Re: [Audacity-quality] shaped dither too "loud"?

[Gale A. <ga...@au...>]

| From Steve the Fiddle <ste...@gm...> 
| Wed, 5 Sep 2012 17:49:48 +0100
| Subject: [Audacity-quality] shaped dither too "loud"?
> We periodically get complaints on the forum about Audacity adding
> "hiss". This invariably turns out to be dither noise.
> The default is "shaped" dither, which should be the least obtrusive at
> normal listening levels.
> Indeed it is pretty quiet at normal listening levels, but comparing
> with shaped dither from other programs, if applied to a 44.1 kHz
> track, it does seem to be noticeably louder than other
> implementations.
> 
> Looking at the spectrum of shaped dither at 44.1 kHz, the dither noise
> is noticeably higher in the 4 to 10 kHz range than shaped dither from
> other programs.
> Examples of shaped dither, not from Audacity:
> http://shibatch.sourceforge.net/ssrc/gspi_dither.png
> http://www.tnt-audio.com/sorgenti/dither_e.html
> 
> Looking at equal loudness contours
> (http://en.wikipedia.org/wiki/Equal-loudness_contour) hearing  at low
> level is most sensitive at around 3 kHz, so it appears that our
> "shaped" noise at 44.1 kHz is not shaped to the right shape.
> 
> The reason I keep stressing "at 44.1 kHz" is because the shape of the
> dither noise changes dramatically according to sample rate. Should it
> do that?  

There are complaints about every month or so on feedback@ also. 
Many (but not all) are by people reporting bug 22 in regards to
16-bit exports from 16-bit projects: 
http://bugzilla.audacityteam.org/show_bug.cgi?id=22 .

So is Audacity's dither noise at 3 kHz loudest at 44100 Hz sample 
rate, or some other relationship? 

If I'm reading Plot Spectrum (at 44100 Hz sample rate) correctly,
our shaped dither is quietest at 4000 Hz but there is a second 
notch at 12500 Hz; noise at 8000 to 12500 Hz is above the level 
of that notch.  

"Triangle" dither is indeed louder at 44100 Hz than shaped dither 
in the 3kHz - 4 kHz range, but I don't recall complaints about 
dither noise in 1.2.6 where the default dither was triangle. I don't 
think this is just that 1.2.6 did not have bug 22, because most
users will be using default 32-bit project quality. 

The reason we changed to shaped dither for High-quality conversion
and efforts made to "fix" shaped dither which was said to be "broken"
are described here:
http://audacity.238276.n2.nabble.com/Mixer-was-Re-FW-Dither-td251450.html#a251465 .




Gale 



Gale