Preparing the Audio for Encoding - A Practical Guide to Video and Audio Compression - page 606

Clipping region

0Db

-3Db

Correct level

Peak

Input level

Figure 37-8 Clipping the audio.

This might introduce audible noise that was too quiet to hear previously. Use of spe-
cial noise-gated gain filters and compressor limiters may yield a sufficiently loud output
without bringing up the background noise. It all depends on what has been recorded and
the quality of the original source material. The argument applies equally well to video and
audio content.
This may become quite complicated when you are processing surround sound with
5 or 6 tracks of audio that must remain mixed to the right level, must not be allowed to go
out of sync or out of phase, and may contain sounds of varying volume. That peak-level
setting is an aggregate. Don't check just one channel; there may be a louder sound in one
of the others.
In the same way that video gets crushed if you set the white level too high, audio
squares off the tops of the wave shapes when clipping occurs. The effect of this clipping
sounds horrible and can damage the speaker drivers on the system playing the audio.
It is worthwhile to use a specialized tool for normalizing the audio.

37.8 What Is a VU Meter?

You monitor levels using VU meters on your audio console. They may be small mechani-
cal meters or a set of LEDs arranged in a bar. Figure 37-9 shows you what to look for on
your audio console.