Saturday, 25 August 2012

Changing the frame rate of a QuickTime movie

Originally published on Oct 8, 2010 @ 23:16

One of the great things about Canon's DSLRs is their ability to shoot video at various frame rates.  The 50 and 60 fps 720p modes can be used to over crank  slow motion by adjusting the frame rate to 24, 25 or 30 fps in post.  The problem for me is that the examples for the mac on the web are using Cinema Tools, which is great if you own Final Cut Pro but I'm not quite at the level where I want to spend that much on software yet.  I have Final Cut Express 4, which is perfectly good to cut your teeth in video editing but lacks all the extra powerful tools that it's big brother FCP provides.  

There is a free way to adjust the frame rate of your quicktime videos without re-compressing and loosing quality.  The QuickTime file format makes use of data types known as Atoms.  These Atoms contain all the data in the quicktime file, the video and audio data as well as various meta data Atoms that contain information such as the dimensions of the movie, the number of audio channels, the audio bit rate etc.  It also contains Atoms that dictate the frame rate of the movie.  Apple have a free application which you can download from called Atom Inspector. By trial and error and some educated guesses I have worked out a method of changing the frame rate of a quick time movie by changing two values.

Using the Atom inspector load up the Quicktime move that you want to adjust. Find and expand the movie header.  This is the Atom named mvhd under moov - Movie.

I have loaded a movie which was shot at 60 FPS.

mvhd - Frame rate at 60 FPS

I'm going to change the frame rate to 25 FPS.  Simply edit the value and change to 25000

mvhd - Frame rate set to 25 FPS

The next value you need to change is under the Atom name mdhd - Media Header which is under the mdia - Media section:

mdhd - Frame rate at 60 FPS

Change the timescale value from 60000 to 25000
mdhd - Frame rate at 25 FPS

Press command S to save and that's it.  The QuickTime movie is now a nice 25 FPS slow motion movie.  You can of course choose any other frame rate and speed up as well as down.


I have only tried this on the H.264 .mov files that are created by my Canon 550D and Quicktime files that have been transcoded into Apple Intermediate Codec. I'm using OS X version 10.6.4.  I don't know if this method works with every Quicktime movie file and what side effects this method might have.  So it's a really good idea to make a copy of your file and edit the copy.  Always make backups of you original files before any messing about such as this.
If you try this on a copy then you have nothing to loose.  It might look a bit complicated and scary, but it's actually quite quick and easy to do.
Good luck.