Digital Audio Fundamentals 431Channel coding is also needed to broadcast digital signals where shaping of the spectrum
is an obvious requirement to avoid interference with other services.
14.9 Audio Compression...............................................................................................
In its native form, high-quality digital audio requires a high data rate, which may be
excessive for certain applications. One approach to the problem is to use compression,
which reduces that rate signifi cantly with a moderate loss of subjective quality. Because
the human hearing system is not equally sensitive to all frequencies, some coding gain
can be obtained using fewer bits to describe the frequencies that are less audible.
While compression may achieve considerable reduction in bit rate, it must be appreciated
that compression systems reintroduce the generation loss of the analog domain to digital
systems.
One of the most popular compression standards for audio and video is known as MPEG.
Figure 14.15 shows that the output of a single MPEG compressor is called an elementary
Figure 14.15 : The bit stream types of MPEG-2. See the text for details.