Specifications

Appendix F: TDM Mixing and DSP Usage 97
Stereo and Surround Dithered
Mixers
The Stereo Dithered and Surround Dithered
mixer plug-ins provide non-correlated dither in
addition to other basic attributes of the mixers
described above. Every output summing point
(whether to an internal bus or an physical out-
put) is dithered in these mixers. This technique
is used to avoid any possibility of audible arti-
facts caused by truncation of extremely low
level data that occurs when signals pass from the
48-bit world of a TDM mixer to the 24-bit world
of a TDM bus connection or a hardware output.
Any material that is truncated, lies below
–144 dBFs (reflecting 24-bit of dynamic range).
There is dissension in the audio community as
to whether or not artifacts that fall within this
area are actually audible in some way. (The nor-
mal dynamic range of human hearing is gener-
ally accepted to fall within a range of around
120 dB, from the threshold of audibility to the
threshold of pain.)
Digidesign has developed a mixer that provides
all of the benefits of the standard stereo and sur-
round mixer plug-ins, but also provide uncorre-
lated dither on any summed output. This pro-
vides a steady dither “noise floor” at extremely
low level, which causes any truncation artifacts
to be converted into steady white noise. How-
ever, the addition of uncorrelated dither re-
quires more DSP horsepower. As a result, the
channel instance count from the dithered ver-
sions of the mixers is around 15% lower (or
more) than the non-dithered standard versions
(this varies with sample rate and mixer type).
For this reason, we offer both types of mixer
plug-in. You may want to use the standard non-
dithered mixer in most applications because of
its greater efficiency (allowing you to mix more
channels without running out of DSP power), or
the dithered mixer because of its theoretical ad-
vantages.
A Note About Dithering to 16-Bit
and Dither Plug-ins
The Dithered mixers provide 24-bit dither at
their summing points. When creating a final
mix that results to 16-bits (for example, for CD
mastering), final dithering should still be han-
dled by plug-ins that provide dither. To properly
dither the final mix result, insert a dither plug-in
on the post-fader inserts of a Master fader. De-
tails about dither and proper usage of the dither
plug-ins can be found in the Pro Tools Reference
Guide.
Mixer Plug-ins
Pro Tools|HD systems come with four different
mixers: Stereo, Surround, Stereo Dithered, and
Surround Dithered.
The standard Stereo and Surround Mixer plug-
ins are installed by default. When you run your
Pro Tools Installer, a copy of the Stereo Dithered
and Surround Dithered mixers are placed in the
folder “Plug-ins (Unused).”