Can the frame rate vary throughout the VOB?

[rozemab]

Over on the Dr Divx forum (www.divx.com) there is a discussion on whether or not the frame rate can vary from chapter to chapter.

What started the thread was a comment made by myself on the encoded audio sounding like the actors were speaking in a large cardboard box, or sounding like an 'echo' or very 'tinny'.

Has others experienced this as well (varying framerate)? Can the framerate vary throughout the video?

[temporance]

Yes, on an NTSC DVD the MPEG-2 picture rate can vary between 20 and 30 Hz.

Essentially you have two sorts of frame with NTSC MPEG-2. You have a 20Hz frame (made of 3 fields) and a 30Hz frame (made of 2 fields). By mixing these up, you can get the framerate you want.

So for a purely 24Hz movie, you just alternate '3' and '2' frames (this is nice clean telecine and will be found on a well-mastered DVD).

In a purely 30Hz TV program, you have only '2' frames.

But an MPEG-2 encoder can mix up these frames as much as it likes - and it will do this if the source is edited film and TV material. So you can get a few frames at 30Hz, a short burst of 20Hz, then a few frames where it's not sure whether we have 24Hz or 30Hz, then a long 24Hz section. This is a nightmare when trying to convert to AVI which is essentially a fixed framerate format.


Note: For simplicicity let's forget 1/1001 NTSC multiplier.

[rozemab]

temporance,

Very interesting point, is it correct to assume when reading the VOB, somewhere in the data is a 'field' that says "hey I'm at a xx fps" etc. and the decoder is suppose to take that into account?

So, I must assume the audio is mastered (on the source) to take into account the varying fps as well?

eeak! Think of the problems at the boundary between different fps and audio.

[temporance]

It's not that bad, rozemab. Don't think about a NTSC VOB file having a certain fps- it's completely variable and can change at any frame.

Each frame either lasts for 2/60 or 3/60 seconds. And that is flagged with the frame's "repeat_first_field" flag. So it's very easy for a player to keep a track of timing.

MPEG-2 streams also have lots of timestamping, to sync up video and audio.

Once you've grasped the concept of variable frame rate, it's hard to go back to thinking about a fixed frame rate format!