Dialogue Editing

228 DAMAGE REPAIR
updated FFTs. The FFT of the noise sample is divided into 2000 (or so)
narrow-frequency bins, in which the noise is reduced to a formula. 9
When the signal is played through the processor it, too, is assessed in the
frequency domain. At each of the 2000-ish bins, the formula for the signal is
compared to that of the noise sample. If the match is suffi cient, the sound
within that bin is attenuated by a user-controlled amount. If there’s no correlation
between the noise formula and the incoming signal, no attenuation
occurs in that bin since the dissimilar signal is likely valid audio rather than
noise. This process is repeated for all of the frequency bins.
At this point there are usually control parameters for threshold and reduction.
As would be expected, threshold determines the sound level at which
processing begins; attenuation (or “reduction”) dictates what’s done within
each bin fl agged as “noise.” If these settings are too aggressive, you’ll hear
very obvious artifacts. Back off fi rst on the attenuation and then on the
threshold.
After noise reduction, the signal must be recorrelated into a “normal” time
domain sound. You usually have some control over this. “Sharpness” controls
the slope between adjacent bins. The steeper the slope during the recombination
process, the more effective the noise reduction but the “edgier” the
sound. If you set the sharpness too high, you’ll hear digital “swimming”
artifacts, often called “bird chirping.” There’s usually a control called “bandwidth”
that determines how much sharing occurs between adjacent bins
during the recorrelation. The higher the bandwidth, the warmer (but perhaps
less articulate) the sound.
Low-frequency (LF) cutoff, if available on your processor, isn’t what you
think: a high-pass fi lter. Rather, it dictates the frequency beneath which
there’s no processing. If you’re attacking traffi c, set the LF cutoff to zero. If all
you’re fi ghting is hiss, set it to 2000 Hz or higher and you won’t run the risk
of damaging anything in your audio source below that level. Many processors
have a high-frequency cutoff, which normally defaults to the Nyquist
frequency. Change this setting if you’re processing only low frequencies and
want to leave higher frequencies unaffected.
All broadband noise removers are shackled by the fact that as they aim for
greater resolution in the frequency domain resolution in the time domain
9 Not all broadband processors use FFT algorithms to transform sound into the
frequency domain. Wavelets, as well as other transforms, are also used in such calculations,
but for the sake of simplicity, we’ll stick with FFT.