Specifications
Appendix F: TDM Mixing and DSP Usage 89
appendix f
TDM Mixing and DSP Usage
Benefits of TDM II
Digidesign’s TDM (or time division multiplexing)
technology is based on the concept of a single,
high-speed data highway, or bus that transmits
data between your Pro Tools CPU, Pro Tools
cards, and the DSP chips on the cards.
Pro Tools|HD cards feature Digidesign’s en-
hanced TDM II architecture. The TDM II archi-
tecture provides many advantages over the orig-
inal TDM (or TDM I) architecture in terms of its
mixing capacity and flexibility.
In TDM systems, individual channels from
sources such as audio tracks, sends, or busses are
sent out from Pro Tools audio cards, and com-
bined together or multiplexed onto the TDM bus
so that all signals can travel simultaneously and
can be accessed within a single sample period.
At the receiving end, the audio cards can listen
to any connection on the bus, and take what-
ever data they need.
The TDM bus runs fast enough to accommodate
many audio signals at the same time. Each sepa-
rate audio signal or stream, takes up a single time
slot on this multiplexed bus.
One of most powerful features of the TDM archi-
tecture is that a single time slot can be used to
“broadcast” data to many destinations simulta-
neously.
With TDM II, not only can it send data to many
destinations simultaneously, but it can also
send signals both bi-directionally and “pri-
vately” between DSP chips, which effectively
provides a much greater number of available
time slots. This provides a greater potential
number of connections for routing, processing
and mixing audio signals within Pro Tools.
TDM (or TDM I)
(Legacy Pro Tools MIX-Series Systems Only)
The TDM I architecture (used with Pro Tools|24
MIX systems) has a single bus where every DSP
within the entire TDM system shares a single
pool of 256 time slots. In practice, this means,
for example, that if an audio track has a Reverb
One plug-in insert, one of the 256 time slots is
used to connect the Engine DSP on one chip to
the Reverb One plug-in DSP on another chip.
The TDM time slot used comes from the pool of
256 time slots, and is unavailable for use by the
rest of the system.
Time slot usage on Pro Tools|24 MIX hardware
1
slot 1 (used)
slot 2 (used)
DSP
1
DSP
2
DSP
3
Engine Mixer ReverbOne
256