[Flac] 24 bit question

[Brian Willoughby]

Nicholas is probably right about noise.  Another factor would simply  
be the amplitude of the resulting file.

A 275 MB 24-bit file which compresses to 110 MB is probably not very  
loud.  I assume that the average level is somewhat low, with few if  
any peaks that reach 0 dBFS.  FLAC is very good at compressing audio  
that is not loud.  In fact, the quieter the recording, the smaller  
the FLAC file.  I've actually mastered some audio where the final 24- 
bit FLAC was larger than the original 24-bit FLAC, simply because the  
amplitude is larger on average after mastering.

When dithered to 16-bit, I'd assume that your WAV output is about 183  
MB, is that right?  Seems like you're saying that this only  
compresses to 112 MB.  That is consistent with a louder file and/or  
one with more noise.  I assume that you may have boosted the level of  
the recording before dithering to 16-bit, or maybe even just  
normalized the 24-bit file.  Audio dynamics compression is expected  
to increase the size of the FLAC output.  If you did not alter the  
volume at all before dithering, then I suppose it's still possible  
for the dither noise alone to reduce the efficiency of the FLAC  
compression.  What was your dithering process?

In general, FLAC compression achieves about 50% reduction in size.  I  
get better than 50% with raw 24-bit recordings, and worse than 50%  
with mastered 16-bit recordings.  In your case, the 24-bit file is  
compressed to 40% of the original size, but the 16-bit is only  
compressed to 61% of the 16-bit WAV.  These values are all consistent  
with my typical experience, especially since there is an incentive to  
make the 16-bit version louder to avoid excessive quantization noise.

Brian Willoughby
Sound Consulting

P.S.  Nicholas, where did you come up with that 14% figure?  A 16-bit  
file is 33% smaller than a 24-bit file, or a 24-bit file is 50%  
larger than a 16-bit file.  I can't find any math that works out to a  
mere 14%.


On Dec 2, 2010, at 07:10, Nicholas Wilson wrote:
> 24bits are only 14% more information than 16bits, not as much as it
> looks. I presume the downsampling introduces some noise which
> compresses poorly (bigger residuals) and pretty much outweighs the
> advantage. This does not happen with a lossy codec, if the same error
> tolerance is imposed on each stage: a 5% noise introduction (say) at
> one stage does not create a problem if a 5% error is allowed to be
> introduced later to discard the noisiest 5% of the data. So,
> heuristically I would expect most all the gains of reduced detail to
> be realised in lossy codecs, and rather little or no space saving with
> lossless codecs.
>
>
> On 2 December 2010 13:15, scott brown <scottcbrown at gmail.com> wrote:
>> Someone sent me a question late last night and I briefly looked at  
>> his file
>> this morning and couldn't figure out the answer, so I'm posting here.
>>
>> A friend has a a ~275MB 24 bit, 48khz stereo wav file of rock  
>> music that
>> when compressed using flac level 8 gives a flac file under 110 MB  
>> in size.
>> When I dithered his file to 16/48 and converted that file to flac,  
>> the
>> resulting flac file was actually 2 MB *bigger* than the  
>> corresponding 24/48
>> flac file.  Does this make sense to anyone?
>>
>> He says that his 24/48 files always compress to around the same  
>> size as the
>> same files converted to 16/48 or 16/44.1.  I couldn't give him an  
>> answer as
>> to why.
>>
>> Does anyone have an answer?