Preparing the Audio for Encoding - A Practical Guide to Video and Audio Compression - page 602

37.4.4 Plug-In Tools

Try to obtain a plug-in for your video-editing software that accomplishes everything you
require. The Waves plug-ins offer a variety of modules that compress, limit, de-noise, and
perform just about any audio processing you might want to do to a sound track. They
work on Mac OS and Windows, so they are available regardless of the platform you are
running your workflow on.

37.4.5 Cyclic Noise

There may be other cyclic noises, such as motors or other ambient sounds, that have been
picked up inadvertently. These might repeat at very low frequencies and may be hard to
remove. But if you have the time and if the sound recording is valuable enough, sampling
some silence from the same source as the original recording and trying to establish what
the noise floor (Figure 37-4) is like without any audio superimposed on it gives you a ref-
erence for designing a more sophisticated de-noiser.
In an ideal situation, the loudest noise source will still be quieter than the quietest pro-
gram material. If that is the case, then a simple noise gate can be applied. Often the noise level
is higher than the quietest passages, so some intelligent level tracking needs to take place.
You can also see that the program-signal levels are much larger in magnitude than
the noise. This illustrates the signal-to-noise ratio.

37.4.6 Noise Gating

Applying a noise gate at the input stage is a good idea. This is when the analog signal is
first digitized. If the gating is processed in the digital domain, it can be applied far more
carefully and selectively. Figure 37-5 shows how this affects a waveform.

Figure 37-4 Noise floor.