Preparing the Audio for Encoding - A Practical Guide to Video and Audio Compression - page 600

you might perform the synchronization before the cropping. You need to check that the
audio is in sync at the start and the end and in the middle of an insert. There are so many
ways that the video and audio might have been worked on that you'll have to work some
of this out for yourself. Playing through the whole piece if the video and audio were cre-
ated separately should reveal any sync problems that you need to correct. Be careful that
in synchronizing one section you don't accidentally push out another. Using multiple
tracks in the editor can help a lot and you'll need to down mix again later to finish it off.

37.4 Audio-Noise Removal (Hum, Rumble, and Hiss)

Leaving noise in the signal just wastes compression bit rate. The encoder is supposed to
be spending time compressing the audible sound that you intend to be heard. If it is
encoding the noise, it is wasting some of its capacity.
In this description, the noise removal is placed before the normalizing step.
However, normalizing the audio levels could bring up the noise levels in quiet passages.
You might revisit the noise-removal process again or place it after the normalization.
It is possible to get tools such as BIAS Sound Soap that will remove all this noise
at a single stroke. For the price of these products, it's a very cost-effective solution and
probably deals with most of the problems you'll have to solve.

37.4.1 Hum Cancelling

Use notch filters (Figure 37-1) to remove hum and cyclic sounds. Set the notching as tightly
as possible, around 50 Hz in Europe or 60 Hz in the United States. These are the mains

Notch frequency

Figure 37-1 Notch filter profile.

BIAS Sound Soap: http://www.bias-inc.com/soundsoap