Chapter 7: HD I/O Calibration Mode Instructions 41
Chapter 7: HD I/O Calibration Mode
Instructions
Before you use Pro Tools | HD I/O, you may want
to calibrate its input and output levels to the level
of your mixing console.
The HD I/O has +4 dBu and –10 dBV inputs, and
+4 dBu outputs, each with its own trim pot for
proper calibration.
The HD I/O is factory-calibrated so that its +4 dBu
input operating level is set for 18 dB headroom
above +4 dBu (maximum input/output +22 dBu).
If you need to recalibrate your HD I/O or other
components of your studio, you can use the align-
ment procedure described in this chapter.
About Calibration
Calibrating levels on a digital recording device is
different from calibrating levels on an analog re-
cording device. Unlike analog devices, most digi-
tal devices do not have a standard “0 VU” level
setting that corresponds to nominal input and out-
put levels. Instead, with an interface such as the
HD I/O, the meters are calibrated in decibels below
peak or dBFS (dB full scale)—digital clipping
level.
Headroom
The concept of headroom is slightly different for
analog and digital devices.
Analog
Most analog devices allow for a certain
amount of headroom above 0 VU. If you send a
signal above 0 VU to an analog recorder, you still
have a margin of headroom, and if tape saturation
occurs, it does so fairly gracefully, giving the au-
dio a compressed sound that some find desirable.
Digital
Digital devices do not allow for signals that
exceed the dynamic range of the input or dBFS (dB
full scale). When a signal exceeds the maximum
input level for a digital device, clipping occurs,
causing digital distortion, which is harsh and usu-
ally undesirable.
The Calibration Process
Analog
To calibrate the input level of an analog
device to a mixing console’s output level, you
would typically send a 1 kHz tone at 0 VU from
the console to the analog deck and align the record-
ing deck’s meters to read 0 VU.
Digital
With a digital recording device such as the
HD I/O, in order to allow for headroom, you must
align a 0 VU tone from the console to a value less
than zero (or below dB full scale [–x dBFS]) on
the HD I/O, by exactly the amount of headroom
that you want.
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