dvdcollection

An ordinary 625 line frame (we'll utilize the PAL standard on this example)takes 1.2 Megabytes of knowledge if no compression can be used. As there are 25 frames per second, this will provide us with: 1.2 x 25 = 30 Throughput

So a DVD with 4.7 Gigabyte of storage gives us: 4700 / 30 = 156.6 seconds or 2.6 minutes of video playback.

Obviously one method or another of reducing the level of data which needs to be stored around the disk is required.

A good way to compress the information stream would be to analyse each frame and remove information that's repeated within the frame. This frame by frame compression is known as JPEG. This might be well suited for individual frames but is not sufficient for movies. Jpeg looks best with compressions of 5:1 or less, even this may only give 13 minutes of video on a DVD.

An improved way is to use MPEG compression. This functions by utilizing the first frame and compressing it above. This frame remains quite large since it contains most of the information regarding the frame, this is known as the I frame. The subsequent frame of picture information is compared to here i frame in support of the difference is stored. There are 2 forms of difference frames, B and P frames, B frames are bi-directional, and P frames are predictive. The fact is that B and P frames could be quitr small with respect to the amount of movement inside the picture information. Typically with MPEG we are able to realise cpmpression factors of upto 30:1.

The grouping of I, B and P frames is called a GOP structure (group of pictures) the default stucture of the GOP within a DVD is generally IBBPBBPBBPBBPBBI, 4 P frames within each I frame sequence, and 2 B frames in between each P frame sequence. A GOP sequence of 16 will give a very good compression ratio and use up less disc space but may present problems as below.

Inside the DVD authoring process certain functions must happen on an I frame so that as each GOP is only able to have one I frame then for some videos an inferior GOP structure is more flexible but use up more space. Foe example an instalment point must happen with an I frame, when the GOP structure was 16 frames many times the nearest I frame upto 8 frames away that might lead to an inaccurate position to begin with a brand new chapter.