Why are professional dvds and bluray's using vbr for video?

[yoda313]

ok I guess is the right way to phrase it. I've used the bitrate display option on my ps3 enough times to see that the data rate for video on dvd and blurays is never a constant number.

Does that mean ALL dvds and blurays use variable bitrate for video? I guess I would expect a constant bitrate on a pro-disc. But does vbr allow for the best use of available space on a disc?

I guess I'm more asking the difference between constant bitrate and variable bitrate. My basic understanding of constant means it will always be a fixed amount while variable, well, varies.

Is that the main reason for the existent of the two? So that they can fluctuate?

Also would it be likely that bitrates would be higher on fast action sequences as apposed to a more static scene (ie higher bitrate for a car chase versus lower bitrate for a talky scene like a court room scene or dinner conversation?).

Input is welcome. Just term clarification in the pro world. I mean I would think that if they claim "25mbps" on a bluray disc that it would always stay at that bitrate but it doesn't seem to always do that.

-----------------------

Or let me try to answer my own question - does the audio fluctuate even on a constant bitrate disc? Meaning it would "spike" at louder sections than lower sounds? Is the audio variable while the video is constant and that is how they could make the statement?

Edit - oh another possiblity - would the ps3 not have a sensitive or accurate enough bitrate display program to be reliable? Would that be something only truly analyzed properly on a computer with some program?

[Cornucopia]

Depends on the application whether it's CBR or VBR.

If you have a bandwidth limitation and you need to maximize it with just one user data stream, or need to have a constant feed because of a very limited buffer, you need CBR.

If you have a space limitation and need to maximize it, or if your bandwidth needs to maximize multiple userdata streams, you need VBR.

Modern video codecs and consumer formats allow for both, and I have personally SEEN and CREATED both pro CBR and VBR for DVD (even though you would normally see a vast majority being done as VBR).
Why? Because like I mention in those first rules of thumb, DVDs (and Blu-Rays) have a finite amount of space. Yes, they also have a finite amount of bandwidth also, but you'll run out of space sooner than you'll run out of bandwidth, so for most needs, SPACE is the limiting factor.

Times when you'll see more CBR than VBR:
1. Using VCDs and other formats that require an EXACT and CONSTANT bitrate.
2. When you have constraints on the ENCODING TIME (2+ pass VBR almost ALWAYS takes longer to encode - usually twice - than CBR).

Times when you'll see more VBR than CBR:
1. When you have a filesize limitation, and specifically want the maximum program time/length and also the maximum quality (given those 2 constraints).

What you mentioned about action sequences, etc is true.

I like to think of the analogy of CBR being like a car's Cruise Control.
And VBR is like a Semi Truck trying to minimize gas usage by COASTING/ACCELLERATING down hills and SLOWING DOWN up hills.

Two different methods for two different reasons/goals.

...

Audio has historically be almost ALWAYS CBR. In VCD and DVD it is REQUIRED. IIRC, in Blu-Ray it is now optional.
But LOTS of players have trouble with playing VBR audio (both software and hardware).
When gauging total bitrate requirements and for budgeting, the usual procedure has been to start with a total, subtracting a CONSTANT Audio (cumulative if more than one stream), and other minor misc. items like subitles, to get the MEAN/AVERAGE required for Video.

HTH,
Scott

[manono]

Does that mean ALL dvds and blurays use variable bitrate for video?

I know nothing about Blu-Ray, but DVDs, usually from the lowest quality DVD production companies, sometimes use CBR encoding. The good DVDs all use VBR encoding. I'm talking DVDs of movies here, and not real short stuff.

Also would it be likely that bitrates would be higher on fast action sequences as apposed to a more static scene?

Of course, if the aim is to provide more-or-less equal quality for all kinds of scenes.

...does the audio fluctuate even on a constant bitrate disc? Meaning it would "spike" at louder sections than lower sounds?

As already answered, no, not for DVDs anyway.

If you'd like to see the bitrate graphs for hundreds of DVDs and Blu-Ray discs, check the DVD Beaver site. For example, here they are for several versions of Leon The Professional, both DVD and Blu-Ray:

http://www.dvdbeaver.com/film/DVDReviews7/leon.htm

[videopoo]

Space constraints are an issue too on many titles. VBR is a must in these siuations. Also both the Sonic SD-1000/2000 and Cinemcraft Xtream Presto are unable to segment re-encode after a CBR encode. Segment re-encoding is done on every title before authoring.

[Cornucopia]

...Segment re-encoding is done on every title before authoring.

