Specifications

Appendix F: TDM Mixing and DSP Usage 93
Setting up Sessions to Use DSP
Efficiently
The dynamically configurable mixing environ-
ment in Pro Tools lets you make choices based
on the type of setup you want to have—such as
how many inputs you want for your mixer, how
many plug-ins you want to use, or how many
sends you need.
For example, you could allocate all of your DSP
power to create a large mixer with dozens of
channels—but you would not be able to use as
many busses, sends, or TDM plug-ins. Alterna-
tively, you could create a mixer with a smaller
number of mixer channels plus some sends and
Auxiliary Inputs for returns, and TDM plug-ins
on several tracks.
You can set up your session by choosing one of
the session templates supplied with your system
as a starting point, or by building it from
scratch. If you are starting from scratch, a good
rule of thumb is to start by building your mixer
first, since at least one of the DSPs in your sys-
tem is automatically dedicated to mixing. Start
with audio tracks, then add sends and Auxiliary
Inputs, and finally add plug-ins as available DSP
allows. Master Faders do not use additional DSP
power.
DSP Usage and Mixer Plug-Ins
Understanding Mixers
Pro Tools|HD systems include two mixer plug-in
“flavors”: stereo and surround (both of which
are available with dither or without). Pro Tools
uses these plug-ins to create “dynamic mixers,”
meaning that the mixer size can expand or con-
tract as mix channels are added or deleted in
Pro Tools. More channels take up more DSP
power from your Pro Tools hardware. This is dif-
ferent from hardware mixing consoles where
hardware (analog or digital) creates “fixed” mix
configurations consisting of an unchangeable
number of master outputs, busses or sends.
DSP allocation for mixing in a Pro Tools|HD sys-
tem is based on the concept of DSP summing
mixers. Every send bus or output mix that you
use requires that a summing mixer exists for
those signals. Every single signal path that is
mixed together requires the use of a mixer plug-
in (whether a main output that goes to hard-
ware, a bus or a send). This is even true for an in-
dividual signal that travels from hard disk to an
individual hardware output. These individual
dynamic mixers are created using the appropri-
ate TDM mixer plug-in (stereo or surround) that
is installed in your Plug-Ins folder.
An “input” can be an audio track, send, or inter-
nal bus connection. Adding an output or bus
path (mono or multichannel) adds the require-
ment for DSP power to mix the signals together.
Each Pro Tools|HD card has nine DSPs, which
can power a certain number of signal processing
tasks. DSP resources are dynamically allocated as
the number of mixers and inputs increases.
The TDM mixer provides basic building blocks
by which applications such as Pro Tools can cre-
ate a wide variety of mixer configurations.
RTAS (Real-Time AudioSuite) plug-ins, as
well as AudioSuite
plug-ins, do not use DSP
and are always available. Refer to your
Di-
giRack Plug-Ins Guide
for more information.