Guessing mpeg2 file sizes

mel58i wrote on 7/21/2007, 6:02 AM
using the "bitrate calc" is fine if you want to use CBR - but I guess us lot use VBR in Vegas. So we can't use the above prog to do that.
I had a 2hr 10min video to convert to DVD the other day and it took a real educated guess to hope it would fit onto a 4.7GB DVD. The actual render took in excess of 6hr to complete, and all the time I was wondering if it would fit. I was rewarded at the end of 6 hrs to find it took up 4.02GB! Just imagine the thoughts running through my mind if it had been in excess of the DVD size.
The point I am trying to make is - could it not be possible to either give a good guess in Vegas as the rendering is progressing, so you can abort a short way into the render OR Vegas can give a good guess as to file size as you type in the MIN/MAX and AVERAGE bit rate.
I do know that using VBR the bit rate varies depending on the motion in the content. Pinnacle ( whatever that was) does give a warning, but again it only uses CBR.
Anyone else been in a similar situation when doing a big VBR render?

Mel.

Comments

Chienworks wrote on 7/21/2007, 6:35 AM
Use the bitrate calculator to figure out the average bit rate and you should end up very very close. I tend to set the max rate at about 50% higher (not going over 8Mbps) and the min rate at about 2Mbps. The average is the critical number and it determines the final file size.
GeorgeW wrote on 7/21/2007, 8:14 AM
As mentioned by Cheinworks, for VBR encoding the AVERAGE bitrate (times your video length) should give you a good idea how large the resulting file will be. For some reason, I think Vegas does NOT maintain the AVERAGE bitrate for single-pass VBR encoding. But for 2-pass VBR encoding it does appear to maintain the AVERAGE bitrate...

Chienworks wrote on 7/21/2007, 8:25 AM
Well, technically, it can't maintain it in a single pass render, it can only guess and try to come close. Since Vegas has no idea what is still yet to come in the video it doesn't know how many bits it can afford to "waste" now or how many it has to conserve for later.

The whole purpose of a two-pass encode is that the first pass makes all the decisions of where the bits are needed. Vegas can then calculate precisely how many to use when to maintain quality and hit the target average bit rate.
GeorgeW wrote on 7/21/2007, 9:39 AM
Yes, I understand what's going on. Just wanted to let folks know at least one reason why VBR encodes can result in "Unexpected" file sizes (which then causes people to think VBR is unpredictable). But in reality, using the same AVG VBR vs. CBR bitrate should result in roughly similar file sizes...

rs170a wrote on 7/21/2007, 7:58 PM
The bitrate calculator I use (the link is to a 640 KB zipped file) has never let me down with CBR or VBR settings.

Mike