[Flac] 24 bit question

[scott brown]

Thanks for the replies!

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.

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.

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...

Thanks,

Scott


On Thu, Dec 2, 2010 at 10:45 AM, Brian Willoughby <brianw at sounds.wa.com>wrote:

> 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?
>>>
>>
>
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