Specifications

Appendix B: TDM Mixing and DSP Usage 605
Trimming Input Summing Levels
With many inputs that contain high-level sig-
nals, it is possible to clip the input summing
stage of a MIX-series mixer plug-in. This could
easily happen with the 16-bit Optimized Mixer,
but is very much less likely to occur with the 24-
bit Optimized Mixer because of its 30 dB of
headroom.
To reduce the levels going into the input sum-
ming stage of the Mixer plug-in, lower the fader
levels of all track faders using the All Mix/Edit
group. If you are using mix automation, you will
need to use Automation Trim mode or the Trim-
mer tool to lower the overall volume of all tracks
until the levels are low enough to avoid over-
loading the mixer input. You can see this clearly
by using a Master Fader for that mixer sum
point, and examining its meter.
Note that clipping the “input” side of a mixer is
not a concern on Pro Tools HD-series systems. It
is virtually impossible to clip the input of any
HD-series mixer plug-in, because the 48 dB of
headroom provided prevents any possibility of
overload, even with a maximum number of in-
puts 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 versions of the mix-
ers is near sample-accurate (as is pan automa-
tion on the stereo versions). In addition, DAE
provides 24-bit interpolated values between mix
breakpoints, which provides near “analog-like”
resolution. 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 transi-
tions on the DSP hardware with 24-bit preci-
sion, which provides extremely smooth volume
changes. In addition, DAE “de-zippers” any
“live input” to the mixer so that fast, real-time
fader changes that come in from fader move-
ments (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 be-
low –144 dBFs (reflecting 24-bit of dynamic
range).