About the Frame Sync Setting

The Frame Sync setting defines an H.264 group of pictures (GOP). One difference between MPEG-2 and H.264 encoding is that a frame within a GOP can reference frames other than those that it is closest to, which means the decoder must store all potential reference frames in order to be able to access them. The first frame of an H.264 GOP is a special I-frame known as an instantaneous decoder refresh (IDR) frame that also flushes the decoder’s reference frame buffers.

Figure. Diagram showing GOP, Frame sync interval, GOVU, and partial sync key frames.

This means that frames within the GOP defined by the Frame Sync setting can reference any other frames in that GOP, but not those outside the GOP. The result is that a longer frame sync setting creates a larger GOP, with more frames available for referencing, while a shorter setting results in a smaller GOP and fewer frames that can be referenced.

Additional I-frames, referred to as partial sync key frames, are added to the GOP. These standard I-frames define a group of video access unit (GOVU).

Important: To conform to the HD DVD specification and its requirements for maximum GOVU length, Compressor inserts a partial sync key frame every 0.5 seconds.