VERY NOT TRUE. Segment re-encoding is used for when certain particular sections of a title do not maintain the expected level of quality (usually during high action scenes) and IF improvement can be made by adjusting parameters such a Average or Peak Bitrate, or Quantization level then those parameters are adjusted. Hopefully, this won't raise the overall average bitrate to exceed the allotment in the bit budget...

Many professionals can anticipate where the needed sections are and mark it ahead of time (between 1st and 2nd passes) so the title will be encoded CORRECTLY on the first try.
Other "professionals" are so hampered by time constraints (and time is money, so budget constraints) that they'll just leave it as is, even for Hollywood movies. That's why Internet sites devoted to quality review are full of listings of titles that are "sub-par".

And, like I mentioned before, there ARE professional titles that are done with CBR (though not many). If the title is a corporate video, or music/concert video, it is QUITE LIKELY that runtime is less than 60 minutes, and in that case, there is NEVER a NEED TO USE VBR, because CBR at the highest bitrate allowed(which will only give the best the system is available to give) STILL won't use up all the availabe space. I myself have done scores if not hundreds of these, in addition to all the VBR longer form titles I've done.

There are also VERY FEW encoders which support Segment-based reencoding and they are $$$. My guess is that is too much for the OP, so why even mention that kind of feature?

Scott

[videopoo]

VERY NOT TRUE. Segment re-encoding is used for when certain particular sections of a title do not maintain the expected level of quality (usually during high action scenes) and IF improvement can be made by adjusting parameters such a Average or Peak Bitrate, or Quantization level then those parameters are adjusted. Hopefully, this won't raise the overall average bitrate to exceed the allotment in the bit budget...

Many professionals can anticipate where the needed sections are and mark it ahead of time (between 1st and 2nd passes) so the title will be encoded CORRECTLY on the first try.
Other "professionals" are so hampered by time constraints (and time is money, so budget constraints) that they'll just leave it as is, even for Hollywood movies. That's why Internet sites devoted to quality review are full of listings of titles that are "sub-par".

And, like I mentioned before, there ARE professional titles that are done with CBR (though not many). If the title is a corporate video, or music/concert video, it is QUITE LIKELY that runtime is less than 60 minutes, and in that case, there is NEVER a NEED TO USE VBR, because CBR at the highest bitrate allowed(which will only give the best the system is available to give) STILL won't use up all the availabe space. I myself have done scores if not hundreds of these, in addition to all the VBR longer form titles I've done.

There are also VERY FEW encoders which support Segment-based reencoding and they are $$$. My guess is that is too much for the OP, so why even mention that kind of feature?

Reading the OPs post, it seems he's talking about the DVDs and Blu Rays he purchases. Unless he's really talking about his Aunt Sally's wedding video, the small distrubution outlet who uses Compressor and DVDSP or the average corporate video producer, I think we'll have to assume he discussing studio grade encodes. Would the OP correct me if I'm wrong?

Every studio title I work on requires segments encodes......Its just a part of the the production process. (at least where I work) I guess I assumed he was talking about studio titles. By "Pro" I naturally thought studio level encoding and authoring.

[lordsmurf]

Matrices often get adjusted, too, as well as the GOP parameters, when a scene does not meet quality expectations -- it's not always JUST about bitrate.

[videopoo]

Matrices often get adjusted, too, as well as the GOP parameters, when a scene does not meet quality expectations -- it's not always JUST about bitrate.

Yep. Even Run different segments through different pre-processing settings.

[edDV]

...
And, like I mentioned before, there ARE professional titles that are done with CBR (though not many). If the title is a corporate video, or music/concert video, it is QUITE LIKELY that runtime is less than 60 minutes, and in that case, there is NEVER a NEED TO USE VBR, because CBR at the highest bitrate allowed(which will only give the best the system is available to give) STILL won't use up all the availabe space. I myself have done scores if not hundreds of these, in addition to all the VBR longer form titles I've done...

This would also apply to hand held, noisy consumer camcorder video. Since handheld introduces X,Y and rotation movement into every frame there are no low motion scenes that would benefit from VBR. If one sets say 5 Mb/s average VBR, all that happens is high motion bit rate is constrained to near 5Mb/s thus lowering quality. Better to raise average bit rate to near maximum >9Mb/s which is the equivalent of CBR.

If a user needs more than one hour per DVDR, he should shoot for VBR by using a tripod or other camera stabilization, better lighting and minimal pan and zoom like the pros.

[yoda313]

Thanks for all the info everyone.

And for clarification yes I mean store bought hollywood dvds and blurays.

I appreciate the insight.

[edDV]

Thanks for all the info everyone.

And for clarification yes I mean store bought hollywood dvds and blurays.

I appreciate the insight.

They already spent tens of thousands to give you an optimal VBR.