This tool is still being built
It is deliberately unlinked and kept out of search engines. What you see here can change at any time or not work at all.
DJ Tool
MIDI Monitor
See and hear what your deck sends
The tool — try it now
Plug your controller in via USB and the browser listens in: every button, every fader, every jog wheel arrives as a MIDI message and appears line by line in the log — raw as bytes and translated into channel, number and value. On a drawn two-channel deck — jog wheels, transport, pads, mixer with EQ, faders and crossfader — whatever you touch lights up; the monitor learns each of the 45 elements, the way every DJ application does when mapping, and from then on the picture follows your hand. Without a controller, a deck made of pixels takes over and sends the same bytes. And once you turn the sound on, your controller drives two running loops: the crossfader blends, the EQ cuts, the pads hit, the tempo fader pulls — “CC 31 = 64” turns into something you can hear.
What you learn with it
What really happens between your hand and the software. A controller is no magic box but a device that sends numbers — three bytes per move. Once you have seen that, you understand why a mapping in the software works, why a fader sometimes sends two numbers, and where to look when a button does nothing.
How to use the tool
Switch the controller on and plug it straight into the computer via USB — then click “Connect controller” in the browser and confirm the permission
Move any control: a line with channel, number and value appears in the log right away
Click an element in the picture and move exactly that element on the gear — faders over their full travel, buttons pressed once
Check the mapping: the bar has to follow your hand, the button has to light up when pressed
Map as many as you like — the mapping stays in the browser, it is still there on your next visit
No controller around? “Try it without a controller” switches on a deck made of pixels that sends the same bytes — drag the controls, read the log, practise mapping
Press “Turn sound on”, start a deck and mix with the controls you mapped — headphones on
Play ten rounds of blind reach: the monitor names an element, you reach for it without looking
Map the crossfader, both channel faders and the EQ knobs first, then turn the sound on. From there you are no longer practising on a picture but on a sound — and you hear at once what a LOW kill really does.
Common mistakes
Touching the wrong control while learning. The monitor picks the element that moved the most — if you quickly nudge the volume during “Move it now”, you map the gain knob.
Dismissing the permission once and then assuming the page is broken. The browser does not ask again; the setting sits behind the lock icon in the address bar.
Plugging the controller in only after the DJ software is already running. On Windows it often keeps the port to itself, and the monitor sees a device that says nothing.
15–30 minutes
Take the time in one go — hearing and timing develop through repetition, not through reading.
You have it when you can predict, while moving any control, which line is about to appear in the log — and when you get through ten rounds of blind reach without looking at the controller.
Questions about this tool
The monitor cannot find my device. Why?
Three things, in order. Is the controller switched on and plugged straight into the computer via USB — not through a hub that only charges? Does your browser support Web MIDI — the monitor tells you that itself when it opens? And have you granted permission — after one click on “Block” the browser does not ask again; the lock icon in the address bar reopens the setting. On a Mac, Audio MIDI Setup shows whether the system sees the device at all: the DDJ-FLX4 outputs its controls as MIDI over USB according to its manual and shows up there as “DDJ-FLX4”. On Windows, a running DJ application may keep the port to itself.
What do the three numbers in the log mean?
The first byte says which kind of message it is and on which of the sixteen channels it arrives. The second names the control — a controller number for faders and knobs, a note number for buttons and pads. The third is the position or the velocity, zero to 127. A fader moving slowly sends dozens of these messages in a row; a button exactly two, pressed and released. Fine faders often send two numbers at once, a coarse and a fine one — the monitor takes the coarse one when learning.
Can the monitor control rekordbox or play music?
No, and that is deliberate. It only listens: not a single message goes to the device, and no sound comes out of it. Your DJ software keeps running alongside — on a Mac, several programs may read the same MIDI input at the same time, so you can leave rekordbox open and watch here what your hand is doing. A sound of its own in the browser, driven by the controller, is the next step and not this tool.
What happens to what the monitor reads?
Nothing leaves the browser. The messages only exist in the log on your screen and are gone as soon as you close the page. The only thing stored is the learned mapping — one entry per element in your browser's storage, so you do not have to learn it again next time. One click on “Clear mapping” removes that too. There is no server that sees any of it, and no analytics that counts it.
What exactly do I hear when I turn the sound on?
Two loops generated in the browser — kick, bass, hi-hats, chords, nothing downloaded. Your controller is the mixer for them: the crossfader blends between the decks (at constant power, which is why the middle is not half as loud), the channel faders bring them up, HI, MID and LOW cut real bands down to a kill, the filter closes and opens, the tempo fader pulls ±8 per cent, the jog wheel nudges, and the eight pads per deck hit kick, clap, hi-hat, bass, stab and pad. CUE, SYNC and BROWSE stay silent: cueing needs a second output a browser does not have, and a SYNC without a second real track would be a claim. Nothing sounds before you press “Turn sound on”, and nothing at all from an element you have not mapped.
What is the blind reach game for?
Phase 2 states the goal like this: while mixing you no longer look at the controller, you feel it. Until now there was nowhere to measure that — blind reach measures it. The monitor names an element, the clock runs, and what counts is whether you hit it first try and how fast. Ten rounds, a streak bonus, and the best score stays in your browser. Only elements you mapped are asked for — asking about a button whose message nobody knows would mean counting every correct reach as a miss.
I do not have a controller right now. Is the tool still any use?
Yes, through demo mode. “Try it without a controller” switches on a deck made of pixels: you drag faders, turn jog wheels and press PLAY with the mouse, a finger or the arrow keys, and the stand-in sends the same byte sequences a real device would — down the same path, into the same log. That lets you practise everything that counts later on the gear: reading the three numbers, learning a mapping, recognising a jog wheel by its values. The only thing missing is the proof that YOUR controller talks to YOUR computer — that takes the cable.
Tool tried — what’s next?
“Knowing every section of the FLX2” is the matching lesson from phase 2: Hardware Setup (13 lessons). The learning path builds on it: step by step, with comprehension checks and progress that is saved.
- —Progress, XP and streak are saved
- —12 phases from your first track to a club set
- —Free — no download, no payment details
The tools stay free to use without an account.