Change FPS without getting audio out of sync.

[James H.]

Hi there folks,

this should be an easy one, yet I can't seem to figure it out myself. I'm working with Avidemux 2.6.4 and I'm trying to change FPS to make the video faster. But, after that, the audio gets out of sync and I haven't found an option to speed up the audio as well. If I could pin the audio to the video track to they'd get sped up simultaneously , but how.. Thanks for help,

James

[DB83]

You are not making sense.

Any substantial change of frame rate to make video faster will obviously cause sync issues with a related audio track.

If the audio was already muxed with the video, or 'pinned' using your terminology, a change in frame rate will make the soundtrack un-audiable.

But do explain in more detail what you are trying to achieve just in case we are not seeing the whole picture.

[Cornucopia]

While multimedia containers may make it look to average users that V+A are combined, they are different beasts with different requirements. Audio is (usually) sample-accurate, while Video is frame-accurate. Multi-channel/multi-track audio is fairly common, while multi-track video is relatively rare (stereo 3d most common exception).
If you speed up the video, the rate of motion changes. Too much and you have silly "keystone cops" type of motion. A similar thing happens with audio: if you speed up the audio, the pitch & tempo changes. Too much and you have silly "chipmunk" voices.
For them to maintain sync, they have to start at the same time and keep the same duration. Changing video's FPS changes its duration, so you'd have to change audio's duration the same percentage as was done to the video to keep it in sync with the video. E.g.: 24FPS -> 25FPS = 4% (faster, aka SHORTER). 5 minutes of audio would become 4.8min.
While there might be some apps that do BOTH kinds of speedup/slowdown, the better tools work specifically with one media type or the other (A or V).
For example, with audio, most people prefer that when speeding up/slowing down that the pitch remain the same. That is often not possible with general-purpose apps, but with audio-specific apps there are a number of pitch-correcting duration/tempo change options. Some of them are more processor intensive in order to avoid non-musical artifacts.

So the best thing to do is: demux your V and your A to separate streams, fix your V, then accordingly fix your A (using separate tools) and then remux them together again.

Scott

[jman98]

If you demux the audio you can use something like Audacity to make the change, but you will very likely have to do the math yourself to tell it how to do this. You'll have to change the pitch AND duration both or just the duration if you can live with the higher pitch. It might help if you gave more details like what your starting frame rate is and what you want to change it to. For example, if this is a simple NTSC to PAL conversion, you can probably do that pretty easily with an existing tool, but if you're doing something like changing 29.97 fps to 20 fps, that's complicated.

REALLY pressed for time to watch anime?

[budwzr]

What you want to do is called "Undercrank". Any NLE can do it.

[James H.]

Thanks for the replies. What I'm trying to do is to change the current fps (35) to something like 50. It's a game-play recording and some parts of it are a bit boring, that's why I want to speed them up (not too boring to be cut). So the higher pinch shouldn't matter since there's no voices in the audio track, just sounds.

[budwzr]

Then you want to "Overcrank".

Audio is not affected, either way, so you will have to crop the speeded up part.

Visual it as two gears of equal size, 30 teeth each. If you make one of the gears half size, it plays back at half speed. Get it? Yes, Virginia, you CAN play 24, 15, 10 or whatever, inside a 30p video.

That's what telecine does too. Conforms 24p to 60i.

Lay out your project as clips on the timeline, then make the changes where needed. You don't have to re-jigger the whole thing.

AviDemux is a great tool, but it's not a full editor. Probably need Sony Vegas Studio.