CBR VBR Discussion

[josel]

Hi All,
Does anyone know of any discussion forums or websites that can explain in depth and until your heart is content on bitrates, cbr and vbr.


Cheers Josel

[jagabo]

There's not a lot to discuss:

Constant bitrate uses the same bitrate for the entire video. A featureless black segment will take just as many bits as a highly detailed, high motion segement.

Variable bitrate will allocate more bitrate to scenes that need it, less to scenes that don't.

So for the same file size (same average bitrate) a VBR video can be better quality than CBR. Or it can have just as good quality with a smaller size (less bitrate).

[bendixG15]

Just go to the top of the screen and stick the word ...bitrate...in the little rectangular box and then click on...... search all forums.......You will get opinions that do not agree with the previous post.......

[ZippyP.]

....You will get opinions that do not agree with the previous post.......

I didn't see anything in jagabo's post that wasn't pretty much just accepted fact. Not much in it to dispute as far as I could tell, unless I missed something.

[jagabo]

Note I've been speaking of 2-pass VBR vs. 1-pass CBR. There are some programs that use a single pass VBR and those generally don't do as good a job as the 2-pass VBR encoders.

One place where it may not make sense to use VBR is when you are putting less than an hour on a single layer DVD. You might as well save time and just use a high constant bitrate (8000+ kbps) encoded in a single pass.

[ZippyP.]

Note I've been speaking of 2-pass VBR vs. 1-pass CBR.

Doesn't matter, your statements are true either way. You said it "can be better quality" and "can have just as good a quality with a smaller file size", which is what VBR is all about...if it's done right.

[edDV]

One place where it may not make sense to use VBR is when you are putting less than an hour on a single layer DVD. You might as well save time and just use a high constant bitrate (8000+ kbps) encoded in a single pass.

For example, you might go for high bitrate CBR for shaky handheld camcorder video. Shaky video with lots of pans and zooms will defeat most MPeg encoder motion detection and vbr attempts. The argument is that you should max CBR bitrate to ~9500 Kb/s + AC-3 audio or ~8200 Kb/s + LPCM audio and settle for ~1hr per DVD.

VBR will be more successful if the video was shot from a tripod and with controlled pans and zooms.

[whitejremiah]

edDV, yes......and also with most retail dvd's...the main thing to note, though, is this, as far as audio tracks are concerned, you will always want to use CBR on the audio, as vbr can cause synch issues and such......

[guns1inger]

I have never seen an encoder than can do VBR audio for DVD. I don't believe the DVD spec allows for it.

[Cornucopia]

It doesn't.


Scott

[PeterK]

Hi,
I have 60 min. of video to convert to mpeg for making dvd.
Can I use constant bitrate of 9250 + audio mpeg 224.

Thanks,
Peter.

[jagabo]

Yes, but that might be pushing your luck. Some of players don't handle R and RW media as well as pressed DVDs. leaving more headroom will help them out.

MP2 audio is part of the PAL DVD specification. It is not required of NTSC players although many (most?) of them can handle it.