Specifications

Digidesign Plug-Ins Guide62
How Broadband Noise Reduction
Works
The Broadband Noise Reduction module uses a
proprietary technique called Dynamic Audio Sig-
nal Modeling
™
to intelligently subtract the noise
from the digital audio file. Noise is removed
with multiple downward expanders that lin-
early decrease the gain of a signal as its level
falls.
Creating a Noise Signature
The first step in performing broadband noise re-
duction is to create what is called a noise signa-
ture by selecting and analyzing an example of
the noise within the source material. Using this
noise signature, a noise contour line is created
which is used to define the thresholds for the
downward expanders that will perform the
broadband noise reduction. The noise contour
represents an editable division between the
noise and non-noise audio signals.
At the same time, DINR also creates a model of
what the non-noise audio signal looks like.
DINR then attempts to pull apart these two
models, separating the bad from the good—the
noise from the desired audio. The noise portion
can then be reduced or eliminated.
The noise reduction itself is achieved through
the use of multiple downward expanders. The
threshold of these expanders is set so that the
noise signal will fall below them and be de-
creased while the desired audio signal will re-
main above them, untouched.
The Contour Line
Once the signal level has fallen below the speci-
fied Contour Line (which represents BNR’s
threshold), the downward expanders are acti-
vated and decrease the gain of the signal as its
level falls. Over five hundred individual down-
ward expanders are used linearly across the au-
dio spectrum to reduce the effects of unwanted
noise.
Psychoacoustic Effects of Noise
Reduction
One of the psychoacoustic effects associated
with broadband noise reduction is that listeners
often perceive the loss of noise as a loss of high
frequencies. This occurs because the noise in the
higher frequency ranges fools the ear into think-
ing the original signal has a great deal of energy
in that range. Consequently, when the noise is
removed it feels as if there has been a loss of
high frequency signal. DINR’s high-shelf EQ is
useful for compensating for this effect. See
“High-Shelf EQ” on page 64.
Limitations of Noise Reduction
It is important to understand that there is a cer-
tain amount of trade-off inherent in any type of
noise reduction system. Implementing noise re-
duction means that you have to choose the best
balance between the following three things:
◆ The amount of noise removed from the signal
◆ The amount of signal removed from the signal
◆ The number of artifacts added to the signal