This setting specifies the number of frames within a GOP. Because exactly one I-frame
exists per GOP, longer GOP sizes generally provide greater compression, because B- and
P-frames are smaller than I-frames.
For most media, spacing I-frames about 1/2 second apart gives good results. This equates
to a GOP size of 15 frames for NTSC and 12 frames for PAL. The DVD-Video specification
prohibits GOP lengths from being much longer than this. Generally, only material with
scene changes occurring less than 1/2 second apart frequently throughout the video will
benefit from shorter GOP sizes.