A musical writeup presented in the time honored question and answer format.
Q: So, what is this harmonic mixing thing?
A: Before I get started, we should make sure that you are fairly aquainted with the concepts of DJing and beatmatching.
Q: Yes, I am.
A: Good. Harmonic mixing is the next step beyond simple beatmatching and blending. It basically involves choosing tracks to mix together based on what key the tracks are in, and whether or not the keys are compatible together.
Q: But electronica is all just boopity-boom computer music.
A: Go to hell.
Q: That wasn't an answer.
A: That wasn't a question.
Q: Fair enough. Isn't electronica all just boopity-boom computer music?
A: Thank you. The answer to that is yes, and no. I'll concede that electronica is made with computer equipment, and that it goes 'boom' along with the occasional 'boopity,' but 'just' isn't exactly an appropriate modifier. An electronic music composition, especially a good one, is a composition of music in many ways similiar to other forms of music, for example, classical. Wouldn't you think it would be a good idea for the DJ as well as the producer to have a working clue of the musical theory behind the composition?
Q: I'm asking the questions here, pal.
A: Fair enough. The answer to my question is yes, it would be a good idea for any DJ to have a working knowledge of music theory, and to incorporate those concepts into his/her performances.
Q: What's the alternative?
A: Well, if you mix two tracks together that are in incompatible keys, it sounds like ass, to put it succinctly. The majority of DJs don't bother to understand it, and pretty much discover which tracks go well together by trial-and-error and random luck. Even then, occasionally they will end up putting two tracks together that are in clashing keys, and you end up with a sour mix, no matter how well they are beatmatched.
Q: What can you do about that?
A: You can learn how to mix harmonically by being aware of what keys your tracks are composed in, and being aware of how those keys interact together to create an aurally pleasing mix.
Q: Oh. What if you don't want to do that?
A: Well, you can cut out the melody and the bassline, and ride the majority of the mix on the percussion line if it turns out that the mix is coming out sounding terrible due to incompatible keys. It still sounds pretty bad, though.
Q: That's kind of whack.
A: Yes, it is. Even worse, it's cutting yourself off at the knees as a musician. The intelligent use of different keys is key (no pun intended) to creating a compelling tapestry of sound as a DJ.
Q: How so?
A: For one thing, the key of a composition helps to determine the mood of the piece. For example, a step up in key will result in renewed energy on the dancefloor, while switching from a major to a minor key would bring about an eerie, mysterious feel. This ability to create changes in mood is a great skill to have as a DJ. Being able to create mixes that don't sound like someone running a jackhammer into your inner ear is a nice bonus as well.
Q: So this isn't about playing in the same key all the time?
A: No. Playing in the same key all the time is, again, cutting yourself off at the knees. You can seamlessly switch the root key, but you have to do it within certain parameters. This is the idea behind harmonic mixing.
Q: So what are all the gory technical details behind this?
A: A discussion of the technical concepts behind music theory is beyond the scope of this writeup. Try looking there, to start.
Q: Righty-O. Anything else I need to know?
A: As far as DJ and mixing specific information goes, you'll need to know that the pitch fader can change the key that a track is in. Basically a pitch adjustment of around 6% is equivalent to one half-step. For example, C major with a +6% pitch shift is D flat major. Some equipment has circuitry that allows you to change the tempo without changing the pitch, but if you don't have this option, you'll need to keep this in mind.
Q: How do you know what key a track is in to begin with?
A: Again, this is a question beyond the scope of this writeup. It involves using a keyboard to play certain chord progressions while listening to the piece, and determining which ones best fit in the track in question. Ideally, a DJ will have already done this long before his set, and have the keys of each of his tracks marked on either the record label or another easily referencable location.
Q: How do you know which keys go well together?
A: Again, this is a concept that has to do with the broader concept of music theory, which is beyond the scope of this writeup. If you want to learn how to mix harmonically, you're going to need to suck it up and learn music theory.
Q: This is really hard.
A: Nobody said DJing was easy.
Q: One last thing.
A: Yes?
Q: Will you play some 50 Cent? I want to grind with this girl on the dancefloor.
A: No.