Why do some 2 hour movies take 4.5 GB and others 6 GB ?

[tvtuna]

I just got my QUE! DVD Burner. I grabbed a 2 hour DVD movie, found out it had 4.5 GB, made a backup copy and thought "This is great !" Then I grabbed another movie of the same length and found it needed 6 GB.

Can anyone enlighten me as to why the size difference, and if there is anyway to squeeze this onto a 4.7 GB blank?

[big_hed]

I'm just a newbie and will be surely be corrected, but I think it may be since MPEG is a compressed format, it only stores the differences between frames. I would imagine the bigger file has more differences between frames such as action movies, and probably not a talking movie.

[pacmania_2001]

Some movies are of a higher quality "transfer" than others therefore meaning a higher bitrate was used when the DVD was encoded therefore taking up more space. If you want to see whether a DVD is under 4.7GB than look on the back of the case and it should say something along the lines of "Single Sided, Single Layer Disc"

[poopyhead]

don't forget the special features take up space too...so if a dvd has alotta special features, it will be larger than a dvd of the same movie length, but w/ no special features

[tvtuna]

Thanks for the input.

A question on bit rate. Are DVD's recorded at a constant bit rate, yet vary from one to another ?

If a movie includes 10 minutes of fast action, does the whole thing have to be recorded at a faster bit rate to be able to present the rapid motion in one segment ?

A month ago I was trying to squeeze things onto a VCD. Somehow there is never enough room.

[Shabubu]

Nope, DVD's a VBR. But if a movie has a lot of fast motion (believe it or not a field of corn blowing in the wind counts as this !!!) it will require more space cause more sections need a higher bitrate. Basically DVD mastering uses as much disc as they can (like we do) so they can get the best quality. If thye have 9Gb to use, they'll try and fill it.