Why do DVD's use VOB's?

[MrMungus]

Just curious. Why use VOB's that combine different files instead of keeping the individual files (mpegs) separate?

[freestyler]

Do some reading and check out the DVD Demystified FAQ http://www.dvddemystified.com/dvdfaq.html

[MikieV]

Just curious. Why use VOB's that combine different files instead of keeping the individual files (mpegs) separate?

The files need to be "multiplexed" - multiple-files mixed together and "sent" at the same time - so they can be placed on the DVD.

Most computer "video"-files contain a mixture of audio and video data i.e. "elemental streams", so most of us forget they are "multiplexed" together.

On a DVD, not only is the audio and video mixed together, but it is also possible to have alternate language tracks, alternate audio tracks (Dolby 5.1 or THX, etc), alternate viewing angles, and sub-titles.

All this has to be "mixed" together, so that a single laser reading data from a single spiral-groove track of data can be delivered fast enough for all the selected parts to be "de-multiplexed" (un-mixed).

Even the options you are not using are still read from the disc. All the sub-titiles and alternate language tracks are still being read from the disc - since they are all mixed together. The laser does not "hop-around" from sector-to-sector like a hard-drive's read/write head.

That is why the discussions here often get into data-transfer "bit-rates", and what the maximums are - since they change with how much other data has to be transferred by the laser at any given moment. Less subtitles and alternate language tracks means more "bit-rate" for audio/video.

As an example: for every second of "real-time" playback, the DVD has to transfer 1-second of main video, 1-second of stereo audio, 1-second of each alternate language, 1-second of each subtitle track, 1-second of each alternate view.

This is why most DVDs are burnt as one unit, not file-by-file like a hard-drive - because all the data needs to be present so it can be "multiplexed" into one contiguous (sp?) file.

Mike

[MikieV]

Something else that may help in under-standing the "mixing" of files.

One reason there is enough "space" in the data-stream to allow the "inter-mixing" of the data:

Since most video is designed to run on interlaced systems, i.e. NTSC, PAL, etc. there are "gaps" in the data-stream for video, during the times the electron-beam in the CRT is turned off (when the beam is moving back to the start of the next scan-line, and moving back to the top of the screen aka "Vertical Blanking Interval"). These "beam-off" periods may seem very short, but they allow enough time for the other data to be sent, especially at "computer" speeds.

Mike