Friday, May 05, 2006

High Quality Movies - Back of the Napkin calculations

Discussion of Blu-ray and HD-DVD got me thinking, and I made the following back-of-the-napkin calculations:

A DVD9 disc holds roughly 9 billion bytes, 9×1000×1000×1000.

If you dedicate an entire DVD9 disc to a movie (i.e. put all special features, fancy menus, etc. on another disc), and assuming your movie is 135 minutes (which is a pretty safe assumption - there are longer movies but they're rare), that means you can have an average bitrate of 9×1000×1000×1000×8÷60÷135 = 8888888 bits per second or nearly nine megabits.

Now, let's say you have a 384kbit Dolby Digital or DTS 5.1/6.1/7.1 soundtrack, and a 192kbit stereo foreign language soundtrack (say French, or Spanish) and a 192kbit stereo Director's Commentary soundtrack. That leaves you with 8888888-384000-192000-192000 = 8120888 or just over 8 megabits for video.

Now, regular "NTSC" DVDs contain 480p content (720x480). A higher quality HDTV standard is 720p (1280x720). 1280×720÷720÷480 = 2.66. Or in other words, 720p content is 22/3 times the size of 480p content, or 166% bigger.

That means that the 8.12 megabits of video, if it were 720p content, would have an equivalent bitrate for 480p of 8.12÷2.66 = 3.05 megabits.

Now, 3.05 megabits average is too low for high quality DVD-resolution MPEG2 video. However, it's not totally unreasonable for high quality MPEG4 or VP7 or similar advanced CODEC video at that resolution. The 8.12 megabit 720p video would be as high quality, or higher, than a 3mbit MPEG4/VP7 encoded DVD. I encode most of my DVDs to around 2mbit for acceptable quality. I think 3mbit would be virtually indistinguishable from the source material with these codecs.

Please note that when I discuss equivalent quality, I'm not talking about the overall quality of the video itself - I'm talking about the distortions pixel level. Because 720p is a much higher resolution, you would get a much higher quality even if you lose a little more information at the pixel level. What the equivalent quality tells you, is whether you get the full improvement of the higher resolution of HDTV or only part of it. If 3mbit VP7 is equivalent in quality to 8mbit MPEG2 for 720x480, then using VP7 on 720p at this bitrate will mean you get the full benefit of HDTV. If it's slightly inferior, you still get a benefit, but not as much as you would from going to a higher capacity medium which allows you to have a higher bitrate, such as HD-DVD.

My point? Using existing DVD physical storage and modern CODECs, it should be possible to store and play back high quality HDTV content. All that would be required on the player is a more powerful/advanced decoding chip for the newer CODECs. (At the same time, it might make sense to install newer audio codecs too, to squeeze some extra bits in. For example Ogg Vorbis audio for stereo soundtracks is very reasonable at or below 160kbit).

I think this solution would make an excellent intermediate product between regular DVD video and Blu-ray/HD-DVD. The discs would cost no more than regular DVD discs, the players would not cost much more - they could use exactly the same components except for the decoding chip(s) and the DAC which drives the video outputs. Until Blu-ray and HD-DVD are ready, ubiquitous and cheap, why not?

1 Comments:

At 6:06 PM, Blogger Murdoc said...

Nicholas: Email me, will ya?

 

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