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Playback speed control for editing

User avatar unfa
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Playback speed control for editing

Tue Mar 13, 2018 7:01 pm
Hi!

I'm often working with big amounts of spoken video material (like 1h-2h). I think that I could be more effective with editing, if I could change the playback speed.

How I imagine it could work is I'd have a "playback speed" silder somewhere near the timeline preview window That I could set from 100% to (say) 500% with precision.
I'd also like to have a toggle switch so I can quickly switch between sped up and regular speed playback, also with a hotkey.

I know Kdenlive has a "Speed" effect. There are two problems with it:

1. It will affect the pitch of the audio, so fast playback will inevitable make the speech unrecognizable very quickly.
2. It is not possible to use it temporarily to only speed up preview playback, but have the rendered video play at normal speed.

I am using Proxy footage anyway for editing, and even on a machine with a Ryzen 7 1700 CPU, still compositing frames seems to be the biggest bottleneck. Even with proxy in place I usually have no more than 12 FPS playback if I composite anything. So I mostly just disable a bunch of tracks and edit using the audio waveforms, audio cues from the recorded speech and a part of the captured video proxy.

I am not sure if adding a sped-up playback overhead on top of that would serve me any better (it would not, unless I can get at lest 30 FPS of regular playback anyway).

VLC seems to handle playback changes pretty well with time stretching in place, so the audio pitch is not affected - I don't know if they use librubberband or what.

I guess fast playback of non-proxied video streams could be tricky, and add a big overhead since the inter-frames need to be processed way faster - or if you just skip between keyframes it should be faster and require less disk IO, but the playback will be very choppy. For proxy footage I guess it should not be a problem since all frames are encoded independently (for fast scrubbing).

What do you think of that?
User avatar unfa
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PS: I've just learned that Kdenlive does support faster playback with JKL keys.

The problem is that the audio gets all choppy and I can't understand the speech while using it.
Also maybe if the speed-up steps were smaller that wouldn't be such a big deal?

It sounds like the audio stream is hard-cut to play only every Nth block. With some crossfading between these blocks I guess the sped-up playback audio quality should be greatly improved. I have no idea how to implement that however.

 
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