I have a looped drum groove sample that I want to place in a song, but the tempo of the groove does not match the tempo of the song. How can I alter the sample so that the groove plays in time with the song?
You will want to use the Time Warp DSP function in the Sample Editor for this. Using Time Warp will allow you to change the length of time it takes to play the sample without changing its pitch. However, the difficult thing is to determine the amount of Time Warp you should use.
You need to figure out how much time is required for the loop at the new tempo. The formula to do this is:
|(1/target tempo) x (60) x (number of beats) = (new length of sample in seconds)|
This will give you the number of seconds for the loop at the new tempo.
For example, let's say you need to change the tempo of a loop to 120 BPM. At 120 BPM there are two beats per second, which means each beat is 1/2 second. Therefore if you have 8 beats, that will take 4 seconds:
(1/120 BPM) x 60 seconds = .5 seconds
.5 seconds x 8 beats = 4 seconds
Keep in mind that you want to count the number of beats in the actual sample, NOT the number of beats you are holding the note for. In other words, if you have an 8 beat loop that you are holding for 32 bars, then the sample loops back to the beginning after 8 beats (2 bars) and plays the loop 16 times. You want to use 8 beats in the calculation, NOT 128 beats (32 bars at 4/4).
When you do the math, depending on the tempo, you may not get an number that rounds out to the millisecond (which is the highest resolution you can enter for the new target time). For example, lets say you have an 8 beat loop that you need to change to 158 BPM:
(1/158) x 60 x 8 = 3.0379746835443
The closest value you could enter in the time warp function would be 3.038. So if you hold the note, having it loop back to the beginning over and over, it will eventually drift slightly in relation to the metronome. But it will take quite a few times of looping around before the drifting is noticeable. To compensate for this problem , you have two options. One is that instead of holding a single note and having it loop over and over again, you could have a new note for each two bars so that the sample never loops. You could probably even get away with having a new note every 8 bars, or even more, since the drift is small.
The other option is to tune the sample after it has been time warped. You could tune the sample up 1 or 2 cents. This is small enough amount to make the sample just slightly quicker (and therefore closer to 3.0379746835443 in length), but you aren't likely to notice any pitch change at that small amount.
You should also be aware that if you try and time warp too great of a change from the original time, you will start to hear artifacts in the sample.
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