I *still* don't understand the diff between ABR and VBR!

[LisaB]

According to the Doom9 FAQ:

************************************************** *****
CBR means that the stream's bitrate is constant and never changes.
VBR means that the stream's bitrate is variable, and changes in according to the amount of information that needed to be encoded.
ABR has a variable bitrate for each frame, but its average bitrate is a constant.

Conclusion:
CBR is a private-case of ABR which is a private-case of VBR.
************************************************** **********

OK, so CBR means every frame is the same size
and VBR means each frame may vary in size

so what is ABR? Did they mean to say that ABR has a variable bitrate "within" each frame, but each ABR frame is the same size?

And, I don't understand why they claim VBR has no average bitrate...if I add together the total size of all the frames of a VBR stream and divide by the duration, then *that* will give me the average bitrate for the VBR stream!

Maybe if I knew what "private-case" meant, I wouldn't be so confused.

[iantri]

AFAIK, ABR always gives you the average bitrate you want -- if you pick 128kbit ABR, you will get an average of 128kbit, though individual frames may vary.

VBR is more like what you might think of as CQ mode (or VBR mode in Ogg Vorbis).

No, I'm not sure why they define it this way..

[lordsmurf]

ABR sounds made up. I don't know.

Don't forget CVBR (Constrained Variable Bitrate) which is like CBR with some elbow room.

[BJ_M]

CBR means that the stream's bitrate is constant and never changes

is not correct anyway

CBR really means that the VBV buffer remains constant fullness , but the actual bitrate may vary a little ..

[iantri]

I did a bit of googling and apparently this is included in the usage.txt file with lame:

ABR at this average bit-rate usually sounds better than VBR, by definition. ABR is sort of an enhanced CBR, and has not much to do with VBR. The difference is that VBR depends on LAME's psychoacoustic model. If the psy-model says that something is easy to encode, VBR takes a low bitrate frame.

But flaws in that model could lead to the situation where something is hard to encode in reality, but the psy-model misinterprets the signal and says "easy to encode". Then VBR chooses a low bitrate where it really would need a 320 kbit frame.

With CBR and ABR, this can't happen. --abr 256 always takes a bitrate that's high enough and doesn't rely on the psy-model (which is far from perfect). Yes, you'll waste bits at those places in a song where you don't need many bits, but you also have better overall quality .. ABR yields better sound quality [tho many people won't hear the difference].

I can't honestly say, though, what this really means.. it sounds like it is like VBR except it keeps the bitrate from varying too wildly.

My earlier guess was, well, completely wrong.

[LisaB]

Well, I did understand that ABR guarantees a bitrate, while VBR is, as mentioned, a constant quality mode...so, yes, the algorithms used to create abr and vbr is very different....but the outputs are both vbr streams.

In other words:

cbr = cbr stream constrained to a certain size
abr = vbr stream constrained to a certain size
vbr = vbr stream constrained to a certain quality

with abr, you tell the encoder what bitrate you want
with vbr, you tell the encoder what quality you want
but the output of both is a vbr stream.

I guess, practically, my question is: if I give you a stream, and don't tell you how I created it, would you be able to tell the difference between abr and vbr? I'm thinking not. Just like you couldn't tell the difference between a single-pass and two-pass xvid just by examining the output file.

[DVD_Ripper]

CBR means that the stream's bitrate is constant and never changes

is not correct anyway

CBR really means that the VBV buffer remains constant fullness , but the actual bitrate may vary a little ..

I have noticed that too... some frames can vary a few percent from one another.

Also, I have never heard of ABR (just C and V)

[BJ_M]

i have heard of ABR only in relation to mp3's

[FOO]

Maybe ABR means Audio Bit Rate , i dunno

[iantri]

ABR is actually defined as "Average Bit Rate" somewhere.

I get it now -- with LAME, you specify a minimum bitrate and "quality" in VBR mode. The bitrate will be at least the minimum bitrate but spike as necessary. In ABR mode, you also specify the average bitrate; and it tries to keep it close to that, unless otherwise necessary.

Or something like that?

[bugster]

CBR means that the stream's bitrate is constant and never changes

is not correct anyway

CBR really means that the VBV buffer remains constant fullness , but the actual bitrate may vary a little ..

AFAIK, that would be correct for video, but this is the audio forum. For all I know the definitions may vary a little in the audio world..

[BJ_M]

CBR means that the stream's bitrate is constant and never changes

is not correct anyway

CBR really means that the VBV buffer remains constant fullness , but the actual bitrate may vary a little ..

AFAIK, that would be correct for video, but this is the audio forum. For all I know the definitions may vary a little in the audio world..

NEAT -- i didnt even notice which forum this was

[bugster]

CBR means that the stream's bitrate is constant and never changes

is not correct anyway

CBR really means that the VBV buffer remains constant fullness , but the actual bitrate may vary a little ..

AFAIK, that would be correct for video, but this is the audio forum. For all I know the definitions may vary a little in the audio world..

NEAT -- i didnt even notice which forum this was

I wouldn't worry about it, (as I am sure you are not ) I make that same mistake frequently. Browsing the forum via the 'LATEST POSTS' link tends to be a major factor