Why does music often "stutter" when the BPM is manipulated in ITCH?

Question

I've been using ITCH 2.0 with my Twitch for a little while now, and have been running into an issue where audio playback is simply unpleasent.

Every now and then I'll be playing a track, and will have to adjust the BPM of the next track in order to adequately beat match it for crossfade purposes. Occasionally doing so will cause the next track to have an obnoxious "stutter" effect during playback, that goes in and out in relatively steady intervals, though not directly evenly on a beat.

Depending on the type of song, and the amount of BPM change, this stutter can be more or less apparent; But it's completely obvious and obnoxious to my inside my headphones.

I've even run into situations where this problem doesn't occur at all, even when the song's speed is lowered/raised significantly more than usual. (> 30-40 BPM difference).

What could be causing this stutter, and what can I do to mitigate it?

Answer 1

I think what you're experiencing is the side affects of changing the speed of a clip without changing the pitch. Typically if you try and slow down or speed up a piece of music, it's pitch will get higher or lower respectively, like slowing down a 12" vinyl record. This happens because when you slow down a piece of audio you're reducing the frequency of vibrations. And frequency is pitch by definition.

For the software to maintain the pitch while slowing it down, instead of stretching out the clip it breaks it into chunks. It then spreads those chunks out leaving spaces in between them. The added space slows down the clip, but each chunk maintains it's pitch because they haven't been stretched.

So I'm pretty sure the stuttering you're hearing is those little gaps in the track. The effect will be more pronounced with long drawn out notes on an instrument or voice. If you're just listening to percussion, you may not notice the gaps.

Now I'm sure there's more sophisticated software than can then fill in those gaps by guessing what should be there. But now we're getting into territory that is unexplored for me.