What's the skinny on VBV Buffer?

[heavydan]

What does VBV Buffer exactly do?

The site below try's to say that TMPG's VBV Buffer size's in the standard templates are wrong:
http://www.edv-tipp.de/svcd/236_vbv_mux.htm
It is in german and can be translated online here:
http://www.freetranslation.com
The site says MPEG1=46KB & MPEG2=224KB.

Can anyone shed some light?
Also would you need to modify the size of VBV buffer when creating XVCD,etc?
Is there a way to calculate the correct VBV Buffer size for the type of custom encode you are making?

Thanks all.

[disturbed1]

VBV buffer when measured is always half of whats specified. Most encoders are this way. TMPG, and BBmpeg. Set the VBV for 224, and both encoders are measured at 112. Do a VCD, set at 40, it's measured as 20.

Xing's mpeg1 encoder is the opposite. For VCD, instead of 40, it's 20.

Following mpeg guide lines, for a constraint mpeg1, the VBV is 40 (measured as 20) until 1857. Hence the DVD mpeg1 max bitrate of 1856.

What VBV does, it states the max. size for any given frame in a GOP (usually the I frame), it tells the decoder how much to buffer. If it's too large, or too small, it can cause problems. Funny speed up slow down thing.

Simply math
--------------------------------------
~VBV setting = Bitrate x .04.

1150 x .04 = 43.7

1800 x .04 = 68.4

2788 x .04 = 111.5

5600 x .04 = 224

That'll get you by.







<font size=-1>[ This Message was edited by: disturbed1 on 2001-12-16 21:55:17 ]</font>

[heavydan]

Thanks Disturbed1 for your help!

One thing though when I put the numbers in my calculator I got this:

1150 x .04 = 46
1800 x .04 = 72
2788 x .04 = 111.52
5600 x .04 = 224

That 46 tie's in with what that german site was saying for MPEG1. I haven't tested it on standard vcd settings yet. (I'll try later)

I tested different VBV Buffer sizes on my XVCD template and found that a smaller size made blocks appear, dis-colorization, file size shrink, and quality in bitrate viewer got worse. With a larger size buffer nothing bad seemed to happen at all. But I would keep the buffer at it's sweet spot.

If MPEG1 vbv buffer IS: 40
Then a closer formula would be:
VBV setting = Bitrate x .035

This formula seemed to gel with my XVCD findings to! Giving me the "sweet spot".

Again, thank-you Disturbed1 your insight was very helpful.

Does anyone know what formula to use to work out MPEG2 VBV Buffer?