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Old 06-04-2007, 06:37 AM   #1
LegalEyes
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Default CBR v VBR

Hello,

Is it possible to give a bottom line reason to use one over the other. I usually use a CBR for encoding, but I see many here who use the 2 pass VBR and I'm curious about the benefit to trade off ratio.

Thanks
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Old 06-04-2007, 10:04 AM   #2
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IMHO it makes no sense to use VBR over CBR if you can get your video on a disk without going below 7Mbps. Furthermore, if I use VBR, I always use 2-pass VBR to maximize quality.

There is some good advice on setting optimal MPEG bitrates on the MediaChance site -- they are the guys behind DVDLab.
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Old 06-04-2007, 06:15 PM   #3
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So, if you only need to place one hour of video on a standard DVD, perhaps a CBR Grid Encode set a high bitrate would yield the best results, and more quickly at that?

For, lets say, one hour of video, has anyone noticed a discernable quality difference between a high bitrate CBR encode, and the "Mastering" 2-pass VBR encode?
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Old 06-04-2007, 06:48 PM   #4
Khoi Pham
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Quote:
Originally Posted by Thor View Post
So, if you only need to place one hour of video on a standard DVD, perhaps a CBR Grid Encode set a high bitrate would yield the best results, and more quickly at that?

For, lets say, one hour of video, has anyone noticed a discernable quality difference between a high bitrate CBR encode, and the "Mastering" 2-pass VBR encode?
Not for less than 1 hr., if it is less than 1 hr. I usually set it at CBR 8500 kb/s
now if it is over 1 hr. VBR is the way to do, the key to picture quality and space is to set your minimum, average, and max far apart, this way your encoder will really work, for example, a 2 hr. movie, I usually set the minimum at 2000kb/s and the average at 4730kb/s and the max at 9000kb/s
The encoder will try to stay around the average, but if the scene is flat with not much details, it will fall to the minimum, but if there is a sudden change of scene that thas high action and detail, the encoder could peak all the way up to 9000kb/s to compensate for that, you will not see artifacts like macro blocking or anything like that if you have a good encoder and high setting for max.
If you set your bitrate to close to each other like 3000kb/s for minimum and then 5000kb/s for average then 7000kb/s for max, they are to close, it is almost like CBR, you are not taking advantage of VBR encoding and you will see artifacts if the scene changes quickly from no movement low details scene to high movement high detail scene.
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Old 06-04-2007, 10:14 PM   #5
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Khoi,

Thanks. Very useful information on this topic...

Regards,

Jon
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Old 06-05-2007, 02:01 AM   #6
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Thats what I'm talking about.

Great info, Thanks.
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Old 06-05-2007, 02:17 AM   #7
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If there's a lot of variance in the complexity of your video and you have to stay below 6 or 7 Mbps, then it's a good time to use VBR.

By complexity of video, I mean the amount of "action" - be it motion and scene changes. For example, you have a guy sitting on a bench reading a newspaper (without a lot of people walking by or passing the camera) for a few minutes, then suddenly his newspaper shreds apart and turns into an animated paper animal.

The first few minutes of the guy sitting and reading peacefully - this doesn't require a lot of bits to encode as the scene is fairly "calm" - now the later part where his newspaper shreds, that's a lot of action and motion.

In a CBR scenario, equal bits (think of it as "care" or "attention" to the quality) are distributed equally across the entire thing. There's an equal amount bits being dedicated to the peaceful scene as there is to the shredding of the paper.

Obviously, the scene I described is not equal in complexity throughout. That's where VBR comes in.

In single-pass VBR, the bitrate can be increased to compensate for the high-complexity paper-shredding part of the scene. However, because we're working with a net gain here, it would likely "bloat" your output file to a larger size than you want or can use.

In multi-pass VBR, the bitrate is increased to compensate for the high-complexity paper-shredding part of the scene, and it is also decreased to "make up the difference" in the lower-complexity part (sitting reading) of the scene. The net "bit budget" is tuned to stick to the average bitrate specified. More passes generally apply a "smoothing" to the bitrate ramping, so you don't get a detailed explosion then an artifact-laden scene of the debris, which gives you a better aesthetic result.

Of course, by now you might have inferred that the decreases in bitrate of multi-pass VBR can have a detrimental effect. That's the determining factor, and also a reason to not to specify too large a difference between minimum and maximum bitrates in multi-pass VBR. As the saying goes, what goes up, must come down. If you set "up" too high, it's going to hit hard when it comes back down.
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Old 06-05-2007, 02:54 AM   #8
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The way I do it is if a DVD is less than 1hr 30mins, I use CBR...usually around 6500, to 7000 max.

If it is more than 1hr 30 mins, I use 2pass VBR with a proper calculator. I leave some headroom (around 60MB) on the DVD for compatibility reasons, so my DVD's ALWAYS max out at 4.33GB. I use all the available disc space.
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Old 06-05-2007, 06:07 AM   #9
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stormdave, when you do 2 pass VBR, what min and max do you normally use?
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Old 06-05-2007, 11:45 PM   #10
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I have a constant problem with this aspect of production and there are several comments made here by

Khoi Pham
Brandon

that are both logical but also contradictory.

I have no answer here, who does? I see both as right and it is down to trial and error for each job for me. ie more encodes not just one to get the best final.
Not very efficient! I would like to know more to change this.

Stormdave' comments on using a calculator:
I haven't had success on this either, since there is always (in my case) about 200mb difference (for eg) between what is indicated by PCE as the final file size you have your settings set up for using 2 pass VBR (I use this mostly- nearly always, even for short run work) to what you actually get.

This varies of course depending on the job, so that is where the difficulty lies in maxing out quality to DVD/web space available. I nearly always have to do a number of encodes for this to happen as best possible. Again not very efficient! I would like to know more to change this also.

I personally find it hard to know what settings to use and have yet to quantify in my own workflow and options selectable, a basic rule of thumb to apply. I see there is really none, eacg job is different.

The problem is, you have so many permutations that you can test for a given job in terms of what to select within PCE (and the differing results that gives you) that I simply have no answer on this, and each encode is always a problem for me and one that usually takes a number of goes to get the right one. ie best quality to space available.

I don't do a lot of this, but DVD burning for me is an increasing part of my work and the most troublesome/ time wasting. I have the same problen in encoding to web space since I have little to work with/available to use.
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