Compression Primer
As we've said in the past, compression itself is a great thing. It allows you to store and transfer audio and video content that otherwise would be way too big. It's when there's too much compression that things go bad. Yes, in this case there can be too much of a good thing. Our analogy has always been that of a beach ball. It's a great device that provides hours of fun for all ages, but in it's fully inflated state, it's very difficult to store or to transport. You want to be able to deflate (or compress) it to store it in the garage when it isn't in use, or put it in the car to take it to the beach or park, then re-inflate (or uncompress) it when it's time to use it.

There are two basic types of compression, lossless and lossy. In lossless compression you can compress the content all you want and when you decompress it, it will look exactly like it did originally. So in this case every time you re-inflate the beach ball, it looks and functions exactly the same. In lossy compression, when you decompress the content, it doesn't quite look as pristine as it did before it was compressed, you lose some quality.

So why use lossy compression? It sounds like a really bad deal. The fact is that you can compress content to much smaller sizes with lossy compression and still preserve most of the quality. The trade off between size and quality is much better. Mainstream lossy compression started with the MPEG1 format back in 1992, grew into MPEG2 in the mid nineties and is now largely MPEG4 based. We have more detail on all three codecs in Episode 13.

The three codecs included in both the HD-DVD and Blu-ray specifications are MPEG2, MPEG4 AVC and VC1. MPEG2 is quickly becoming yesterday's news because MPEG4 allows the same quality playback at about half the file size, so it's a no-brainer. Early on there were issues with this because of the additional hardware requirements for MPEG4. As you can imagine, getting the compression that good takes a lot of computation, and a lot of processor horsepower.

The bottom line is that compression allows us to watch high definition video in our homes. Without it, a high definition movie would take up terabytes of storage space and would never fit on a Blu-ray or HD-DVD disc. Forget about trying to broadcast an HDTV show to your house.