[Flac] 24 bit question

[Brian Willoughby]

On Dec 2, 2010, at 07:55, scott brown wrote:
> My first thought was that the file had low levels (before he sent  
> me the file), but that's definitely not the case with this file.   
> There are many peaks that reach 0dBFS.
Live, uncompressed music often has peaks that are 4 dB higher, or  
more, than a typical commercial CD.  Such peaks are brief, and would  
not really affect the total size of the FLAC file.  Really, the  
average level is what determines whether the FLAC file ends up being  
smaller than 50% as it is in this case.


> He sent me the original wav this morning and I loaded it into Wave  
> Editor on OS X.  I dithered to 16 bit using MBIT+ (high/ultra  
> setting) and saved the 16 bit file.  I did nothing else (no  
> normalizing or any other processing).  I can't give you the 16 bit  
> size right now since I'm at work and the file is on my Mac at home,  
> but I can report back tonight.

Impressive!  I have read many comments that MBIT+ is the best.  I've  
only recently licensed it myself, so I have not yet had time to form  
a personal opinion as to whether it is better than the dither that I  
have been using for years.

No matter how good the dither is, though, it's still noise.  The  
human ear and brain system cannot hear MBIT+, but FLAC is just a  
mathematical process.  Dithering from 24-bit to 16-bit is equivalent  
to increasing the quantization noise by about 48 dB!  It's actually  
quite impressive that you can add 48 dB of noise and the FLAC file  
only increases in size by less than 2%.

Thanks for the details.  I'm curious about the file size, but  
uncompressed WAV should be exactly as I predicted.


> Whatever my process is, though, the guy who originally recorded the  
> file gets the same results with whatever method he uses to convert  
> to 16 bit on Windows.  I can ask him what his 24 > 16 bit process  
> is.  I can also just truncate the file down to 16 bit and report  
> back on the resulting flac file size.  Would you expect that flac  
> file to be around the same size as the 24 bit?  In my experience,  
> my 24/48 flac files are always substantialy bigger than my 16 bit  
> flac files, which is why this case confuses me...
Personally, I rarely pay close attention to the exact compression.   
I'm happy just that FLAC is smaller and lossless.  But I am still  
curious about the various reasons why some files turn out bigger or  
smaller than others.  I tend to do everything in 24-bit, even final  
mastering, so I have not looked at 16-bit in a long while.

I can say that DTS surround music disc, which is 14-bit data in 16- 
bit CD format, does end up with a FLAC that is almost exactly 87.5%  
of the WAV.  This makes perfect sense, because the DTS data looks  
like random white noise to FLAC, and the only thing FLAC can do is  
compress those 2 unused bits.

It's tempting to look at the 24-bit to 16-bit conversion as simply  
dropping 1/3 of the data, since files are based on 8-bit bytes.  But  
one way to look at this is that FLAC deals with audio samples as if  
they were all 32-bit.  The 16-bit samples simply have more noise.  It  
will be interesting to see what happens with your 16-bit truncation  
test.  I would expect that the FLAC would only get smaller, not  
increase in size, if all you do is truncate.

Fortunately, FLAC is quite smart about bit utilization, and can even  
detect 16-bit samples in a 24-bit file.  I don't think that's  
happening with your files, though, because I would expect the 24-bit  
FLAC to be around 33% of the 24-bit WAV if it actually only had 16- 
bit samples, instead of the 40% that you're seeing.  But I suppose  
that's always possible.

Brian Willoughby
Sound Consulting

P.S.  I'm still curious where Nicholas came up with the 14% value.   
Is that based on decibels, bits, or some other metric I haven't  
thought of?