 2 and the crossfader

Setup 2 collides with how Fusor Studio drives progress. The crossfader is the progress actuator: in Manual Run Mode your hand on it is what advances the transition, and in Auto Run Mode Fusor Studio moves it so it stays aligned with progress. "Park the crossfader near centre and mix on the deck faders" and "run a transition" therefore can't both be true of the same crossfader at the same time.

In practice that leaves you two workable ways to run Setup 2:

Run in Auto Run Mode and let Fusor Studio sweep the crossfader. You mix on the deck faders as usual; the crossfader travels on its own. On a non-bypassed constant-power curve that travel is audible, so it becomes part of the transition whether you wanted it to or not.
Switch to Setup 3 for the transition. With the crossfader bypassed, its travel is silent and the deck faders own the audio outright. This is the setup most DJs who mix on faders end up in, and Per-app bypass toggles below can flip it for you at the start of a run.

Setups 1 and 3 have no conflict. In Setup 1 the crossfader is meant to travel; in Setup 3 it's audibly inert, so whether the engine drives it or not has no audio consequence.

How a fusor behaves under each setup

What a loaded fusor actually does to your audio depends on whether the fusor authored a deck-level domain.

If the fusor doesn't author deck levels

The fusor leaves deck volumes alone. Deck levels come entirely from your DJ app, so whichever of the three setups you're in determines what happens. This is common for fusors that only modulate EQ, stems, FX, or filters and trust the user (or the crossfader) to handle volume.

If the fusor authors deck levels

The fusor's deck-level lanes emit MIDI to the deck-volume controls.

Setup 1. The crossfader and the fusor are both reaching the deck-volume controls. The audible result is the sum of what the crossfader's curve produces and what the fusor's lanes produce. This can be deliberate, if the author shaped the lanes assuming a normal crossfader underneath, or it can produce a fight between two competing curves on the same controls.
Setup 2. Same coexistence as Setup 1, with the added wrinkle above: the crossfader is travelling too, so its curve and the fusor's lanes are both moving the deck volumes.
Setup 3. The fusor's lanes are the sole source of deck-level change during the transition. The crossfader is inert; the deck-level faders sit wherever the user left them.

Disabling the fusor's deck-level domain stops the fusor sending MIDI to those controls at all. The deck volumes are then governed by your DJ app's hardware (crossfader, faders) without any input from the fusor.

The constant-power deck-level pattern

A common authoring strategy is to include a deck-level domain with a constant-power curve in the fusor. The shape mirrors what most DJ apps' constant-power crossfader curves do: a smooth outgoing fade-out paired with a smooth incoming fade-in that sums to a roughly even total level across the transition.

When a fusor is authored this way, the user picks which setup they want and toggles the domain accordingly:

Using the crossfader to control deck levels (Setup 1). Disable the deck-level domain. The crossfader handles volume; the fusor handles everything else. Output stays consistent, assuming the user's crossfader is also on a constant-power curve.
Using the deck-level faders to control volume (Setup 2 or 3). Leave the domain enabled and the fusor's lanes ride the deck levels through the transition; the user doesn't move the faders mid-mix. Or disable it and ride the faders themselves.

This pattern is the closest a single fusor gets to "works for any setup." It isn't mandatory, and some fusors are authored for one specific setup, but it makes a good default.

When the fusor's level shaping is essential

Some fusors modulate deck levels in ways the crossfader can't reproduce: deliberate dips, pumps, half-second drops to silence, mid-transition swells. A constant-power crossfader curve can't mimic those, so disabling the fusor's deck-level domain in favor of crossfader control will lose what the author intended.

If you're authoring a fusor like this, say so in the fusor's notes: state explicitly that the fusor expects crossfader bypass on and the deck-level domain enabled. The user reads notes before arming (see Reading the fusor's intent) and can configure their setup accordingly.

Per-app bypass toggles

Flipping Crossfader Bypass on or off mid-set is cumbersome on many DJ apps: buried menu, modal dialog, easy to miss. Fusor Studio ships a Traktor bypass-toggle action at samples/actions/crossfader pass thru.faction that flips the setting via MIDI, and a fusor's pre-run actions can run it automatically at the start of a transition. The goal is parity across all supported apps, so any fusor can declare its preferred bypass state and have it set for the user without manual intervention. Equivalents for other DJ apps will land alongside their setup guides.

That shipped action is one-way: it switches all four decks' assignments off and nothing puts them back. For a bypass that lasts only as long as a transition, you want a pair — one action that remembers your assignment before switching it off, and one that restores what it found. Bypass the Crossfader, and Put It Back builds that pair step by step (no licence needed), and Build a Constant-Power Crossfade wires it into a fusor as pre-run and post-run, including why the deck-staging step has to come first.