Towards Compressed Video

Video compression can take away the requirement for the very high data rates and move video transmission and storage into a very similar regime to that for audio. In fact, in terms of tolerance for poor quality, it seems humans are better at adapting to poor visual information than poor audio information.

A simple minded calculation shows the amount of data you might expect, and is shown in table 4.5.

 

Table 4.5: Liberal Estimate for Uncompressed Video Data Rate

1024 x 1024 pixels,

3 bytes per pixel (24 bit RGB)

25 Frames per second

 

yields 75Mbytes/second, or 600Mbps - this is right on the limit of modern transmission capacity. Even in this age of deregulation and cheaper telecoms, and larger, faster disks, this is profligate. On the other hand, for a scene with a human face in, as few as 64 pixels square, and 10 frames per second might suffice for a meaningful image.

  

Table 4.6: Cautious Estimate for Uncompressed Video Data Rate

yields 122KBytes/Second, or just under 1 Mbps - this is achievable on modern LANs and high speed WANs but still not friendly! Notice that in the last simple example, we did two things to the picture.

1.

We used less "space" for each frame by sending less "detail".

2.

We sent frames less frequently since little is moving.

This is a clue as to how to go about improving things. Basically, if there isn't much information to send, we avoid sending it. Spatial and temporal domain compression are both used in many of the standards.

If a frame contains a lot of image that is the same, maybe we can encode this with less bits without losing any information (run length encode, use logically larger pixels etc. etc.). On the other hand, we can take advantage of other features of natural scenes to reduce the amount of bits - for example, nature is very fractal, or self-similar:- there are lots of features, sky, grass, lines on face etc., that are repetitive at any level of detail. If we leave out some levels of detail, the eye (and human visual cortex processing) end up being fooled a lot of the time. The way that the eye and the ear work (integration versus differentiation) means that video and audio compression are very different things.