Method of coding 32-kb/s audio signals
First Claim
1. A method of coding digitized audio signals, comprising the steps of:
- dividing an audio signal, consisting of a continuous sequence of sample values, into successive blocks of equal length, and performing overlapping windowing;
transforming the blocks into complex Fourier coefficients by means of a discrete Fourier transform;
decomposing the complex Fourier coefficients into magnitude values and phase values;
quantizing the phase values with a linear quantization characteristic which becomes coarser going from low toward high frequencies;
combining the magnitude values into frequency bands which are oriented with regard to predetermined critical bands and become wider toward high frequencies;
fixing quantization levels for each frequency band, taking into account a frequency-dependent, absolute threshold of hearing, such that a frequency range lying below the respective frequency-dependent, absolute threshold of hearing is disregarded, and taking into account a relative threshold of hearing, such that frequency ranges in the immediate vicinity of frequencies with large magnitude values are taken into account to a reduced extent;
fixing a number of sub-bands consisting of single values of the frequency bands, and determining the greatest single magnitude value of each sub-band, andperforming variable quantization of each frequency band by quantizing the greatest magnitude value of a sub-band with a maximum available number of bits and all other magnitude values with a smaller number of bits.
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Abstract
Coding digitized audio signals includes dividing an audio signal, consisting of a continuous sequence of sample values, into successive blocks of equal length, and performing overlapping windowing. The blocks are transformed into complex Fourier coefficients by means of a discrete Fourier transform, which complex Fourier coefficients are decomposed into magnitude values and phase values. The phase values are quantized with a linear quantization characteristic which becomes coarser going from low toward high frequencies. The magnitude values are combined into frequency bands which are oriented with regard to predetermined critical bands and become wider toward high frequencies. Quantization levels for each frequency band are fixed, taking into account a lower, frequency-dependent, absolute threshold of hearing, such that a range lying below the respective threshold of hearing is disregarded, and taking into account a relative threshold of hearing, such that ranges in the neighborhood of frequencies with large magnitude values are taken into account to a reduced extent. A number of sub-bands consisting of single values of the frequency bands are fixed, and the greatest single magnitude value of each sub-band is determined. Variable quantization of each frequency band is performed by quantizing the greatest magnitude value of a sub-band with a maximum available number of bits and all other magnitude values with a smaller number of bits.
27 Citations
9 Claims
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1. A method of coding digitized audio signals, comprising the steps of:
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dividing an audio signal, consisting of a continuous sequence of sample values, into successive blocks of equal length, and performing overlapping windowing; transforming the blocks into complex Fourier coefficients by means of a discrete Fourier transform; decomposing the complex Fourier coefficients into magnitude values and phase values; quantizing the phase values with a linear quantization characteristic which becomes coarser going from low toward high frequencies; combining the magnitude values into frequency bands which are oriented with regard to predetermined critical bands and become wider toward high frequencies; fixing quantization levels for each frequency band, taking into account a frequency-dependent, absolute threshold of hearing, such that a frequency range lying below the respective frequency-dependent, absolute threshold of hearing is disregarded, and taking into account a relative threshold of hearing, such that frequency ranges in the immediate vicinity of frequencies with large magnitude values are taken into account to a reduced extent; fixing a number of sub-bands consisting of single values of the frequency bands, and determining the greatest single magnitude value of each sub-band, and performing variable quantization of each frequency band by quantizing the greatest magnitude value of a sub-band with a maximum available number of bits and all other magnitude values with a smaller number of bits. - View Dependent Claims (2, 3, 4, 5)
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6. A method of coding digitized audio signals, comprising the steps of:
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dividing an audio signal, consisting of a continuous sequence of sample values, into successive blocks of equal length, and performing overlapping windowing; transforming the blocks into complex Fourier coefficients by means of a discrete Fourier transform; decomposing the complex Fourier coefficients into magnitude values and phase values; taking into account a relative threshold of hearing, such that those magnitude values of coefficients which lie in the immediate vicinity of frequency lines with large magnitude values are either fully taken into account or disregarded; dividing the magnitude values into frequency bands which are oriented with regard to critical bands and become wider toward high frequencies; fixing quantization levels for each frequency band, taking into account a frequency-dependent, absolute threshold of hearing, such that a range of frequencies lying below the respective frequency-dependent, absolute threshold of hearing is disregarded, and performing variable quantization of the magnitude and phase values with a quantization characteristic which becomes coarser going from low toward high frequencies, with the value zero being quantized for those magnitude values of coefficients which lie below the relative threshold of hearing or below the absolute threshold of hearing, no value being quantized for the associated phase values, whereby the bits not needed for said associated phase values are used for a more accurate quantization of the nonzero magnitude and phase values. - View Dependent Claims (7, 8, 9)
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Specification