How can I change frame rate without changing audio speed?

Before you modify the clip speed in the timeline, detach the audio from the clip. Right click (Control + Click) on the clip and select Detach Audio from the drop down menu. Once the audio is detached, adjust the video speed.

MtD

It works as expected on my system.

Here is my two cents, feel free to ignore:

Are you playing the portion of the timeline where this problem occurs without rendering the section? If you render the audio and video, does the appearance of sync improve? It may be that your computer does not have sufficient horsepower to slow the video, slow and pitch shift the audio and play this all back correctly.

As a test, if you export that portion of the timeline to a file and play that exported file back in QuickTime Player, does the appearance of sync improve? This combines the speed adjusted elements into a easy to play file and serves as a way to see if it is the processing power of your computer.

Are you confident you can accurately determine the sync when viewing material that has be slowed to 20% of original speed? Detach the slowed audio from the slowed video, if you move the audio earlier or later in the timeline in small increments, can you achieve the appearance of the sync you expect?

Slowing material down that far will acerbate any sync relationship error in the original recording and the pitch shifting process may repeat may introduce a bit more, particularly if the timeline portion is not rendered.

An external microphone recording on a separate audio recorder opens up another can of worms unless you are using an audio recorder that is Timecode locked and synced to the video camera, as is done in professional workflows. Even with that equipment you might still encounter the appearance of an out of sync condition when using the material as you are planning on the timeline.

MtD

This guy keeps telling that 80% is "real time". That is just bogus. 100% is real time.

Besides, audio does not have frame rate. You could record audio together with video in a separate device at 24fps.

You could record audio separately - and in many cases, you'd want to do that - you may have a lapel mic on your talent, or a shotgun mic, or some audio feed from a conference room, that you'd sync to your video in post; you'd want to choose a sample rate (typically 48KHz) but not a frame rate (which does not make sense).

Audio doesn't have a frame rate. It has a sample rate. It recordings in real time, just as 60fps video does. The frame rate doesn't matter, the amount of time matters. If you change the length of time for the video then the audio will change, unless the software can pitch shift it.

Are you using the Automatic Does function in the Retiming menu? In the same menu is Preserve Pitch checked on?

So what are you doing to retime? Select the 60p clip in the 24 project and at the bottom on the Viewer using the Retiming menu to select Additional Speed.

It is unclear what you want to do. Obviously, if you slow the video down, the audio will slow too, or become out of sync. You can slow down somewhat and FCP will try to preserve the pitch to some extent, but there is no way speech or music, for example, will sound natural at that slow speed.

Why does it have to be 24fps, anyway? You make your video at 60fps and that will be smoother than 24fps.

Please don't take the following as a criticism, but rather as a genuine effort to understand what you are trying to achieve, and educate myself. In reality, I'm quite puzzled.

Can you clarify exactly what it means to you to "slow down by 80%"? To me it seems to be to play at 20% of the original speed, that is, 5 times slower. For example, if you film a person walking for 10 seconds, playing it back takes 50 seconds. That amounts to very slow motion. That can't be right.

Maybe you mean slown down to 80%? So those same 10 seconds would now take 12.5 seconds. That could be it.

However, the math does not quite add up. Each frame (which would have been 1/60th of a second in the original) would now be about 1/50th of a second. Or maybe you would slow down to 75% and then each frame would be about 1/48th of a second. However, since your timeline is only 24fps, this means that you'd drop half of the frames that you filmed.

Plus, with 60fps the shutter speed would typically be twice that (1/120th of a second per frame), resulting in much less motion blur.

The end result would be the opposite of smooth... maybe it is cinematic, though, but in Chaplin-esque way (?!).

Just to clarify, you want to slow down your footage but have the audio play in sync with that slowed down footage AND maintain the original pitch of the audio?

MtD