Its pretty much what bastardo said..
Mixing is all about matching the beats, using the pitch, which like he said is the tempo, or speed in a way..
The difference being is that your using mp3 as opposed to vinyl..
It maybe a whole different ball game, as thats what i was raised on, the principal of mixing is to take 2 completely different tunes, and...Well i always got the pitch even, by matching the kick of the bass drum..
Im sure ive said this before ,so forgive me if i have.
A great example of this is getting two records the same, and matching the beat and the pitch, as if in a mix, and it creates 'phase cancellation' You end up with a weird echo like effect..
But even with two tunes the same you still have to get them matched perfectly, and its not just using the pitch, its little dabs from your finger on the decks, to push or slow the record with real acute fine touches..
This clip of my mate jonny, you can see him on the spindle that holds the 12", just twisting it or holding it, to get the beat aligned , and then he like feathers it at the edge, to get it bang on...
Its a art really..
http://www.youtube.com/watch?v=kTRODZ74rHw&feature=channel_page
Mixing with cd's is basically the same philosophy, just a different technique in principle..
A lot of its done in the headphones, and just by knowing the right place to start and finish the mix, theres always a point in a tune that allows for this..
I guess getting the tracks and matching all the bpms would work, but i always thought it was part of the battle getting the bpms the same, working the mix in mid flow and so on..
And tel ben honest mate with only really slinging vinyl, anything i say would be guessing with cd's..
But it must be pretty much the same principle as mixing vinyl...
(was this a load of waffle or did it make any friggin sense) .?:confused:
Im almost talking aloud here on the fly...