Press the Smart Fader button on a DDJ-FLX4, slide the crossfader across, and two tracks at different tempos blend without you touching a pitch fader or an EQ knob. It feels like a trick. It is a set of automatic moves, and once you know what they are, you can hear every one of them on a recording.

So record the same short set twice, once with Smart Fader on and once with it off, and listen back. Before you do, know what the button does in your app (it changes from one app to the next) and why that matters once you are standing in a booth.

What Smart Fader is

The Pioneer DJ DDJ-FLX4 carries a Smart Fader button, and so does the AlphaTheta DDJ-FLX2. Pioneer's product page says it "automatically adjusts the volume, bass volume, and BPM of the music so you can smoothly transition from one track to the next by simply moving the channel faders or crossfader."

That description fits rekordbox. Load a different app and the same button can do a different job, a point Pioneer makes itself in a footnote on the DDJ-FLX2 page. The manuals document:

  • rekordbox (Mac/Windows and iOS/Android): handles tempo, bass and volume for you and adds an automatic echo. The full Smart Fader.
  • Serato DJ Lite on the DDJ-FLX4: the button applies an effect preset as you move the faders. Press it for an echo type, hold SHIFT and press for a reverb type. The manual mentions no tempo or bass handling.
  • djay on the DDJ-FLX2: the button turns on Crossfader Fusion, with SHIFT changing the effect type.

The rest of this article assumes rekordbox, because that is where the tempo and bass behaviour lives.

What happens during a Smart Fader transition

The DDJ-FLX4 manual is far more specific than the marketing copy. Four things happen.

Beat Sync switches on and stays on

Press the button and Beat Sync comes with it, locked on for as long as Smart Fader is. The manual is blunt: "Beat Sync can't be canceled." Meanwhile you also lose control of EQ LOW and some of the effects from the rekordbox screen.

The tempo follows the fader

This is the part that is easy to miss. Press normal Sync and the two decks lock to one BPM, which then holds while you mix. Smart Fader works the other way round. Wherever the fader sits sets the tempo, so as your hand moves, the BPM moves with it, from the start of the blend to the end.

Pioneer's own example uses a 100 BPM track on deck 1 and a 120 BPM track on deck 2. Fully on deck 1, you hear 100 BPM. With both decks at equal volume, both play at 110 BPM, the midpoint. Fully on deck 2, you are at 120. Push the gap too far and you run into the manual's warning: with tempos that are "significantly different", Smart Fader "may not work properly".

The bass swap happens for you

rekordbox takes over EQ LOW. Split the crossfader throw into thirds and, in the manual's crossfader example, the swap lives in the middle one: across that stretch, deck 2's low end climbs from minimum up to centre while deck 1's drops from centre to minimum. Look at the screen and the EQ knobs sit still, even though you can hear the bass moving.

An echo is added

"The echo effect is automatically applied according to fader operation," says the manual. Push the crossfader all the way across and the track you are leaving gets the echo. Pull back from that edge and there is no echo on the way.

You can tune some of this. In rekordbox for Mac/Windows, Preferences > Controller > Mixer > SMART FADER lets you choose whether it reacts to the crossfader, the channel faders or both, and whether to use the recommended Smart Fader curve. On iOS and Android the fader choice sits in the DDJ-FLX4 settings.

Why it matters once you leave the bedroom

Smart Fader is a good first step. You get musical transitions on day one, which keeps you playing.

The catch shows up later. Pioneer's DJM club mixers and CDJ players have no Smart Fader button, so in most booths everything it does becomes your job. You set the tempo. You make the bass swap, at the moment you choose. You decide whether an echo belongs there at all. And Pioneer does not describe Smart Fader aligning phrases, so starting the next track in the right place is already on you, with or without it.

None of that is a reason to stop using it tomorrow. It is a reason to find out what it is covering for you.

The practice method: record it both ways

Hearing your own transition speed up halfway through teaches more than any description. Record two short runs of the same material and play them back.

