In the realm of digital DJ equipment two types of tempo manipulation rein supreme. I’m going to look at both time stretching and pitch shifting and explain their strengths, weaknesses and how to effectively utilize them.
Time stretching or Audio timescale-pitch modification is defined by Wikipedia as “…the process of changing the speed or duration of an audio signal without affecting its pitch.” This means you can use it to speed up a recording without getting the high pitched “chipmunk” effect or slow it down without making the recording sound deep. The time stretch effect works by sampling small sections of a recording and then playing them back to fill the gaps that are left by slowing a recording down and by cutting pieces of a recording out to conform to higher speeds. This process isn’t quite as simple as I’m making it out to be however, for our purposes this definition will suffice. This process works in real-time on most pieces of equipment. Pitch shifting on the other hand, is the normal effect created by speeding up or slowing down a recording and will create the “chipmunk” effect when sped up and will make the recording sound deep when slowed down. Most modern digital DJ equipment will be able to play music in both time stretch and pitch shift modes.
The benefit of using time stretch mode is that you’re playing the song in the key that it was originally intended to be played in regardless of the speed you’re mixing at. This is crucial when key matching songs. If you have two songs that are 5 BPM (beats per minute) apart and both are in C-sharp they wont clash harmonically when the tempo is manipulated. The downside of this is that you’re going to have degradation in the quality of the recording. You will hear a kind of skipping effect on the deeper tones and kick drums however, if the song leans more towards the higher frequencies this effect isn’t as noticeable.
The pitch shifting effect is what happens when vinyl records are sped up or slowed down and it’s something that DJ’s have been using since DJing was invented. Regardless of the speed you play the recording at it’s quality wont degrade due to time manipulation like in the time stretch method. The basslines will sound full and the kick drums will sound punchy without clicking or skipping. The downside to pitch shifting is that the key or tone of the recordings, when manipulating time (tempo), may clash harmonically. Picture two guitar players who haven’t tuned their instruments, even if they play the same notes they wont be in harmony with each other due to the instruments natural tenancy to become detuned. The two records might be in the same key on their respective tempos but when a tempo change is administered to one in order to mix, the key will change causing the two records to be out of tune with each other.
Personally I use time stretch only when playing songs that have a lot of higher frequency sounds… ripping synths and slashing high hats or when i hear the songs substantially clashing harmonically in my headphones. I use the pitch shift method when playing deeper songs with big bouncy kicks and smooth thick basslines that aren’t covered by higher frequency synths. Many DJ’s either chose to use time stretch or Pitch shift exclusively but I think that both methods have a place in the DJ’s arsenal and shouldn’t be overlooked. If you’re still confused about what method to use or when to use it just get behind the wheels and try for yourself! Try mixing different songs of different tempos applying both methods and note the overall characteristics of the songs. Are they deep with thick basslines? Or are they sharp with a lot going on in the higher frequencies? Feel it out and find out what works best for you. Good luck!
