Image Processing Reference
Original signal level
Figure 37-5 Threshold noise gating.
Operating System Support
Modern operating systems support a lot of audio-processing capabilities, and in the case
of Mac OS 10.4, there are sample accurate-audio-processing frameworks that would allow
you to construct a sample player that could loop a repeating background noise track and
then add an out-of-phase copy to your recording. This would neutralize the noise without
damaging the audio you want to keep. Designing software that does this kind of thing is
a specialist art, and there are commercially available tools that do it very efficiently.
Applying some pre-emphasis to the audio may help it make the best of the target platform.
Music producers do this with pop songs when they equalize them so they sound great on
small speakers. They do this because most of the time, people are listening in the car or on
portable radios—with small speakers.
Modern codecs such as AAC
use a technique called spectral band replication that
puts back some information at the decoding end, based on what it finds in the encoded bit
stream. The output is therefore not an exact copy of the input material but is a simulacrum
of it that the human ear finds pleasing.
Some of these operations increase noise components that were previously masked.
To avoid this problem, be careful where you place this pre-emphasis operator in your
workflow. It should not affect the audio being routed to every platform. Instead, you
should apply this in a platform-specific way in the workflow path that is unique to each
target device. Pre-emphasis is not always necessary if the destination playback system has
its own equalization (EQ) capability. Also, be aware that pre-emphasizing the bass and
outputting it on a high-powered system will cause it to boom and growl somewhat.