Get the recording route right

The DDJ-FLX4 does not create the mix. Your laptop, iPad or phone does that in software, and Pioneer's wording is that the computer "will supply both power and sound" to the controller. What the FLX4 adds is the sound card: the finished mix arrives over USB and leaves through MASTER and PHONES. So plugging the controller's USB cable into an iPhone running Ohm will not record your mix. Nor will running rekordbox for iOS and Ohm on one phone, because Ohm cannot record another app's output on the same iPhone.

The route that works:

  1. Run your DJ software on a laptop, an iPad or a second phone, connected to the DDJ-FLX4 by USB as usual. If the host is a phone or tablet, give the FLX4 its own power through its power port.
  2. Connect the FLX4's MASTER output (a pair of RCA sockets) to the line inputs 1 and 2 of a class compliant USB audio interface. The DDJ-FLX2 is different: its MASTER output is a single 3.5 mm stereo mini jack, which needs a cable that splits into two RCA plugs or two jacks at the interface end.
  3. Plug the interface into the iPhone running Ohm. Ohm picks up the USB device automatically and shows its name on the Record screen. There is no input to choose.
  4. Monitor as you normally would, with headphones in the controller's PHONES output.

Expect a strong signal. At a rated 2.1 Vrms, the FLX4's MASTER output is hot enough to overload the line inputs on some interfaces. So turn the controller's MASTER LEVEL well down first, set the gain on the interface, and only then raise the master, with one eye on Ohm's meters. Our guide to gain structure for DJ recording walks through it, and if you need hardware, see the best USB audio interfaces for recording DJ sets.

And since the file is a copy of the MASTER output, every tempo glide, bass swap and echo Smart Fader makes is captured in it. That is the point.

Run the test

  1. Choose three or four track pairings, and make sure one of them has a big difference in BPM so the glide is easy to hear.
  2. Run one, Smart Fader on. Record it in Ohm, then rename it in the Library (hold the title) to something like "Smart Fader on".
  3. Run two, Smart Fader off. Repeat the tracklist exactly. Match tempo with Sync or by ear, swap the bass yourself on the EQ LOW knobs (our guide on how to mix with EQ covers the technique) and start each incoming track on a phrase. Rename it "Smart Fader off".
  4. Back in the Library, play each recording. To reach a given transition in each file, scrub the waveform or jump with the 15 second skips.

Keep both runs on the same Ohm quality setting so the only difference is your mixing. If you have Auto-Stop switched on in Settings, a long pause between runs will end the recording after five seconds of silence. That gives you two separate files, which suits this exercise.

What to listen for on playback

Take it one transition at a time, Smart Fader run first.

  • Tempo drift. Does the outgoing track speed up or slow down while the two tracks overlap? With Smart Fader on, it should. In your manual run, the tempo should hold steady.
  • Pitch moving with the tempo. If key lock (Master Tempo) is off, a tempo change shifts pitch too. In Pioneer's 100 to 120 BPM example, deck 1 at 110 BPM runs 10% above its own tempo, which lifts it a little over one and a half semitones. Listen hardest on vocals and pads, where the shift is easiest to hear. Look at each deck to see if Master Tempo is lit, and read how key lock affects recording quality for the detail.
  • The bass hand off. Smart Fader does its swap while the crossfader is in its middle third. Is there a stretch where two kick drums fight, or one where the low end vanishes? Set that against the point where you swapped the bass yourself in run two.
  • The echo tail. Is it on every transition? Do you want it on every transition?
  • Level dips or jumps. A blend that sags or bulges in the middle points to the fader curve, yours or Smart Fader's.
  • Phrase landing. Did the new track arrive at the start of a phrase? Our piece on why phrase mixing matters when you are recording explains why it shows on a recording.

When to switch it off for good

The test answers this for you. Once the recording where you did everything yourself holds up next to the Smart Fader one (steady tempo, a bass swap you chose, no level dip, transitions on the phrase), you have learned what the button was doing.

You might still switch it on for a relaxed party set after that, and there is nothing wrong with that. The difference is that it becomes a choice. Run the test again every few weeks; the gap between the two recordings is the most honest progress report you will get.