system
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human
ELI5: what exactly are DJs such as Skrillex and Deadmau5 doing when they perform live besides just playing the songs from their album?

Just from watching videos, it looks like they are busy turning knobs and flipping switches, but then I've seen clips where they will dive into the crowd for several minutes with no discernible affect on the music.

I realize that the songs are probably mixed differently from what's on the album, but couldn't they just be playing a pre-recorded mix and pretending to be manipulating the equipment?
gpt
Back in the days, when laptops were not used for live performances, the main task for a DJ was basically to beatmatch 2 tracks.

This was done by listening to the track you want to play next on your headphones and concentrate on the beat to get an approximation on the bpm (beats per minutes). Now you check out the track that is currently playing and do the same. If you're really good at bpm counting you can get a pretty precise result that is +/-1 of the exact bpm the track really is.

All these information tell you which track is slower and which one is faster.

Now you either fade in the song that is currently playing to your headphones or just slide one side of the headphone off of an ear and listen to the speakers. The tricky part is to pitch THE NEXT TRACK (you will never want to pitch the current track, because the audience would hear it) so it aligns with the current track playing. Once the beats are matching (meaning: there is no stuttering, or the beats don't sound like a "horse-gallop") you need to keep listening to this for some more and hear if the tracks stay beatmatched. This is often not the case, especially at the beginning of this process. If you hear the tracks not aligning anymore, you need to repitch (run slowlier/faster) the next track. Think of 2 marathon-athletes: The guy in the lead is running slowlier than the guy behind him. Naturally, the guy behind him overtakes him. At the moment where he's overtaking him they are "beatmatched". Now: if they would run the same speed, they'd run next to each other (beatmatched) forever, but since one guy is faster, he will overtake the guy in the lead and start running in front of him. To make them run next to each other again we'd have to tell the guy now in the lead to run slowlier, or the guy now behind to run faster.

You do all this with the pitch slider, which controls how fast a track is playing, and by touching & scratching the scratchpad of a CD player or even more oldschool: by touching/scratching/pushing a vinyl itself.

You also need to find good spots to drop the next track in, because just randomly throwing in the new track -even if beatmatched- will most likely sound awful. You need to be aware of the structure of tracks. A good rule of thumb in dance music is to align 2 tracks on the 32nd beat, because that's where normally a new sample is introduced to a track or something happens.

So that's just beatmatching which a DJ has to do in order to please the audience.

Now with the mixer and all his knobs he can add some effects and manipulate the Equalizer (EQ). One thing that is often done is to kill the low end of the EQ which basically eliminates the bass of a track (When a DJ grabs a knob on the mixer and is quickly turning it to the full left it is most likely a kill). This helps in a transition f.e. so you don't have 2 beats at once which might cause distortions and unpleasant sounds.

Nowadays however everything is automatic or you know all the information ahead: -mixers and CD Players show you the bpm

-mixers/CD Players beatmatch for you

-laptops beatmatch for you and even fade in/out for you

The skillful and impressive thing these days though is, sampling while mixing. DJs can create samples (small soundbites) & loops (soundbites which can play in a loop -a bassline f.e.) at home on their laptops with the necessarry programs, which they can then throw in while playing tracks to a live audience. Some DJ's are so creative, that they create whole new tracks live using all these samples they pre-created at home. To do all this, DJs use launchpads. These are boards with a lot of buttons (each representing a sample f.e.) connected to a laptop.

To answer the question of the pre-recorded mix playing: it is absolutely possible someone made a mix at home, burned in on the CD or even simpler saved it on his laptop which he uses in his performance and presses play.




TL;DR;ELI5

In the past: You had to listen to the tracks and estimate the speed of the tracks and then make the next track as fast as the currently playing track by pitching them manually.

You have some effects and an equalizer to play around with.

Today: Beatmatching is mostly done by the machines themselves.

The focus lays on sampling, which means putting together completly new tracks or remix a playing track on-the-fly with samples (soundbites) and loops (repetative soundbites).