User`s guide

Pro Tools Reference Guide386
DSP Usage with TDM
Mixers
Understanding Mixers
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 demands that a summing
mixer exist for that group. Each pair of out-
puts or busses that you use requires its own
mixer.
An “input” can be a disk track, a send, or an
internal bus connection. Adding a pair of
outputs or busses adds the requirement for
more DSP power.
A MIX card has six DSP chips, and a DSP
Farm card has four DSP chips, each of
which can power a certain number of sig-
nal processing tasks. DSP resources are dy-
namically allocated as the number of mix-
ers and/or inputs increases.
The TDM mixer provides basic building
blocks by which applications such as Pro
Tools can create a wide variety of mixer
configurations. Each TDM mixer is of the
dimensions “N x 2,” meaning that it mixes
a variable number of inputs to an output
pair.
For example: a session with 6 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, 2 mixers are re-
quired—one 5x2 mixer routed to output
1–2, and one 1x2 mixer routed to output
3–4.
The important concept here is that every
pair of outputs (whether they are I/O or bus
outputs) requires that a mixer exists for
that output pair. This means that creating a
send to bus 1 requires that a mixer be cre-
ated for the bus 1-2 outputs, and that mixer
will have one input.
Types of Mixers
Pro Tools offers two different mixer plug-
ins: a “24-bit optimized” mixer and a “16-
bit optimized” mixer. These mixer modules
use DSP power at slightly different rates on
DSP Farms, but at the same rate on MIX
cards, as shown below:
DSP Farm:
16-bit Optimized mixer: 36x2 per chip
24-bit Optimized mixer: 25x2 per chip
MIX Card:
16-bit Optimized mixer: 59x2 per chip
24-bit Optimized mixer: 59x2 per chip
If the DSP is also being used for I/O process-
ing, then some of the chip’s capacity will
be devoted to I/O, yielding slightly lower
mixer count (these figures represent maxi-
mum numbers when I/O is declared in the
Hardware Setup dialog):
DSP Farm with one I/O declared:
16-bit Optimized mixer: 30x2 per chip
24-bit Optimized mixer: 20x2 per chip
MIX card with one I/O declared:
16-bit Optimized mixer: 51x2 per chip
24-bit Optimized mixer: 51x2 per chip