[Flac-dev] Variable Bit Rate

[Brian Willoughby]

On May 23, 2011, at 11:35, Dennis Brunnenmeyer wrote:
> I'm well aware how compression works. But images and document files  
> do not depend on the relative timing of the data to reproduce  
> themselves. They are in essence only two-dimensional in space,  
> whereas the data in a sound file is time-dependent.
Images are three-dimensional or maybe five-dimensional,  
mathematically, because the pixel value at each two-dimension point  
can have any value (monochrome) or color (three-dimensional RGB).

Documents and sound files are two dimensional.  You cannot change the  
position or value of a character in a text file without losing  
information.

The key point here is that the timing you refer to in a sound file is  
not really so special.  It is merely another dimension of the data.   
It is preserved in FLAC.  Of the various methods for drawing sound  
files on the screen, they are all at least two-dimensional, if not  
more, which should be a clue that sound files are two-dimensional.


> The question really has more to do with the decoded FLAC stream  
> output, which I presume is a linear PCM file, e.g. WAV.  If FLAC is  
> lossless and created from an original CBR WAV file, is is true that  
> the decoded output is also CBR when played?
>
> That is, WAV in = WAV out, where both are CBR?
Yes, an uncompressed sound file is CBR, unless you're talking about  
LDPCM.  FLAC is compressed, though, and thus it must be VBR in its  
compressed form.  The Variable in VBR ranges anywhere from slightly  
above the CBR of uncompressed audio (including overhead) to  
approximately half that rate (on average) or even sometimes lower.


> Thanks for any insights on this matter. I've been told that because  
> a FLAC stream from a server to an application is VBR, that certain  
> transients are not handled correctly, like the ringing of bells. If  
> this were true, FLAC would not be lossless in this application.
You have been told wrong.  If such things happen with streamed FLAC,  
then there is a flaw in the streaming software.

One thing to keep in mind is that a VBR format like FLAC requires  
latency when streaming.  If the streaming software is not designed  
with adequate latency, then you could have artifacts when the data  
does not appear in time.  But that is not the fault of the format,  
but rather that the playback is trying to get ahead of the format -  
which is impossible.

Brian Willoughby
Sound Consulting