Group of Pictures
A MPEG compression technology, the GOP concept reduces the temporal redundancy across frames (from frame to frame) for video content and consists of I, B, P frames.
This is the grouping of I, B and P pictures into a specified sequence known as a group of pictures (GOP). The group must start and end with an I picture to allow for random access to the group, and contains B and P pictures in between in a specified sequence (determined by the designer). A group can be made of different lengths to suit the type of video being encoded, for example it is better to use a shorter group length for a film which contains a lot of fast moving action with complex scenes. A group length is typically between 8 - 24 pictures. Commonly used GOP sizes are 12 for 50 Hz systems, 16 for 60 Hz systems. GOPs are optional in an MPEG2 bitstream, but are mandatory in DVD video, to achieve an SMPTE timebase. A bitstream with no GOP header can be directly accessed at a specific point using the sequence header.