Setup guide

Appendix F: TDM Mixing and DSP Usage 107
Note that clipping the “input” side of a mixer is
not a concern on Pro Tools|HD systems. It is vir-
tually impossible to clip the input of any
Pro Tools|HD mixer plug-in, because the 48 dB
of headroom provided prevents any possibility
of overload, even with a maximum number of
inputs being fed by full-code signals with fader
gains at maximum. While it is possible to clip
the “output” side of the mixer, you can safely
use a Master Fader to trim your mix bus back to
avoid clipping (by simply examining the Master
Fader meter for clipping, and pulling back the
fader until it disappears; make sure that any
plug-ins you may have on the Master Fader in-
serts are not the cause as well). The 48-bit preci-
sion of the mixer allows gain adjustment on the
Master Fader with no loss of data integrity or au-
dio quality, so there is no need to trim the indi-
vidual input faders back to avoid clipping.
Mixer Automatiion
Volume automation on all mixer versions is
near sample-accurate (as is pan automation on
the stereo versions). In addition, DAE provides
24-bit interpolated values between mix break-
points, which provides near “analog-like” reso-
lution. This process of interpolation means that
a smooth “data series” is created between any
two breakpoints that you specify in Pro Tools.
DAE calculates these smooth transitions on the
DSP hardware with 24-bit precision, which pro-
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.)
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).