Audio Engineering

Other Digital Audio Devices 623

Layer I is used in home recording systems and for solid-state audio (sound that has been
recorded on chip memory, used for automated voices, etc.).

Layer II offers more compression than layer I and is used for digital audio broadcasting,
television, telecommunications, and multimedia work. The bit rates that can be used
range from 32 to 192 Kbit/s for mono and from 64 to 384 Kbit/s for stereo. The highest
quality, approaching CD levels, is obtained using about 192–256 Kbit/s per stereo pair of
channels. The precise fi gure depends on how complex an encoder is used. In general, the
encoder is from two to four times more complex than the level I encoder, but the decoder
need be only about 25% more complex. MPEG level II is used in applications such as
CD-i full-motion video, video CD, solid state audio, disc storage and editing, DAB,
DVD, cable and satellite radio, cable and satellite TV, ISDN links, and fi lm sound tracks.

Layer III offers even more compression and is used for the most demanding applications
for narrow band telecommunications and other specialized professional audio areas
of audio work. It has found much more use as a compression system for MP3 fi les (see
21.5 MP3).

MPEG-1 is intended to be fl exible in use, so that a wide range of bit rates from 32 to 320
Kbit/s can be used, with a low sampling frequency (LSF) of 8 Kbit/s added later. Layer
III allows the use of a variable bit rate, with the fi gure in the header taken as the average.
Decoders for layers I and II need not support this feature, but most do.

Filter

bank

Input Channel

Psycho-

acoustic

model

Multi-

plexer

Q

Q

Q

Figure 21.2 : Block diagram for MPEG encoding.