Most effective time shrinking for phase coherence between clips

[Sean Michael Robinson]

Hello CWBBLabers!

Let's say you had an excellent multi-mic recording — in this case, close-miced acoustic guitar, upright bass, and accordion — that you were really happy with, save for one thing — the tempo of the performance is just a hair too slow. So you'd like to speed it up just a bit, without affecting the pitch, and you turn to your trusty CWWBL to make it happen.

I have experience doing this with great results to individual isolated tracks-- but in this case all three tracks (again, acoustic guitar, upright bass, and accordion) have some bleed between each close mic. That is, there's a substantial amount of acoustic guitar in the upright bass mic, and a minimal amount of ac. g and bass in the accordion mic as well.

Given that there are phase relationships to preserve between these three spot mics, what would be the very best method to shrink/alter the tempo of this audio and not create a newly-phasey mess? This of course is also applicable time you'd want to stretch or shrink stereo piano or drum mics as well, so I'm guessing whatever the solution is here is also the best method to that situation as well.

Would appreciate any and all advice on the topic!

All the best,

Sean

[57Gregy]

If it's just a slight increase in speed you want, have you tried Process>Length on a stereo mix of the song?

If that's still a thing in CbB.

[RBH]

I haven't tried this myself - so I'm hypothesizing, I think the way to address this is to alter the tempo map. It should act as a master offset of sorts for all tracks and hopefully not alter the phase relationships.

[Sean Michael Robinson]

Update: Whatever was glitchy before when I was attempting to select and stretch or compress multiple clips is...no longer glitchy

I selected a small segment of all three tracks, went to the Process -> Length menu, selected the desired duration and tried out the new Elastique Pro algorithm. Nice and duration shifted with no noticable phase difference, either to the ear or according to Span. So either the amount of bleed is negligible for this purpose, or the new algorithm is doing it's job. Either way, mission accomplished!

[Noel Borthwick]

Elastique preserves phase coherence so you should have no problems stretching multiple clips. You dont even need to do it destructively via process Length.

You can select all the clips and slip stretch them all by the same amount. The shortcut key to do is hold down CTRL-Shift and then Drag the right edge of the clips to slip stretch them to the amount you need. Make sure the stretch method in the clip inspector is elastique pro. This will dynamically and non destructively stretch the audio to the new length.

[RBH]

Noel - thanks for that info - I've never tried to stretch multiple tracks or clips across multiple tracks simultaneously. 

[Sean Michael Robinson]

Noel-- thank you SO much! I don't know if I ever would have found that shortcut. I've used the slip stretching on single clips but hadn't been able to tease out how to get it to fly with multiple selections. Seems very straightforward. Thanks for the help!

I'm very interested in the maths behind these processes. Going to do some reading about the different types of algorithms now.

[Noel Borthwick]

Hi, 

Please read these help topics for more details.

Slip-stretching audio

zplane élastique audio stretching and pitch shifting

 

[Sean Michael Robinson]

Hello Noel!

I'm  trying and failing to get the slip stretching to work on multiple clips. Even with Ctrl-Shift held, and all clips selected, it'll just slip-stretch the one. Any guess what I might be doing wrong?