Archived from groups: rec.video.desktop (

More info?)

On 3 May 2004 23:23:21 GMT, Leonid Makarovsky <venom@cs.bu.edu> wrote:

>Bariloche <bariloche@bariloche.com> wrote:

>: CQ shall always give you the best result, for the same given average

>: bitrate, because it is the only mode that gives the same quality to

>: each frame.

>

>But what is given average bitrate if I specify min 2000, max 8000 and say 90%.

>How do I calculate the average bitrate?

What I said would apply to the following: you perform a CQ encoding,

then you look at the log file to find out what the resulting average

bitrate has been. You can then encode in VBR mode, using that figure

as the average bitrate. Theoretically you would get the same encoding,

but in practice you don't, even if you get the same file size.

Now, the problem you wonder about is right the opposite: if I know the

average bitrate, how do I know the CQ mode that ends up using that

same average bitrate? If you have some experience with CQ encoding,

you can more or less know the bitrate given by each quality

percentage. But of course, it depends completely on the film, so you

cannot know beforehand and precisely, the bitrate attained with CQ

encoding.

This has been dealt with formerly in the newsgroup, and apparently

people is doing a tentative encoding of an Avisynth script, that can

give you the bitrate attained by a certain CQ mode, within a

reasonable degree of exactitude. Something like

AviSource("TheClip.avi")

SelectRangeEvery(300,15)

shall give you a clip made by 15 frames from every 300 (you should

encode the same number of frames that comprise a GOP, which would be

15 for PAL). If you encode this one, it can give you a reasonable

insight on the bitrate you would get when encoding the whole. Of

course, if it's not the bitrate you want, you would have to encode at

a different percentage of CQ, and keep trying, etc. If your concern is

getting an exact file size, it may not be worth that much effort, and

you should just use VBR mode instead.

>: CBR represents the most unequitative use of the available bits.

>

>True, but if you just want to put 50 minutes of video on 1 DVD, that's the

>most proper thing to do.

No. CBR allows you to calculate exactly the file size -but VBR does

exactly the same for you. You just multiply the constant bitrate of

the CBR mode, or the average bitrate of the VBR mode, by the seconds

of movie, and you get the size of the resulting mpeg video. The

mathematical definition of "average" is the very guarantee of that.

Now, let's not forget that if you intend to make a menu, that shall

also take some space on disk.