four decks, nothing hidden
Every four-deck controller I have used banks two decks onto one set of controls. You press a button, the knobs change meaning, and half your rig becomes invisible. In a studio that is fine. In a dark booth at 1am it is a small tax you pay on every single move.
So I specced the opposite: a control surface where every control for every deck is permanently live. No deck-switch button, no shift layers, no menu diving.
the shape
It sits on a decksaver, on top of a CDJ you are not using — 320 by 415mm, the CDJ-3000 footprint exactly, about 45mm tall. Four identical vertical strips. Library across the top, then per-deck transport and loops, then dual FX, then the mixer section reading top to bottom the way a channel strip should.
That is 40 potentiometers, 5 encoders, 38 buttons and 4 toggle switches. Around 290 dollars in parts.
the part I like most
Each channel gets a toggle that flips its three EQ knobs between standard 3-band tone control and stem isolation — vocals, drums, bass.
The knobs do not move. The firmware just sends different CC numbers depending on the switch position. Your DJ software learns both sets once, and one physical switch swaps an entire channel strip between two completely different jobs. It is the cheapest feature in the build and probably the most useful.
what it deliberately does not do
No audio. It is a control surface only, with the interface as a separate box on the same powered hub — if one fails you swap that one thing rather than the whole rig.
And it will not replace a jog wheel for scratching. The per-deck encoders are for scrubbing and needle-search. If you scratch, this is not your controller.
the honest status
This is a design, not a finished object. It is specced to the point where the next step is ordering parts, and I am publishing it in exactly that state — gaps included — because build references written after the fact quietly omit all the decisions that were hard at the time.
The slowest phase, incidentally, is the MIDI mapping, and it does not need the board to exist. That is what the prototyping piece is about.
Full reference — layout, control inventory, the Teensy brain and its iOS gotcha, multiplexing 40 pots onto three pins, the button matrix, MIDI map, bill of materials and build order — is in the Lab: 4-Deck MIDI Controller Build Reference.