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Pro Tools|HD User Guide92

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.

Note that clipping the “input” side of a mixer is

not a concern on Pro Tools|HD systems. It is vir-

tually impossible to clip the input of any

Pro Tools|HD mixer plug-in, because the 48 dB

of headroom provided prevents any possibility

of overload, even with a maximum number of

inputs being fed by full-code signals with fader

gains at maximum. While it is possible to clip

the “output” side of the mixer, you can safely

use a Master Fader to trim your mix bus back to

avoid clipping (by simply examining the Master

Fader meter for clipping, and pulling back the

fader until it disappears; make sure that any

plug-ins you may have on the Master Fader in-

serts are not the cause as well). The 48-bit preci-

sion of the mixer allows gain adjustment on the

Master Fader with no loss of data integrity or au-

dio quality, so there is no need to trim the indi-

vidual input faders back to avoid clipping.

Mixer Automatiion

Volume automation on all mixer versions is

near sample-accurate (as is pan automation on

the stereo versions). In addition, DAE provides

24-bit interpolated values between mix break-

points, which provides near “analog-like” reso-

lution. This process of interpolation means that

a smooth “data series” is created between any

two breakpoints that you specify in Pro Tools.

DAE calculates these smooth transitions on the

DSP hardware with 24-bit precision, which pro-

176.4 and 192 8x6

Surround

Dithered, 7.1

44.1 and 48 34x8

88.2 and 96 20x8

176.4 and 192 4x8

HD Accel card, Dithered mixers

Mixer

Sample Rate

(kHz)

Usage per Chip

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.

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