Bitrate, what to use for a DVD when Divx,Xvid etc. is source

[kristian1]

Hi

I have thought of this for a while. When I make DVD's out of the various other moviesources like DivX, Xvid, Mpeg's etc I never know what bitrate to use. If the movie is long I will generally just use a bitrate that fits snuggly. But ... the original movies sometimes have a much lesser bitrate than what I use to encode it to the DVD.
What is the differance when just increasing the bitrate so much, I mean what does the extra bytes do to the video? And what is a good bitrate? Are there any do's and don'ts here? What bitrate is an industrial copy encoded with what do you use?

/Kris

[willmcmanus]

why dont u let tmp dvd template do it for you just adjust bar to 100%

regards will

[kristian1]

Well yes I can do that, this is what I do every time. But I want to know limitations, recommendations and guidelines ....

[SaSi]

In general, your question has no answer other than trial and error, however I will try to give you some guidelines.

The reasons why there is no definite answer to the question bears on the various compression characteristics of each codec you mention. As I have done the same thing with DivX movies, I have collected some "rules".

Let's suppose that the movie is two hours long. This gives 180.000 frames @ 25fps (2x60x60x25). Let's also suppose that the source DivX avi is 2.6Gb long.

Dividing the size by the number of seconds (2.600.000.000 / 7200) gives us the average bps of the movie (in this case 360kbps). (It would help to know if the compression is of CBR or VBR type to go more precice from now on, but ok).

Now the "rules". A 360kbps Divx movie is of average quality. This means that when compressing into MPEG-2 you don't have to go for a quality index of higher than 70. If you do you would be wasting bitrate.

I contrast, a 500kbps (VBR) Divx movie is of excelent quality (actually not worse than the original DVD - unless the contents is very demanding). Such a video requires a quality index of at least 80.

Apart from the quality index, you must also specify min and max bitrate. Min should obviously be as min as possible, but 0 may cause problems. Using 500 as a minimum is ok. Once you set the quality index at any level, you can have the max bitrate set to 8000 (no more or you may end-up with a total bitrate out of spec). You don't have to worry about wasting bitrate there. The encoder will never use more than 8000, but will hardly reach this rate if you select a quality index of (say) 70. The best (I have seen it go) is around 5.000 ~ 5.500 and that only for movie parts with high motion.

The max bitrate may be used as a hard limiter to the compression process. If for example you set a quality index of 90 and a max bitrate of 5.000, you will get the distinctive square texture of jpeg in every frame. This is because the encoder tries to create a sharp and good image (generating many bits) but the max bitrate setting cut's it off, losing the least significant ones.

Try to experiment along these lines and the appropriate rules for you will become apparent.