Back in 1988, the first work began on video compression standards that would ultimately result in MPEG. The Motion Picture Experts Group, in cooperation with the International Organization for Standardization (ISO), created multiple standards for video compression, now known as MPEG-1, MPEG-2 and MPEG-4/AVC. MPEG was devised as a means of compressing broadband video into a small bit stream that could fit in extremely narrow broadcast or satellite transmission channels. As such, it is well-suited for distribution purposes, not for applications that require internal switching and processing.
Because MPEG is a motion image compression technology, it works on a sequence of video frames, known as a group of pictures (GOP). (See Figure 1.) A processor examines several frames of video and assigns one frame as the reference frame for that group (the I-frame). The GOP also has several predictive frames, or P frames, which use information from the I frame and previous P frames to construct images. Finally, there are bidirectional predictive frames, or B frames, which look at preceding and following I and P frames. Motion is analyzed and the motion vectors, which predict the offset from the current frame to the reference frame, are estimated down to a quarter pixel. The motion vectors and the difference between the actual images are used to reconstruct video that looks good at low bit rates.
Problems with MPEG encoding arise when there is a scene change or where a large percentage of the image changes at once. The processor can’t predict the movement, and the system creates a distortion known as macroblocking. (See Figure 2.) Raising the bit rate adds more detail in each 8 x 8 block of pixels, which can reduce the number of P frames. But there will always be P frames — and the potential for macroblocking — in all implementations of MPEG.
Another major failing of MPEG compression is that it is difficult to edit and switch cleanly. With P and B frames, editing systems and switchers have trouble finding a clean frame on which to edit. MPEG also doesn’t react well to being repeatedly encoded and decoded. It works best when the video is encoded once and decoded at the viewing location.