I think it was the legendary DJ Larry Levan, famed for his decade long residency at the New York club Paradise Garage in the 80's, who said, "I don't play tunes I create atmospheres." To my mind that sums up a DJ's job whether it is creating a convivial ambience in a bar or filling the dance floor in a club. Sometimes the atmosphere makes you want to dance, some time just tap your feet and nod your head.
The way most DJ's do this is a simply matter of matching the beats per minute (bpm) of one track to the next so that the mix from one to another is fairly seamless. Sometimes they get it wrong and the rhythms collide with each other creating a 'train crash'. Sometimes DJ's mix whole tracks together. Sometimes this is called a Mashup other times it is more a matter of adding for example some latin percussion from one track to an electro arpeggio for another to create an interesting juxaposition.
More and more DJ's nowadays are producers in their own right and will break down their own or other peoples tracks into small components which they can digitally sync with a time signature and drop in an out. A good friend of mine does with this with what I have described as mashed up ghetto funk. He takes components of 70' Funk and whether it is an orchestral stab or a percussion riff and loop while adding other often familiar samples on top. This is driven by two incredible percussionists into a danceable frenzy. Fabulous, but you probably have to be there to appreciate the wonder of what he does.
Sometimes the mix bit is a simple transition, aided with a bit of EQing ie taking out the bass on one track as it is brought in. A lot of mystic has built up around the mix and some DJ's are rightly exalted for their ability to create interesting mixes. Sometimes the mystique of the mix hides the fact that they DJ doesn't really have very good taste in music but has rather selected a bunch of tracks which are all of roughly the same bpm and can be easily mixed with each other.
Very simply I tend to divide DJ's into Mixers, Knob Twiddlers and Selectors. The best DJ's combine all three skills. Although nowadays more and more DJ's work digitally rather than with vinyl the principles of mixing are pretty much the same and some DJ are amazing Mixers, demonstrate incredible acrobatic skills in balancing several tracks and components together. Sometimes I am awe-struck by their abilities but sometimes too I find myself thinking, "Mme very clever but it's a bit of a cacophony."
DJ Mixing Skill
Some DJ's are very good at Knob Twiddling, messing around with
the EQ on each track so they whoosh and splinter in spectres of the melody before coming crashing back in on a bass kick. Sometimes I have observed DJ's so engrossed in their knob twiddling that they seem oblivious to the people they are playing for and I think that if they got more interesting music to begin with they wouldn't have to do so much twiddling to make the track sound more interesting.
I suppose I would place myself in the Selector camp in that to me the most important element is interesting tracks. Yes I can mix and I do a bit of knob twiddling from time to time is if I feel that a track would benefit from it but my main concern is the quality and originality of the tracks. I estimate that I might spend around 20 hours a week hunting for new tracks which is another reason why it can be a bit frustrating when people come and ask for requests of some tired old track that has been played to death.
I once heard a story about a Japanese food critic who went to visit a restaurant which claimed to serve the best sea food in the whole of Japan. To test them out he decided to order a basic dish of king prawns. The chef came and stood in front of him, lit a small flame and then slowly turned the King Prawns on a skewer over the flame. He then presented these on a plate to the critic who in a voice of outrage said, "You claim to serve the best sea food in the whole of Japan yet all you did was toast three King Prawns over a flame." "Yes sir, thats right", said the chef, "First we find the best sea food in the whole of Japan and then we cook it." I like to apply this same principle to DJing. Find the best tracks and then play them (in some sort of progressive order)
Because of my style and the venues I play in I am not tied to the tyranny of 128 bpm, or whatever the standard club speed of the time is, and so, while I am overjoyed when I find two tracks which can segue in an interesting fashion with one melody growing out of the other I do not like to confine my mixes to same bpm. I love it, for example, when I find one track that ends by breaking down into a solo piano and mix this with another track that begins with a solo piano. If this works properly you can take people from a downbeat tempo of around 100bmp up to 128 with no one being aware of how it happened.
Because I like to play eclectic sets I like the idea of framing genres. That is to say there might be people who while they enjoy jubilant gospel house with vibrant singing would say that they don't particularly like the form of stripped back electro known as Minimal Techno (or simply Minimal to the congnicenti). However, if done properly I think that it is possible to take people across genres without them being aware of it with the Minimal providing a clean and refreshing respite from the full on bounce of gospel house. For me the best DJ's not only create a great atmosphere but also take you on something of a musical journey.