Setup guide

Pro Tools|HD Setup Guide104
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.
Mono and Stereo
Each TDM mono or stereo mixer is of the di-
mensions “N x 2,” meaning that it mixes a vari-
able number of inputs to an output pair. For ex-
ample: A session with six tracks routed to
Output 1–2 would require a single 6 x 2 mixer. If
one of the six tracks is assigned to Output 3–4,
however, two mixers are required—one 5x2
mixer routed to Output 1–2, and one 1x2 mixer
routed to Output 3–4.
Multichannel Surround
Each Surround mixer can have a variable num-
ber of outputs as well as a variable number of in-
puts. For example, the 7.1 format requires eight
outputs. A single mono track assigned to a 7.1
Output or Bus path would require a 1x8 mixer,
while one mono and one stereo track assigned
to a 7.1 Output or Bus path would require a 3x8
mixer.
The important concept here is that every output
(whether they are I/O or bus outputs) requires
that a mixer exists for that output. This means
that creating a send to bus 1 requires that a
mixer be created for the bus 1–2 outputs, and
that mixer will have one input.
Mixing with Sends and Busses
Each send will add an input to the destination
output pair. For example, a send to output 1 will
add another input to the output 1–2 mixer. If
the send destination doesn’t already have a
mixer for its output pair, then a new mixer will
be created. A send to bus 3 will make a 1x2 mixer
for bus 3 if no other bus 3 sources have been cre-
ated yet.
In addition, adding a new track and assigning its
input to a bus source will create a mixer for that
bus pair if there is not one already.