CortexDJ
In your browser · free · beta

Make a 15-minute set from your own music

Pick 10 to 20 of your tracks. This page analyses them right here, puts them in a mixable order by tempo, key and energy, and plays them as one continuous, beat-matched set. Then hear where each transition lands.

Drop 10–20 audio files here

MP3, M4A/AAC, WAV and OGG work in every current browser; FLAC and AIFF depend on the browser. Each track needs at least a minute of audio.

Your music stays on this device. The files are read and analysed by this page, in this tab. Nothing is uploaded or sent anywhere, and nothing is kept after you close the tab.

What this page can and can’t do

It is a preview, so you can hear the idea on your own music before a 370–388 MB download. The app does the same job properly.

This browser version

  • Up to 20 tracks and about 15 minutes, played live in this tab. It is not recorded, saved or exported.
  • Tempo is matched by changing the playback speed, so the pitch moves with it (no keylock). Small tempo gaps sound natural; big ones don’t, which is why the order favours close tempos.
  • The beat grid comes from the kick drum. There is no downbeat or phrase detection, so a blend can start on the wrong beat of the bar.
  • The key is a rough estimate from one minute of each track, and energy here is just loudness. Key tags in your files are not read.
  • Every transition is the same kind: an 8- or 16-bar crossfade with a bass swap. Each track plays an excerpt, not the whole song.
  • Keep the tab open and the screen on. Phones may run out of memory on long or high-resolution files.

The CortexDJ app

  • Sets of 15, 30, 60 or 120 minutes from your whole library, up to 20,000 files in a folder and its subfolders.
  • Detects each track’s Camelot key, BPM, a 1–10 energy level and its structure, and reads key tags from Mixed In Key or Beatport.
  • Transitions of 32, 64 or 128 beats, beatmatched on the kick drum with one bassline at a time; it leaves out tracks whose beat grid it can’t lock.
  • Changes tempo without changing pitch, and renders the set to WAV or FLAC with a timestamped tracklist. Or play it live on two decks, or export it to rekordbox, Serato and five more DJ apps.
  • Runs offline on Windows 10/11 and on Apple Silicon Macs (beta). Free for home use; your first set needs no key.

Make the 2-hour version in the app

Same idea, your whole library and a finished file at the end. Free for home use, with no key and no email for the first set.

Make the 2-hour version in the app

Windows 370 MB · macOS beta 388 MB · or mix two tracks by hand in the web mixer

Questions

Are my files uploaded?
No. The page reads the files you pick with your browser’s own audio decoder and analyses them in this tab. There is no upload, nothing is stored on our server or in your browser, and closing the tab forgets everything.
Why was a track left out?
The page says why next to each file: the browser could not decode it, it is silent, it is shorter than a minute, or no steady beat was found (“BPM not detected”: ambient, beatless or very loose tracks). The app’s analysis is stronger, but it also leaves out tracks whose beat grid it can’t lock.
It sounds off in a transition. Is that what the app does?
Probably not. The browser version bends pitch with tempo, guesses the key from one minute of audio and doesn’t know where the bar starts. The app changes tempo without changing pitch, analyses each track’s structure, and its beat and key analysis is stronger.
Does it work on a phone?
Yes, in a current Safari or Chrome. Pick the files from the Files app, keep the screen on, and on an iPhone turn the ringer on if you hear nothing. Very long or high-resolution files can run a phone out of memory.