624 Chapter 21
Table 21.1 shows the relative complexity of encoding and decoding for the three levels of
MPEG-1. The encoding process is always more complex, but the relative complexity of
the decoder is less.
MPEG-1 coding can be applied to mono or stereo signals, and the stereo system makes
use of joint stereo coding, a system that achieves further compression by seeking out
redundancy between the two channels of a stereo signal. The system supports four modes:
Table 21.1 Comparing Complexity of Circuitry for MPEG-1 Levels
Complexity
Layer Encoder Decoder
I 1.5 to 3 1.0
II 2 to 4 1.25
III 7.5 or more 2.5mono stereo joint stereo
(intensity stereo or mid/side stereo) dual channel (two independent channels,
e.g., for two languages)When the digital signal has been encoded, it is divided into blocks of 384 samples
(layer I) or 1152 samples (layers II and III) to form the unit MPEG-1 frame. A complete
MPEG-1 audio stream consists of a set of consecutive frames, with each frame consisting
of a header and encoded sound data. The header of a frame contains general information
such as the MPEG layer, the sampling frequency, the number of channels, whether the
frame is CRC protected, whether the sound is an original, and so on. Each audio frame
uses a separate header so as to simplify synchronization and bit stream editing, even if
much of the information is repeated and hence redundant. A layer III frame can achieve
further compression by distributing its encoded sound data over several other consecutive
frames if those frames do not require all of their bits.
One important point about all digital audio systems is that the analogue concept of S/N
ratio is no longer relevant, and so far no replacement has been found. If we try to measure
S/N in any of the ways that work perfectly well for analogue signals, the results are
widely variable and have no correspondence with the signal as heard by the listener.