Why is my audio out of sync with video? It's probably Variable Frame Rate

Variable Frame Rate (VFR) refers to the active change in frame rate during video playback. This stands in contrast to Constant Frame Rate (CFR) where the frame rate remains constant during video playback. Variable Frame Rate is useful when a video, such as a recorded presentation, contains a large number of static frames which can be dropped to improve compression rate.

Networked applications like video streaming, calling etc. where frames are being sent over a network, like the Internet, take advantage of the higher compression ratio provided by Variable Frame Rate to ensure a smooth experience for users with slower connections.

Variable Frame Rate videos typically require less disk space when stored locally. That being said, editing Variable Frame Rate videos in a video editing program is quite challenging. As frame rates change throughout video playback, they aren’t accurately represented in the editing program’s timeline. Variable Frame Rate videos should be converted to Constant Frame Rate before being dropped into an editing program.

What is Frame Rate?

Frame rate refers to the number of individual frames or images that are displayed per second in a video. It is commonly expressed in frames per second (FPS). The concept of frame rate is integral to creating the illusion of motion in video production. By displaying a sequence of still images rapidly, the human eye perceives continuous motion rather than individual frames. This phenomenon is similar to how a flipbook works, where quickly flipping through pages with incremental changes in images creates the appearance of movement.

Constant Frame Rate (CFR)

Constant Frame Rate (CFR) refers to a video recording method where each second of video contains the same number of frames, resulting in a consistent and uniform frame rate throughout the entire video. This method is traditionally used in video production to ensure compatibility and smooth playback across various devices and platforms.

CFR is particularly beneficial for maintaining synchronization between audio and video, which is crucial in professional video editing and broadcasting environments. It is commonly used in television broadcasts, films, and other media where consistent quality is essential.

Common frame rates include 24 FPS, 30 FPS, 60 FPS etc.

Variable Frame Rate (VFR)

Variable Frame Rate (VFR) allows the frame rate to change dynamically throughout a video. This means that different parts of a video can have different frame rates, which can be adjusted based on the complexity or motion within each scene.

VFR is supported by some video container formats and is particularly useful for optimizing compression efficiency and visual quality. Variable Frame Rate is used extensively in streaming services, video calling and video recording on portable devices where video compression is necessary.

Variable Frame Rate (VFR) videos don’t have common frame rates as it dynamically changes throughout the video.

Advantages of Variable Frame Rate (VFR)

• Improved Compression Efficiency: By allocating higher frame rates to scenes with more motion or complexity, VFR can enhance video quality without significantly increasing file size. This is especially useful in video streaming and video calling.

Challenges of Variable Frame Rate (VFR)

• Audio Sync Issues: Since the frame rate varies throughout the video, aligning audio with video can become complex. If not handled properly, this can result in audio being out of sync during playback or editing.
• Dropped Frames: Editing software might drop frames or duplicate them to match the timeline's frame rate, leading to jittery playback. Using features like frame blending or optical flow can help smooth out these issues

How to edit Variable Frame Rate videos

Editing variable frame rate (VFR) videos can be challenging due to issues like audio sync problems and playback inconsistencies. But as variable frame rate (VFR) videos have immense benefits in practical applications, it’s best to learn how to work with them.

1. Test if video is Variable Frame Rate (VFR)

Before importing your video into an editing program, check if it has variable frame rate. Tools like TimeBolt can help you determine whether your video is VFR or CFR for free. Here’s a quick rundown of how you can test if your file has VFR in TimeBolt.

1. Download and open TimeBolt.
2. In the menu bar, go to Tools → Variable Frame Rate Detector.
3. Select your video file and let TimeBolt finish analyzing it.

Once the analysis is completed, TimeBolt will give you a percentage denoting how much of the video is Variable Frame Rate (VFR) with an option to convert it to Constant Frame Rate (CFR), if necessary.

2. Convert Variable Frame Rate (VFR) to Constant Frame Rate (CFR)

The best way to avoid any issues that can arise when working with Variable Frame Rate (VFR) files in any editing program is to convert the video to Constant Frame Rate (CFR). This bypasses any audio sync issues, timing issues, dropped frames etc. that can be encountered in the editing program.

The conversion might not be an exact 1:1 as many frames that might have been dropped in the Variable Frame Rate (VFR) might need to duplicated to maintain Constant Frame Rate (CFR).

How to Convert Variable Frame Rate (VFR) to Constant Frame Rate (CFR)

Variable Frame Rate (VFR) videos can be converted to Constant Frame Rate (CFR) videos with most video editing programs.

As discussed in the previous section, if you use TimeBolt to detect Variable Frame Rate (VFR)—the program will give you an option to re-encode the video file to Constant Frame Rate (CFR) immediately after or you go to Tools → Re-encoder in the menu bar.

You can also use programs like HandBrake etc., to re-encode the file to Constant Frame Rate (CFR). For programs like Premiere Pro, Davinci Resolve etc. where you have a timeline—just place the Variable Frame Rate (VFR) video in the timeline and export it without any cuts. This will export a Constant Frame Rate (CFR) video.

3. Record In Constant Frame Rate (CFR)

Variable Frame Rate (VFR) is great for streaming and video calling but when it comes time to use these videos for editing, they have a host of issues. Hence, it might be best to record in Constant Frame Rate (CFR) in the first place.

Video calling programs like Zoom come with special features that save video recordings in Constant Frame Rate (CFR) so that they can be easier to edit in video editing programs.

How to Optimize Zoom Recording Settings for Constant Frame Rate (CFR)

• Open Zoom and go to Settings.
• Navigate to the Recording tab.
• Activate Optimize for 3rd party video editor.

Conclusion

Variable Frame Rate (VFR) is a powerful tool in video production that offers significant advantages in terms of efficiency, quality, and flexibility. By dynamically adjusting the frame rate to match the complexity of the video, VFR enables creators to produce high-quality videos while optimizing file sizes and bandwidth usage.

That being said, editing a Variable Frame Rate (VFR) video is comes with a host of challenges like audio-sync issues and dropped frames. The only real way to deal with these issues is to convert to Constant Frame Rate (CFR) before starting the editing process. Programs like TimeBolt, HandBrake etc., come in handy when working with Variable Frame Rate (VFR) videos.