[Flac] 24 bit question

[Brian Willoughby]

On Dec 2, 2010, at 14:53, scott brown wrote:
> original 24/48 wav file: 264,904,968 bytes
> flac level 8: 105,992,780 bytes
>
> dithered 16/48 wav file:173,885,996 bytes
> flac level 8: 108,700,948 bytes
>
> truncated 16/48 wav file: 173,885,996 bytes
> flac level 8: 105,224,448 bytes
>
> RMS level of original 24 bit: -15.3dB with peaks at -0.3dB
>
> if I normalize the original file to a max of 0.0, the resulting  
> flac file is 192,798,482 bytes. weird....
Normalizing will probably always make the FLAC larger.  Of the many  
components of the FLAC algorithm, one is to use differential values  
rather than absolute, and another is to use variable-length coding  
(Rice coding).  Quieter files have smaller differential sample  
values, and thus compress more.

I am surprised that a mere 0.3 dB normalization would add over 80% to  
the FLAC size.  Perhaps you missed something?  Can the RMS level  
actually reach -0.3 dB without serious distortion?  ... or did you  
mean -15.3 dB RMS and -0.3 dB PPM?

Comparing the 24-bit WAV to the 16-bit WAV, it looks like as much as  
4 MB of non-audio data is in the file.  You may be looking at  
waveform overviews or other extra chunks in the WAV file which are  
usually discarded by FLAC.  But this only explains a small part of  
the surprising numbers.

One thing that stands out to me is that your original 24/48 WAV may  
not actually have 24-bit samples in it.  I wrote a program which  
could detect this situation (16-bit samples in a 24-bit file), but I  
do not know of any other available tool to test this.  It seems very  
suspicious that the 24-bit FLAC and truncated 16-bit FLAC are within  
0.73% of each other (i.e., less than 1%).  I have a suspicion that  
your friend is recording 24-bit files from a 16-bit A/D converter  
interface, or at least the software is set for 24-bit files while the  
interface is set to 16-bit mode.  You should have the recordist  
double-check all settings.  A mistake here would easily explain the  
strange FLAC compression ratios.

Brian Willoughby
Sound Consulting