User guide

Appendix E: TDM Mixing and DSP Usage 93
vides extremely smooth volume changes. In ad-
dition, DAE “de-zippers” any “live input” to the
mixer so that fast, real-time fader changes that
come in from fader movements (on the Pro Tool
user interface or control surfaces), do not cause
audible artifacts as the mixer tries to “catch up”
to fast changes that it receives.
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.)
Avid has developed a mixer that provides all of
the benefits of the standard stereo and surround
mixer plug-ins, but also provide uncorrelated
dither on any summed output. This provides a
steady dither “noise floor” at extremely low
level, which causes any truncation artifacts to be
converted into steady white noise. However, the
addition of uncorrelated dither requires more
DSP horsepower. As a result, the channel in-
stance count from the dithered versions 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).”