Specifications

Pro Tools Reference Guide600
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 with TDM Mixers
Understanding Mixers
Pro Tools TDM Systems include two mixer plug-
in “flavors”: stereo and surround versions. The
mixer plug-ins are used by Pro Tools to create
“dynamic mixers,” meaning that the mixer size
can expand or contract as mix channels are added
or deleted in Pro Tools. More channels take up
more DSP power from your Pro Tools HD-series
or Pro Tools MIX-series hardware. This is differ-
ent from hardware mixing consoles where hard-
ware (analog or digital) creates “fixed” mix con-
figurations consisting of an unchangeable
number of master outputs, busses or sends.
DSP allocation for mixing in a Pro Tools TDM
system is based on the concept of DSP summing
mixers. Every send bus or output mix that you
use requires that a summing mixer exist 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, a send, or an
internal bus connection. Adding an output or
bus path (mono or multi-channel) adds the re-
quirement for DSP power to mix the signals to-
gether.
An HD card has nine DSPs, a MIX card has six
DSP chips, and a DSP Farm card has four DSP
chips, each of which can power a certain num-
ber 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 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 would re-
quire 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.