MPEG-4 Audio

Adaptive filter banks. In the basic configuration, all filter banks described above feature a time/frequency decomposition which is constant over time. As mentioned above, there are possibilities to switch the characteristics of a filter bank, going from one time/frequency decomposition to another one. We explain the basic principle using the example of MPEG Audio Layer 3 ... 

Brandenburg and Bosi, 1997] Brandenburg, K. and Bosi, M. (1997). Overview of MPEG audio Current and future standards for low bit-rate audio coding. J. Audio Eng. Soc., 45(l/2) 4 - 21. 

The filter bank used in the MPEG/Audio coding system will be used as an example. A 511 tap prototype filter is used. Figure 2.7 shows the prototype filter (window function). It has been optimized for a very steep filter response and a stop band attenuation of better than 96 dB. Figure 2.8 shows the frequency response of the filter bank. In addition to the attenuation requirements it was designed as a reasonable tradeoff between time behavior and frequency localization [Dehery, 1991],... 

As discussed above, the model of hearing built into a perceptual coding system forms the heart of the algorithm. A lot of systems (like MPEG Audio, see below) just define the transmission format, thus allowing changes and improvements to the perceptual model even after a standard is fixed and a lot of decoders are deployed at the customers. 

The frequency domain representation of the data is calculated via an FFT after applying a Hann window with a window length of 1024 samples. The calculation is done with a shift length equal to the block structure of the coding system. As described below, the shift length is 576 samples for Layer 3 of the ISO/MPEG-Audio system. 

Fuchs, 1995] Fuchs, H. (1995). Improving MPEG audio coding by backward adaptive linear stereo prediction. In Proc. of the 99th. AES-Convention. Preprint 4086. 

Non-uniform scalar quantization. While usually non-uniform scalar quantization is applied to reduce the mean squared quantization errors like in the well known MAX quantizer, another possibility is to implement some default noise shaping via the quantizer step size. This is explained using the example of the quantization formula for MPEG Layer 3 or MPEG-2 Advanced Audio Coding ... 

Johnston et al., 1996] Johnston, J. D., Herre, J., Davis, M., and Gbur, U. (1996). MPEG-2 NBC audio - stereo and multichannel coding methods. In Proc. of the 101st AES-Convention. Preprint 4383. 

The optimization of a freq, Ot time and y is performed using the subjective audio quality database that resulted from the ISO/MPEG 1990 audio codec test [ISO90, 1990]. The optimization used the standard error of the estimated MOS from a third order regression line fitted through the PAQM, MOS datapoints. The optimization was carried out by minimization of the standard error of the estimated MOS as a function of O.freq, (X time, Y... 

Figure 1.8 Relation between the mean opinion score and the perceptual audio quality measure (PAQM) for the 50 items of the ISO/MPEG 1990 codec test [ISO90, 1990] in loudspeaker presentation.

MPEG-1 perceptual model 2. As an example for actual perceptual models we give a description of the perceptual model 2 as it is described in the informative annex of MPEG-1 audio [MPEG, 1992],... 

Most systems apply uniform quantization. One exception to this rule is the application of non-uniform quantization with a power law in MPEG-1 and MPEG-2 audio. 

Vector quantization. In vector quantization, not the individual filter bank output samples are quantized, but n-tuples of values. This technique is used in most current speech and video coding techniques. Recently, vector quantization has been applied in a scheme called TWIN-VQ ([Iwakami et al., 1995]). This system has been proposed for MPEG-4 audio coding (see [MPEG, 1997b]). 

Since 1988 ISO/IEC JTC1/SC29 WG11, called MPEG (Moving Pictures Experts Group) undertakes the standardization of compression techniques for video and audio. Three low bit-rate audio coding standards have been completed ... 

MPEG-2 Audio [MPEG, 1994a] consists of two extensions to MPEG-1 ... 

Both extensions do not introduce new coding algorithms over MPEG-1 Audio. 

MPEG-2 Audio. MPEG-2 audio coding contains two large additions to the MPEG-1 audio standard ... 

ISO/IEC IS 13818-7 is called MPEG-2 Advanced Audio Coding and covers a new, non backwards compatible audio coding system for flexible channel configurations including stereo and multichannel services. 

The newest coding system which is reported here is currently still under development. MPEG-4 audio, planned for completion in late 1998, will actually consist of a family of coding algorithms targeted for different bit-rates and different applications. 

Figure 2.21 shows a block diagram of the configuration for scaleability of the planned MPEG-4 audio standard. 

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