[music-dsp] 32 bit floats >>> 24 bit integer
earlevel at earlevel.com
Thu Jan 19 18:03:25 EST 2006
Generally speaking, there is no sense in wasting computing power to
dither a signal to 24 bits. If you're going to play back the 24-bit
value at some point, good luck getting a D/A and repro system whose
noise floor wouldn't bury the effects of the dither--short of using
cryogenics (the LSB is -144 dB full scale). 32-bit floats have
roughly the same resolution at "full code" as 24-bits (24-bit
mantissa plus sign bit, compared to 24-bits including sign for 24-bit
ints); floats have an advantage in dynamic range, but if you do your
final adjustment for audio level before the conversion, as you
should, the two come out even. Dither when you need to output to
shorter word lengths.
There are other issue in the conversion--under windows & intel, it
can be really slow to do it the obvious way in C (because assignment
of float to int in C is by truncation, and microsoft libs have
convergent rounding IIRC as the default--Visual C/++ makes a lib call
to change the rounding mode, which take an incredible amount of time
due to emulating ancient math coprocessor hardware...). It's not a
problem on PowerPC chips and other things, but for wintel you can
avoid the mode change by using assembly language. You should find
something on musicdsp.org, or searching the list archives.
On Jan 19, 2006, at 2:16 PM, kernel wrote:
> hello, what's the best way to convert from 32 bit floating point
> audio to 24 bit integer whilst retaining the best audio quality.
> are we talking simple truncation and a splash of dither? these are
> multichannel files so I guess it would be best to dither the mix
> bus instead.
> thanks, stephen.
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