DVD Demystified

102
Birds Revisited: Understanding
Audio Compression
Chapter 3
Audio takes up much less space than video, but uncompressed audio coupled
with compressed video uses up a large percentage of the available
bandwidth. Compressing the audio can result in a small loss of quality, but
if the resulting space is used instead for video, it may improve the video
quality significantly. In essence, reducing both the audio and the video is
more effective. Usually video is compressed more than audio, since the ear
is more sensitive to detail loss than the eye.
Just as MPEG compression takes advantage of characteristics of the
human eye, modern audio compression relies on detailed understanding of
the human ear. This is called psychoacoustic or perceptual coding.
Picture again your telephone conversation with a friend. Imagine that
your friend lives near an airport, so that when a plane takes off, your friend
cannot hear you over the sound of the airplane. In a situation like this, you
quickly learn to stop talking when a plane is taking off, since your friend
will not hear you. The airplane has masked the sound of your voice. At the
opposite end of the loudness spectrum from airplane noise is background
noise, such as a ticking clock. While you are speaking, your friend cannot
hear the clock, but if you stop, then the background noise is no longer
masked.
The hairs in your inner ear are sensitive to sound pressure at different
frequencies (pitches). When stimulated by a loud sound, they are incapable
of sensing softer sounds at the same pitch. Because the hairs for similar frequencies
are near each other, a stimulated audio receptor nerve will interfere
with nearby receptors and cause them to be less sensitive. This is called
frequency masking.
Human hearing ranges roughly from low frequencies of 20 Hz to high
frequencies of 20,000 Hz (20 kHz). The ear is most sensitive to the frequency
range from about 2 to 5 kHz, which corresponds to the range of the
human voice. Because aural sensitivity varies in a nonlinear fashion,
sounds at some frequencies mask more neighboring sounds than at other
frequencies. Experiments have established certain critical bands of varying
size that correspond to the masking function of human hearing (Figure 3.6).
Another characteristic of the human audio sensory system is that sounds
cannot be sensed when they fall below a certain loudness (or amplitude).
This sensitivity threshold, as with everything else, is not linear. In other
words, the threshold is at louder or softer points at different frequencies.
The overall threshold varies a little from person to person—some people
have better hearing than others. The threshold of hearing is adaptive; the