What is the purpose of average bitrate with VBR?

[michelle]

A lot of encoders require you to specify an average bitrate when using VBR. What is the purpose of that?

[TelemachusMH]

hey michelle. Welcome to the forums.
Thre reason why abr is used is so give an estamate of the final file size. Not sure if your using GordianKnot or another program, but it is good to have an approximation when you are doing compression checks. Also, if you're using lame, (which I am guessing you are) the -alt-preset abr settings are specially tweaked for quality.

These aren't really big important reasons, but they are useful.

TelemachusMH

[michelle]

Hi,

I'm using the MainConcept encoder in Vegas Video. The default SVCD setting for average bitrate is almost as high as the maximum bitrate. The way I see it, that turns VBR into CBR. I'm more concerned with image quality than size. But I would like the best quality and the smallest size possible. I'm confused about how to set the average bitrate because it seems to defeat the purpose of VBR.

BTW, I use lame for mp3 with '--alt-preset standard' and the result sounds really good.

[Teegedeck]

I'm confused about how to set the average bitrate because it seems to defeat the purpose of VBR.

...and that's pretty much my feeling about it, too. Isn't the only sense in abr that it gives you a roughly predictable filesize? But one doesn't need a predictable audio-size if one encodes audio before video. (--alt-preset fast standard really seems more recommendable.)

[ultimatebilly]

Well, the problem with SVCD is, that it has a relatively low maximum bitrate, namely 2.718 kbit/s for both audio and video.
I usually calculate the average bitrate by the number of CDs I want to use...
If I have a 90 min movie for example, and use audio with 224 kbit/s, then I calculate:
2700-224=2476->max. bitrate (note that many players support higher bitrates, but that isn't standard-compliant...)
Number of CDs:2 = 794mb*2 (SVCDs can hold up to 800MB per 700er CDr...)
Now I calculate the kbits I have:
794*2=1588MB, *1024 = 1626112kbyte, *8 = 13008896kbit...
Now I divide this by the seconds I have: 90*60=5400...
13008896/5400=2409kbit/s for video and audio...
2409-224=2185->average bitrate for video only...
The average bitrate lets you calculate how big the filesize of your final movie will be...
It works somehow like: "Choose the bitrate according to the frame-complexity, but don't go beyond the maximum bitrate, don't go below the minimum bitrate, and in the end, you have to achieve the average bitrate, so the file isn't over- or undersized".
By the way, there is normally no need to set a minumum bitrate, just for some players, like for example the Scott 837, which can't handle bitrates under 500kbit/s very good (audio will stutter)...
So the nearer the average bitrate is to the maximum bitrate, the better the quality will be...
Well, hope that clarifies more than it confuses, and I hope I don't have any errors in my calculation, because normally I use bitrate-calculators for that...

PS.: The way I understand it, a high average bitrate doesn't defeat the purpose of VBR, because all that VBR does, is to save bits where there is the possibility, and spend them where many are needed; to allocate the bits so it fits the needs of the video...
You would defeat the purpose of VBR if you would set the minimum bitrate near to the maximum bitrate (or the average bitrate to the same value as the maximum)...
So even with the average bitrate set near to the maximum, you can have scenes in the video where only the minimum bitrate is applied, and the bits saved here will be spent in high-bitrate scenes, where the bitrate will be then as high as the maximum bitrate allows...

[michelle]

The way I understand it, a high average bitrate doesn't defeat the purpose of VBR, because all that VBR does, is to save bits where there is the possibility, and spend them where many are needed; to allocate the bits so it fits the needs of the video...
I wish I understood that. It's seems to me that if the material requires, let's say, an average bitrate of 1800 and you specify an average bitrate of 2300, then you are forcing the encoder to use more bits than it needs to. But there's no way of knowing that it may only need 1800.

[Ookami]

Originally posted by michelle
I wish I understood that. It's seems to me that if the material requires, let's say, an average bitrate of 1800 and you specify an average bitrate of 2300, then you are forcing the encoder to use more bits than it needs to. But there's no way of knowing that it may only need 1800.

I think you are looking it from a wrong standpoint.

First, there is no "must" bitrate and "needed" bitrate at all. There are only bitrates that:

a) fit into the standard (max. or minimum or standard doesn't even support VBR at all...)
b) that fit your needs/thoughts of quality
etc.

The encoder (in the ideal case, of course), should just do what you tell him, so if you "tell" him to encode a file with e.g. minimum 300 avg. 2000 and max 2000 then it should do it. :)

IMO, the names says it all, avg. bitrate is the one that should come out when you look at the average bitrate of the whole file, not only a part of it.

The maximum bitrate is the maximum bitrate wich should not be exceeded (for one reasons or another).

The minimum bitrate is the one that should, er, stay the minimum.

In the case of SVCD, the standard is pretty limited and even if you encode to max. bitrate, many people would argue that the quality is not satisfactory.

Hope my chaotic and in a hurry written posting is readable and not full of errors ;) .

Cheers,

Mijo.

[michelle]

minimum 300 avg. 2000 and max 2000
In your example wouldn't it be impossible for the bitrate to go below 2000. :)

All the guides I see show a high maximum and average and a low minimum. The way I see it, and I'm not that good at math, if you have a high maximum and average then it isn't possible to have much variability. The bitrate can't go below the average very far or very often. So the advantage of VBR over CBR would be small.

[Ookami]

Erm, I thought it was clear that it was not a real life example, but more a "encoder is stupid and should do what you tell him" example. Oh, well...

Well, IMO, the biggest mistake one can make is to set the minimum too high, and that was very often made by people believing that the overall quality would rise because of that.

What I have got from your postings, you are more looking for a quality based encoding model than a VBR model. If, I understood you well.

Cheers,

Mijo.

[Acaila]

I wish I understood that. It's seems to me that if the material requires, let's say, an average bitrate of 1800 and you specify an average bitrate of 2300, then you are forcing the encoder to use more bits than it needs to. But there's no way of knowing that it may only need 1800.You cannot force a codec to use a higher bitrate than a particular frame requires. It will encode that frame to the highest bitrate it can (reaching maximum quality), any bits that are left after this are available for future frames.

The average bitrate is fixed by the duration of the video and target filesize. The maximum bitrate is (in your case) fixed by the codec/standard you're using. Just because the average is rather close to the maximum doesn't make it bad, in fact that's a good thing. It means that for the target filesize you've set you'll reach a video quality close to the highest quality the codec/standard allows. No bits will be wasted on frames that don't require it, and those that do need it will have a high average bitrate to draw from.

[ultimatebilly]

Maybe I should add, that CBR is not generally a bad thing...
VBR just can make better use of the bits you have because they are allocated better, but if you have a very high bitrate (in this case near to the maximum of the allowed), you could also use CBR...
The strength of VBR lies in low(er) bitrate conditions...
Or, as DVD Demystified puts it: "VBR doesn't directly improve the quality, it only means that quality can be maintained at lower average bit rates. For best quality of long-playing video, VBR is the only choice. For short video segments, CBR set at or near the same data rate of VBR will produce similar results with less work and lower cost".
I hope this reference helps to convince you :D
Good that I had the book in front of me because I needed the dimensions of JACKET-pictures...