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Chapter 11. General Audio Coding > Introduction to Time/Frequency Audio Coding

11.1. Introduction to Time/Frequency Audio Coding

The term T/F coder was chosen in MPEG-4 to refer to coders that adhere to the traditional paradigm of perceptual audio coding by coding a spectral (frequency domain) representation of the input signal rather than the time domain signal itself. This type of coding technology has made tremendous progress in the past 10 years and has become the coder type of choice for music distribution in broadcasting, over the Internet, and on other media. This may be explained by the fact that the T/F coder framework combines both redundancy reduction and exploitation of the potential provided by irrelevancy removal. Coding of a spectral representation is an efficient way of exploiting linear correlation between subsequent samples of an input signal (i.e., the signal's spectral unflatness) [JayN84]. Further redundancy removal can be achieved through entropy coding of the spectral coefficients. A further advantage of a spectral representation of the input signal is that it makes possible good first-order modeling of the limits of human auditory perception. Once the threshold of perceptibility (audibility) is estimated, the precision of the coding process can be easily adapted to match that threshold by adjusting the quantization distortion of the corresponding spectral coefficients.

Although the development of T/F based coding dates back all the way to the 1970s, an important step in its evolution was marked by the development of the MPEG-1 and MPEG-2 Audio standards [MPEG1-3, MPEG2-3]; these standards prepared the way for the large-scale application of this technology. The upper part of Figure 11.1 depicts the basic block diagram of a generic monophonic T/F audio coder:


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