George Ou over on Zdnet wrote an excellent piece outlining why those too-good-to-be-true HD downloads we see in Xbox 360, ABC.com and even Apple TV are a bit bogus. He points out that while these services deliver on their 720p resolution promises, the encoded bitrates are so low, compressing the data to such small proportions, that the image within the said resolution has inadequate fidelity. And though we have some major reservations regarding Ou's theory, it's tough to entirely disagree when you see the cold, hard numbers:
480i DVD movies: 5 to 8 mbps
HD Steaming: 1.5 to 4 mbps
XBOX360 Dloads: 6.8 mbps
HD DVD: 28 mbps (max)
Blu-ray: 40 mbps (max)
Uncompressed 1080p video at 60fps: 3000 mbps
Note: Apple reports HD movies will be about 5GB, which would represent a bitrate more on par with DVD standards.
What's missing from this data, of course, is that DVD is based upon the older, less efficient MPEG2 codec, rather than modern, sexier H.264 and VC-1 codecs. But Ou argues that the bottom line is still the bitrates, and that companies offering HD downloads have compromised too much in their compression to claim HD resolutions. (Then again, it should be noted that you can rip a ton of color information out of an image to cut bitrates while maintaining resolution).
It reminds us of all those new bajillion megapixel point-and-shoots that take craptastic pictures...and we haven't even started on issues of sound. Don't expect Blu-ray's successful fallback of classic, uncompressed PCM to make its way into streaming HD.