Audio Engineering

586 Chapter 19

knew the function to be transformed was only made up of cosines because that would cut
down the maths by half. This is exactly what is done in the DCT. A sequence of samples
from the incoming waveform is stored and refl ected about an origin. Then one-half of the
Fourier transform performed. When the waveform is inverse transformed, the front half of
the waveform is simple ignored, revealing the original structure.

19.5 MPEG Layer 2 Audio Coding (MUSICAM) ........................................................

The MPEG layer 2 algorithm is the preferred algorithm for European DTV and includes
a number of simple enhancements of layer 1 (or PASC). Layer 2 was originally adopted
as the transmission coding standard for the European digital radio project (Digital Audio
Broadcasting or DAB) where it was termed MUSICAM. The full range of bit rates for
each layer is supported, as are all three sampling frequencies, 32, 44.1, and 48 kHz. Note
that MPEG decoders are always backward compatible, that is, a layer 2 decoder can
decode layer 1 or layer 2 bit streams; however, a layer 2 decoder cannot decode a layer
3-encoded stream.

MPEG layer 2 coding improves compression performance by coding data in larger groups.
The layer 2 encoder forms frames of 3 by 12 by 32  1152 samples per audio channel.
Whereas layer 1 codes data in single groups of 12 samples for each subband, layer 2 codes
data in three groups of 12 samples for each subband. The encoder encodes with a unique
scale factor for each group of 12 samples only if necessary to avoid audible distortion.
The encoder shares scale factor values between two or all three groups when the values

1

0

angle

 1

Figure 19.2 : Cosine function.