CBR vs VBR question

[John Rofrano]

[Mark Prebonich] “I used DVD shrink to pull my daughter’s dance video onto the computer.”

Why? You should import the DVD right from Vegas using File | Import | DVD Camcorder Disc…

[Mark Prebonich] “Which of the above settings offer the highest quality video?”

CBR 8,000,000 will be higher because it’s using 8Mbps for every frame regardless if the frame needs that many bits or not. Your other two settings are only using 6Mbps, although technically the VRR 9,500,000 will be higher for any frames that really requires 9.5Mbps.

[Mark Prebonich] “I find it interesting that the setting with the highest total bitrate is the smaller file size and that both the 8,000,000 and 9,500,000 settings generate the same file size.”

You’re not understanding what 9,500,000/6,000,000/196,000 means.

These have exactly the same average bit rate and so produce the same file size:

VBR 2-pass 9,500,000/6,000,000/196,000 (generated file size 116 MB)
VBR 2-pass 8,000,000/6,000,000/196,000 (generated file size 116 MB)

The first number is the maximum bitrate: 9,500,000 and 8,000,000. No second of video can use more than the max bits per second.

The middle number is the average bit rate. 6,000,000 and 6,000,000. Since they are the same the file sizes are the same. The average means average over the entire file. For example (and this is oversimplified to make a point) if the file had 30 frames, and you encoded 15 of them at 8,000,000, the next 15 will be encoded at 4,000,000 regardless of how many bits they actually need because the file MUST average at 6,000,000 bits per second.

It’s as if I gave you a budget of 5 dollars a day for lunch for the work week. I give you $25 on Monday and you buy a $10 lunch. Unfortunately, that only leaves you $15 for the next 4 days so you can only spend $3.75 for lunch. At the end of the week, you still had a $5 budget for each day even though that’s not what you actually spent. This what 9,500,000/6,000,000/196,000 means. Your bit budget is 6,000,000 bits per second. You are not allowed to spend more than 9,500,000 or less that 196,000 bits per second, but at the end of the file, you had better not spend more than 6,000,000 bits per second.

The reason VBR is better with large files is because it will use your “bit budget” more wisely and give you better quality for less overall bits.

The last number is the lowest bit rate to use. No frame will be encoded at less that this number.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Mark Prebonich]

“Why? You should import the DVD right from Vegas using File | Import | DVD Camcorder Disc…”

The import DVD Camcorder Disk breaks the video up into multiple VOB files. Each of these files has an approximate 3 frame segment at the end of each of these segments where the audio is missing. This will then produce a noticeable skip. The DVD Shrink method was mentioned by Mike Kujbida as a way to get around this. This method did indeed work as intended.

When I click on the various file types that I rendered and review the info at the bottom of the screen in Windows 7, there is listed several pieces of date. It lists both the data rate and the total bitrate. In the max VBR 9,500,000 render file, the data rate is listed as 9,500,000 and 8,000,000 in the other file respectively. I had then misunderstood and assumed that this was the bitrate for the entire file. As you can imagine, this can be a bit misleading. Thank you for setting things straight and for your thoughtful answer.

-Mark

[John Rofrano]

[Mark Prebonich] ” Each of these files has an approximate 3 frame segment at the end of each of these segments where the audio is missing.”

Ouch! I’ve never had that happen but that’s not good. I’ll have to remember this if I ever run into that problem.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Mike Kujbida]

[John Rofrano] “Ouch! I’ve never had that happen but that’s not good.”

John, I periodically get asked to extract some clips from a DVD where the source footage is no longer available and ALWAYS run into that problem when I go from one converted VOB file to another one (i.e. a few frames of missing audio).
It’s extremely annoying, especially if music is involved, hence the reason for the solution I offered Mark as it does work.