Audio coders, aimed for generic audio, i.e. all types of speech and music signals, are perceptual audio coders, rather than so-called 'waveform coders'. In a perceptual audio coder, the codec
does not attempt to retain the input signal exactly after encoding and decoding, rather its goal is to ensure that the output signal sounds the same to a human listener. The primary psychoacoustic effect that the perceptual audio coder uses is called 'auditory masking', where parts of a signal are not audible due to the function of the human auditory system. The parts of the signal that are masked are commonly called 'irrelevant', as opposed to parts of the signal that are removed by a source coder (lossless or lossy), which are termed 'redundant
In order to remove this irrelevancy, the encoder contains a psychoacoustic model. This psychoacoustic model analyses the input signals within consecutive time blocks and determines for each block the spectral components of the input audio signal by applying a frequency transform. Then it models the masking properties of the human auditory system, and estimates the just noticeable noise-level, sometimes called the threshold of masking.
In parallel, the input signal is fed through a time-to-frequency mapping, resulting in spectrum components for subsequent coding. In its quantisation and coding stage, the encoder tries to allocate the available number of data bits in a way that meets both the bitrate and masking requirements. The information on how the bits are distributed over the spectrum is contained in the bitstream as side information.
The decoder is much less complex, because it does not require a psychoacoustic model
and bit allocation procedure. Its only task is to reconstruct an audio signal from the coded spectral components and associated side information.
Within the professional and consumer market, four fields of applications can be identified: broadcasting, storage, multimedia and telecommunication. This variety of applications is possible because of the wide range of bitrates and the numerous configurations, allowed within the MPEG Audio standard.
Some of the most important applications are:
Consumer Recording (DCC)
Disc based storage (CD-i
Disc based Editing, audio broadcasting station automation
Solid State Storage Audio
and satellite TV
(e.g. DVB, USSB
Digital Audio Broadcasting (e.g. ADR, DAB, US-Digital Radio, Worldspace Radio)
Computer based Multimedia
Stand-alone electronic information systems
by Leonardo Chiariglione
reproduced with permission.