Setup guide

Pro Tools|HD Setup Guide106
48-Bit Mixing Precision
The Pro Tools mixer plug-ins use a register area
inside of the DSPs on the hardware to hold a full
48-bits of precision when mixing signals to-
gether. This allows a fader to be lowered in level
without any loss of resolution. (Even if the fader
is lowered almost to the bottom, all 24 of the
original 24 bits of the signal are preserved.)
Mixer Headroom
Use of 48-bit precision when mixing allows the
mixer to be designed to provide a very large
amount of headroom, which allows the faders
on the Pro Tools mixer to be placed in the
“sweet spot” position without clipping.
Mixer plug-ins provided with Pro Tools|HD sys-
tems provide 48-bit precision with 48 dB of
headroom. This means that on the “input” side
of the bus (where signals are summed together),
signals can never clip (even if channel faders are
set to a full +12 dB of gain).
However, the “output” side of the summing
mixer (where the signal is sent in the 24-bit
world of a digital output or onto the TDM bus)
can clip. You can use a Master fader (which does
not “cost” any DSP) to scale the output level of
any mix summing point (a bus or physical out-
put). The master fader's meters will tell you if
you are clipping the mix bus, and the fader can
be used to safely scale the level to avoid clip-
ping, with no loss of quality.
HD card (for PCI), default Standard mixers
Mixer
Sample Rate
(kHz)
Usage per
Chip
Stereo 44.1, 48 68x2
88.2, 96 44x2
176.4, 192 16x2
Surround, 5.1 44.1, 48 22x6
88.2, 96 12x6
176.4, 192 4x6
Surround, 7.1 44.1, 48 16x8
88.2, 96 8x8
176.4, 192 2x8
HD card (for PCI), Dithered mixers
Mixer
Sample Rate
(kHz)
Usage per
Chip
Stereo
Dithered
44.1, 48 56x2
88.2, 96 34x2
176.4, 192 14x2
Surround
Dithered, 5.1
44.1, 48 19x6
88.2, 96 10x6
176.4, 192 1x6
Surround
Dithered, 7.1
44.1, 48 15x8
88.2, 96 7x8
176.4, 192 not supported
If you are mixing larger numbers of signals
together, always use a master fader so that
you can monitor levels for the bus (using the
master fader meter), and to trim the result to
avoid clipping. Since Master faders cost no
DSP, there is no reason not to use them.