Key frame Distance in Video Files

Navigation to a specific frame in a video file in FAB Subtitler can be fast or slow. This may depend on the structure of the video file and the codec used in the video file.

Video files contain frames (pictures) and generally 24, 25, 30, 50 or 60 frames per seconds are used in video files. Different codecs are used for different compression types: MPEG1, MPEG2, H.264, H.265/HEVC, VC1 and others. The file extension .MP4 does not define the codec which is used in the video file. MP4 only defines the container format and not the codec (codec=encoder/decoder).

Some frames in the video file are called “Key frames”. To be able to display any frame in the video file FAB Subtitler must search for the last key frame before the selected frame, process the key frame and all following frames until the selected frame. This process will be fast if a key frame is present often in the video file and it will be slow if a key frame is not present very often in the video file. The distance between two key frames is called the “Key frame distance” and is generally expressed in the number of frames.

The examples below show sequences of frames in video files. Frames generally have names (I-Frame, B-Frame, P-Frame) and I-Frames are also called Key frames.

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Therefore for fast navigation the Key frame distance must be as small as possible. Video files which cause slow navigation in FAB Subtitler probably have a high Key frame distance. You can check the key frame distance in FAB Subtitler by opening the video file and clicking on Video / Display video file info.

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In the above example the key frame distance is 373 which is extremely high and the file feels very slow to navigate in FAB Subtitler.

The solution is to use the function “Video / Convert video file” in FAB Subtitler to convert the original video file to a new MP4 file which will have a shorter key frame distance and will be much easier to use in FAB Subtitler. Please make sure to enable “Optimize for fast navigation” when converting the file:

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The resulting file will have the key frame distance of 3 and will feel very fast when navigating to a certain frame in FAB Subtitler:

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