1. Field of the Invention
The present general inventive concept relates to an audio signal coding and/or decoding system, and more particularly, to a method and apparatus to extract an important spectral component of an audio signal and a method and apparatus to code and decode a low bit-rate audio signal using the same.
2. Description of the Related Art
“MPEG (Moving Picture Experts Group) audio” is an ISO/IEC standard for high-quality high-performance stereo coding. The MPEG audio is standardized together with moving picture coding in accordance with ISO/IEC SC29/WG11 of MPEG. For the MPEG audio, sub-band coding (band division coding) based on 32 bands and modified discrete cosine transform (MDCT) are used for compression, and in particularly, a high performance compression is performed by using psychopathic characteristics. The MPEG audio can implement a high quality of sound compared to a conventional compression coding scheme.
In order to compress audio signals with a high performance, the MPEG audio utilizes a “perceptual coding” compression scheme in which detailed low sensitive information is eliminated by using sensitive characteristics of human beings sensing audible signals, to reduce a code amount of the audio signals.
In addition, in the MPEG audio, a minimum audible limit and a masking property of a silent period are mainly used for the perceptual coding using an auditory psychopathic characteristic. The minimum audible limit of a silent period is a minimum level of sound which can be perceived by auditory sense. The minimum audible limit is related to a limit of noise which can be perceived by the auditory sense in the silent period. The minimum audible limit varies according to frequencies of sound. At some frequencies, sound higher than the minimum audible limit may be audible, but at other frequencies, sound lower than the minimum audible limit may not be audible. In addition, a sensing limit of a specific sound may varies greatly according to other sounds which are heard together with the specific sound. This is called “masking effect.” A width of a frequency at which the masking effect occurs is called a critical band. In order to efficiently use the auditory psychopathic characteristics such as the critical band, it is important to decompose the sound signal into spectral components. For the reason, the band is divided into 32 sub-bands, and then, the sub-band coding is performed. In addition, in the MPEG audio, filter banks are used to eliminate aliasing noises of the 32 sub-bands.
The MPEG audio includes bit allocation and quantization using filter banks and a psychoacoustic model. Coefficients generated from the MDCT are allocated with optimal quantization bits and compressed by using a psychoacoustic model 2. The psychoacoustic model 2 for allocating the optimal bits evaluates the masking effect based on FFT by using spreading functions. Therefore, a relatively large amount of complexity is required.
In general, for the compression of the audio signals with a low bit-rate (32 kbps or less), the number of bits which can be allocated to the signals is insufficient for quantization of all spectral components of the audio signal and lossless coding thereof. Therefore, there is a need for extraction of perceptively importance spectral components (ISCs) and quantization and lossless coding thereof.