An Explanation of MPEG-2 for DVD Authoring (page 2)

Not all compression is created equal. A high action play on a televised football game and a newscaster sitting at a desk talking represent significantly different problems for MPEG encoding. Since frames are bundled together, a talking head without much motion can be compressed significantly (the only thing that really changes in great detail is a small portion of the newscaster's face), whereas the streaking motion of a fast-moving sports play requires a lot of updating. For this reason, many MPEG encoders are "variable bitrate" (VBR) -- they'll compress slow-to-change sequences highly (say maybe at 4 megabits per second) and rapidly changing ones less so (such as 8 megabits per second). Typically, these average out to 6 megabits per second over time. Constant bitrate encoders (CBR) work faster than their variable counterparts, but if encoding something with a lot of action, can have compression legacies. The best encoders use multi-pass VBR encoding where the data is first examined on one or more passes and then compressed on a final pass. This assures that the data receives the best compression.

You can put audio on your DVD in one of several formats.

• Uncompressed PCM (Pulse Coding Modulation) -- usually Microsoft WAV format or Apple's AIFF (Audio Interchange File Format): Huge size, 10x larger than AC3.
• Dolby Digital compressed AC3 (Audio Codec 3): Highly compressed, can be stereo or surround, bitrates from 128-384k. Probably the best all around choice.
• MPEG (Layer II) compressed: Not officially supported except in PAL, but most NTSC players can now handle it.
• DTS (Digital Theater Sound) compressed: Similar to AC3 in terms of quality, but it uses higher bitrates.

We're probably in the minority, but we wish that when you put a DVD in, it would just play. No hunting around for the remote, no clicking on the "play movie" option, and, most importantly, no 10 second loop of music to play over and over again, driving us insane while we're out in the kitchen making popcorn. Break the trend -- resist the urge.

However, there are reasons you may want to have your DVD open up to a menu that allows you to, for example, set up different audio features or choose between one of several videos on the same disc.

"So," I said, as my mom came in with a vase of flowers. "Anything else you want to know about MPEG-2 encoding, Aunt Edna, you just let me know." I gave her a kiss on the forehead, and three months later, when she was out of the cast, she sent me a DVD of her snow jump and the unfortunate mechanical bull incident. I was pleased with the technical advisory credit she gave me.

Contributing Editor Kyle Cassidy is a video artist, network engineer and co-author of Enterprise Internetworking and Security.

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