Other Digital Audio Devices 621that was developed from the earlier “ silver disc ” was quite capable of achieving tight
packing of data, suffi cient for the needs of audio.
Data compression was, by that time, fairly well developed, but only for computer data,
and by the start of the 1980s several systems were in use. Any form of compression for
audio use had to be standardized so that it would be as universal as the compact cassette
and the CD, and in 1987 the standardizing institutes started to work on a project known as
EUREKA, with the aim of developing an algorithm (a procedure for manipulating data)
for video and audio compression. This has become the standard known as ISO MPEG
Audio Layer-3. The letters MPEG stand for Moving Picture Expert Group, because the
main aim of the project was to fi nd a way of tightly compressing digital data that could
eventually allow a moving picture to be contained in a CD, even though the CD as used
for audio was not of adequate capacity (see 21.9 DVD).
As far as audio signals are concerned, the standard CD system uses 16-bit samples that
are recorded at a sampling rate of more than twice the actual audio bandwidth, typically
44 kHz. Without any compression, this requires about 8.8 Mbytes of data per minute of
playing time. The MPEG coding system for audio allows this to be compressed by a factor
of 12, without losing perceptible sound quality. If a small reduction in quality is allowable,
then factors of 24 or more can be used. Even with such high compression ratios the sound
quality is still better than can be achieved by reducing either the sampling rate or the
number of bits per sample. This is because MPEG operates by what are termed perceptual
coding techniques, meaning that the system is based on how the human ear perceives sound.
The MPEG-1 Layer III algorithm is based on removing data relating to frequencies that
the human ear cannot cope with. Taking away sounds that you cannot hear will greatly
reduce the amount of data required, but the system is lossy, in the sense that the removed
data cannot be reinstated. The compression systems used for computer programs, by
contrast, cannot be lossy because every data bit is important; there is no unperceived
data. Compressing other computer data, notably pictures, can be very lossy, so that the
JPEG (Joint Photographic Expert Group) form of compression can achieve even higher
compression ratios.
The two features of human hearing that MPEG exploits are its nonlinearity and the
adaptive threshold of hearing. The threshold of hearing is defi ned as the level below
which a sound is not heard. This is not a fi xed level; it depends on the frequency of the
sound and varies even more from one person to another. Maximum sensitivity occurs in