File Size, Picture Quality, and CPU

If you rent one of those 60's artsy French movies where everyone stands stock still and talks for twenty minutes about ennui, then compression doesn't matter. You might just as well look at a still photograph. On the other hand, if you rent a Jason Statham film where in the first 15 minutes they destroy what appears to be an entire week's production from General Motors, then compression is an issue. If you really want to see every bolt fly off of every demolished car, it will take a lot of bytes.

Advanced MPEG 4 can be used to either improve picture quality or else to shrink file size, at the cost of more CPU. Of course, CPU is not a problem if you use a Blu-Ray player or you get assistance on your PC from a modern video card. Once you choose a type of compression, better picture quality means more bytes per second.

1. A DVD movie is Standard Definition recorded in MPEG 2 and uses 4-7 megabits of data per second. As the master version of the DVD is being created, computers or the technicians monitoring the process use a higher data rate when there is a lot of action on the screen, but then turn down the data rate when not much is changing on the screen.
2. HD TV has four times the picture of Standard Definition, but it also compresses better. Broadcast HD programs vary from one network to another. They typically use 10-15 megabits per second of the 20 megabit per second broadcast signal. They also have to squeeze in a Standard Definition TV broadcast for people with old sets, but they don't have to send that picture with as much quality as you would get on a DVD.
3. Blu-Ray movies can come in anything from a 15 megabit per second MPEG 2 format (essentially the same as a network broadcast picture) to 30 megabits per second H.264 AVC. It depends on the material. A movie with a lot of elaborate special effects gets the higher rate, while Chick Flicks can make do with MPEG 2.

Compression means that you cannot have a lot of action on the screen and have every single dot on the screen appear in the sharpest possible HD. However, AVC is a lot better than MPEG, and 30 Mb/s is obviously better than 15 Mb/s. So until you rent a Blu-Ray disk with an AVC movie, you have not seen the best picture your HDTV can deliver.

ATT's U-Verse system has to squeeze the TV signal into a much smaller bandwidth than other systems, so they recompress network TV MPEG 2 digital programs into the more efficient AVC format, and maybe they also give up some picture quality to reduce data size.

Right now TV stations have no particular use for unused bandwidth. If they find a way to make money with extra capacity, they will put pressure on TV networks to increase compression on broadcast TV shows.