Apple Compressor: Change Frame Rate

[ Here’s an article that provides a more complete discussion of what frame rates are. ]

The number one rule in frame rates is: Where possible, always shoot, edit and output the same frame rate. Camera-native frame rates always look better than converted frame rates.

Most of the time, you don’t need to change frame rates, the video editing software you use will change them for you. However, sometimes you need to change them outside of a video editor.

NOTE: Why not just use Final Cut Pro X – or Premiere? Because you have more control in Compressor. Also, Compressor can process clips in batches automatically.

This article explains how to use Apple Compressor to change frame rates.

BACKGROUND

Apple Compressor Help: There are two ways to modify the rate at which a transcoded file plays back:

• You can apply speed effects to slow down or speed up playback
• You can change the frame rate of the transcoded (meaning “converted”) file (which may or may not have an effect on playback speed).

IMAGE QUALITY ISSUES

Apple Compressor Help: Frame rate conversion can have a subtle or dramatic effect depending on how big a difference there is between the original and new frame rates, and also depending on the specific nature of the footage being converted. Footage with a lot of movement yields a much more visible change than footage with little movement in the frame.

Frame rate conversion may also add visible artifacts in the transcoded file; from stuttering (sometimes called “juddery”) playback, to repeated frames, ghost images, or other unnatural-looking elements. These effects can be somewhat mitigated by adjusting the “Retiming quality” property in the Quality section of the Video inspector.

RETIMING vs. FRAME RATE

Apple Compressor Help: The Retiming section in the General inspector allows you to define a new speed setting for the clip. You can specify a percentage, such as 50% to create a slow-motion effect, or 200% to create a fast-motion effect. You can identify a precise number of frames and allow the software to calculate the percentage for you; or you can instruct Compressor to renumber the current frames into a different frame rate in order to convert a file from one frame rate to another without adding or deleting any frames.

In the Video Properties section of the Video inspector, you can set a specific frame rate for the transcoded file. By default, this is set to “Automatic,” which applies the frame rate of the source file to the output file.

If the source file contains audio, retiming modifies the audio speed as well, thereby keeping audio and video in sync. Compressor also automatically corrects the pitch so the audio does not sound artificially high or low.

In the Quality section of the Video inspector you can set the retiming quality to specify the processing method used when you make changes to another settings that modify the speed or frame rate of a clip.

NOTE: If you modify retiming properties with a setting that uses the QuickTime Movie format and choose “Copy audio tracks from source” in the Audio inspector, the audio speed is not changed. Consequently, audio in the output file will not maintain sync with the video.

FRAME RATE EXAMPLE: 59.94 fps to 29.97 fps

Changing frame rate modifies the frames in a clip, without changing the apparent speed of motion displayed by the clip.

Here’s a clip with a 59.94 frame rate that we want to convert to 29.97.

When converting files, its important to transcode them into a high-quality, I-frame video format. For clips that originate in a camera, use ProRes 422. For clips that originate on the computer (say After Effects or Motion) use ProRes 4444.

Select the compression setting: “Apple ProRes 422” in this case.

Click the Video tab in the Inspector, then change the Frame Rate from Automatic to 29.97 fps. (Cutting the frame rate in half does the least amount of damage to the movement in a video.)

NOTE: You use this menu to change from any frame rate to any frame rate. You can even enter custom frame rates, but those may not work on an iOS device.

Next, In the Quality area, I’ve found that setting both Resize and Retiming to Best takes FOREVER!

Instead, I set both Resize and Retiming Quality to “Better.”

NOTE: Adaptive Details uses advanced image analysis to distinguish between noise and edge areas. It, along with Anti-Alias and Details Level only applies when you are scaling an image smaller, not deinterlacing. I recommend leaving all three of these off.

RETIMING EXAMPLE: 23.98 fps to 25 fps

Retiming changes the speed of a clip. We use this when we have very subtle changes to make; say from 23.976 to 24, or 30 to 29.97.

Here, we have a 23.976 fps clip that we want to convert to 25 fps.

In the General tab, use the Retiming menu to speed the clip 4%. Compressor will also alter the audio so that it remains in sync with the video.

Then, go to the Video tab and adjust the Quality settings as outlined above.

SUMMARY

When doing any frame rate conversion, the ultimate quality of the converted files will be highly dependent on the format and quality of your source footage.

Most of the time, you never need to worry about converting frame rates, as your NLE will do it for you. However, in those rate cases where you need to do it yourself, Compressor is a good tool to use.

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