Beatmatching is a mixing technique employed by DJs. While originally it involved counting the tempo with a metronome and finding a record with the same tempo, today it involves changing the speed at which a recording is played back so that its tempo matches that of the song currently playing. In this way, the DJ can either simultaneously play two songs of different original tempos without their beats clashing or "galloping" or can more smoothly transition between two songs. The tempo of the recording can be changed through the use of specialized playback mechanisms. In the case of vinyl records, for example, the turntable would have a separate control for determining the relative speed (typically listed in percent increments) faster or slower the record can be played back. Similar specialized playback devices exist for most recorded media. Changing the speed the record is playing is called pitching or pitch shifting.
For a DJ wishing to beatmatch using vinyl turntables, the general procedure involved is relatively simple to explain, but harder to master.
Assume, as is the case for most DJs, that we have two turntables with records on them, T1 and T2, and a mixer capable of variably blending the outputs of T1 and T2, and also allowing the DJ to cue the music playing on one or other of turntables (listen to it on headphones without outputting the sound to the audience). In order to control the records on the turntables, the DJ must be competent in moving the vinyl record with his hand whilst the platter of the turntable is moving. Typically a slipmat is placed on the turntable between the record and the platter, to reduce the friction between the two, and allow the DJ to search through a record by moving it with his or her hand, as well as momentarily speed up or slow down the record, by pushing/nudging it on, or holding it back (typically by pressing a finger against the side of the turntable platter to reduce its speed).
Now, if T1 is playing to the audience, and we wish to mix the beginning of T2 into the closing sections of T1, we will most likely need to adjust the pitch of T2 in order to match the tempos of the two songs, and the beats.
To do this, the first thing a DJ must do is find the start of the beat in T2. For a lot of dance music, this is typically the first sound of the song, often a kick drum. The DJ can find this beat by playing the record until he hears the sound of the first beat in his headphones, which will be set to cue T2. Once the first beat has been found, the DJ can hold the record with his hand and prevent it from spinning, thus pausing the sound. To start it again, he or she must simply release the record, and the spinning platter will start the record moving again. To find the first beat again once the song has begun playing, the DJ can rewind the record, again with his or her hand. In this way it is possible to start T2, rewind it, start it again, etc...
Once the DJ is confident he or she has found the first beat, and can start it at the exact moment they desire, the process of beatmatching can begin.
The basic process is as follows:
Hold T2 on its first beat (the DJ hears it through headphones, the audience cannot).
Listen to T1, and try to follow the beats to a bar (i.e. know which is the first beat of the bar).
Start T2 on the first beat of a bar (ideally this will be the first bar of a section of T1, see below). Assuming the tempos of T1 and T2 are close, the two tracks will sound 'in time' to begin with.
If the tempo of T2 is too slow, it will fall behind T1 (listening to the drums often highlights this). If this is the case, speed up T2 (see above) until it is again in time with T1.
If the tempo of T2 is too fast, it will overtake T1 (again, the drums are often the easier way to hear this). If this is the case, slow down T2 (see above) until it is again in time with T1.
Repeat this process until you are sure that T2 is definitely slower or faster than T1.
Now, once we know if T2 is too fast or too slow, we can permanently adjust the pitch using the pitch slider (found to the side of all DJ turntables) to adjust the tempo of T2 and bring it closer to that of T1.
Next we must repeat the whole process of testing T2 to see if it is too fast, too slow, or just right to blend with T1, and adjusting as necessary.
After a few iterations of this procedure (the speed at which it is performed varying according to the skill of the DJ) the tempo of T2 should be very close to that of T1. If this is so, the DJ will find that even after listening to T2 for a (relatively) long time, it will not fall out of time with T1. At this point we can be sure our beats match, and T1 and T2 are at close enough speeds that they may be blended.
Now we can simply start T2 at the desired point in T1 to create a nice effect, layering the outro of T1 over the intro of T2.
To beatmatch, a DJ must be aware of both the songs he or she wishes to blend at all times. A fact that makes the life of a dance music DJ a lot easier is that the majority of dance music is based around the 4/4 time signature. In broad terms, this has the effect that the sections of a dance music song will have length equal to some multiple of 4 bars. Most commonly, if you count the bars in a section of a song, they will be 4, 8, 16, 32, or 64 bars in length. This information helps the DJ decide at which point during T1 he must start T2 in order for the sounds of one track to fade as the other builds, or whatever effect is desired.