A family of MPEG formats. Part One: MPEG-1 – 2
Information compression principles in MPEG-1.
As an example, consider the reduced PAL / SECAM format, which is more common than SIF, although both formats are not different from each other with the exception of resolution and frame rate.
The simplified version of the PAL / SECAM format contains 352 ppl (point per line – points per line), 288 lpf (line per frame – lines per frame) and 25 fps (frame per second – frames per second). I must say that the full PAL / SECAM standard has parameters 4 times higher than MPEG-1 (except for fps). Therefore, it is often said that VIDEO-CD has four times worse clarity compared to normal video.
As for the color depth, it is not as simple as in computer graphics, where a certain fixed number of bits is assigned for each pixel. MPEG-1 uses the YCbCr color scheme, where Y is the luminance plane, Cb and Cr are the color planes. These planes are encoded in different resolutions. There are several encoding options, which can be represented as follows:
Encoding option Resolution ratio Сb / Y (Сr / Y) horizontally Resolution ratio Сb / Y (Сr / Y) vertically
4: 4: 4 1: 1 1: 1
4: 2: 2 1: 2 1: 1
4: 2: 0 1: 2 1: 2
4: 1: 1 1: 4 1: 1
4: 1: 0 1: 4 1: 4
As you can see from the table, Cb and Cr are almost always encoded at a lower resolution than Y. The lower the resolution of the color planes, the coarser and more unnatural the color reproduction in video is. Of course, the last option will be the lowest quality, but also the most compact.
Before the start of encoding, the video information is analyzed, the keyframes that will not change during compression are selected, as well as the frames during encoding from which some of the information will be removed. In total, there are three types of frames:
Type I frames – Intra frame. Unchanged compressed keyframes.
P-type frames – Predirected frame. When these frames are encoded, some of the information is removed. When playing a P-frame, the information from the previous I or P-frames is used.
Type B frames: bidirectional frame. When these frames are encoded, the loss of information is even more significant. When playing back a B-frame, information from the previous two I or P-frames is used. The presence of B-frames in a video is the factor by which MPEG-1 has a high compression ratio (but not a very high quality either).
When encoding, a chain of frames of different types is formed. The most typical sequence might look like this: IBBPBBPBBIBBPBBPBB … Consequently, the play queue by frame numbers will look like this: 1423765 …
