Device and method for bandwidth extension for audio signals
View Patent ↗The purpose of the present invention is to more efficiently extend, using a low bit rate, the bandwidth of input signals having a harmonics structure, in order to obtain better audio quality. The present invention is installed in a device that extends bandwidth for audio signal encoding and decoding. This novel bandwidth extension encoding identifies a low-frequency spectrum component having the highest correlation to a high-frequency bandwidth signal among input signals, duplicates a high-frequency spectrum by energy adjustment of said component, and maintains the harmonic relationship between the low-frequency spectrum and the duplicated high-frequency spectrum by adjusting the spectral peak position of the duplicated high-frequency spectrum, on the basis of a harmonic frequency estimated from a composite low-frequency spectrum.
1. An audio signal decoding apparatus, comprising:
a demultiplexer that demultiplexes low-band encoding parameters, index information, and scale factor information from encoded information transmitted from an encoding apparatus that encodes an audio signal;
a low-band decoder that decodes the low-band encoding parameters to obtain a synthesized low frequency spectrum;
a spectrum replicator that replicates a high frequency subband spectrum based on the index information using the synthesized low frequency spectrum;
a spectrum envelope adjuster that adjusts an amplitude of the replicated high frequency subband spectrum using the scale factor information,
a harmonic frequency estimator that estimates a frequency of a harmonic component in the synthesized low frequency spectrum;
a harmonic frequency adjuster that adjusts a frequency of a harmonic component in the high frequency subband spectrum using the estimated harmonic frequency spectrum; and
an output that generates an output signal using the synthesized low frequency spectrum and the high frequency subband spectrum.
2. The audio signal decoding apparatus according to claim 1 ,
wherein the harmonic frequency estimator comprises:
a splitter that splits a preselected portion of the synthesized low frequency spectrum into a predetermined number of blocks;
a spectral peak identifier that determines a spectrum (spectral peak) having a maximum amplitude in each block and a frequency of the spectral peak;
a spacing calculator that calculates spacing between the identified spectral peak frequencies; and
a harmonic frequency calculator that calculates the harmonic frequency using the spacing between the identified spectral peak frequencies.
3. The audio signal decoding apparatus according to claim 1 ,
wherein the harmonic frequency estimator comprises:
a spectral peak identifier that identifies a spectrum having a maximum absolute value of an amplitude at the preselected portion of the synthesized low frequency spectrum and spectra which are positioned at substantially regular spacing from the spectrum on a frequency axis and which have an amplitude whose absolute value is equal to or larger than a predetermined threshold;
a spacing calculator that calculates the spacing between frequencies of the identified spectral peaks; and
a harmonic frequency calculator that calculates the harmonic frequency using the spacing between the frequencies of the identified spectra.
4. The audio signal decoding apparatus according to claim 2 ,
wherein the adjuster comprises:
a low frequency spectral peak identifier that identifies a highest frequency of spectral peaks in the synthesized low frequency spectrum;
a high frequency spectral peak identifier that identifies a plurality of spectral peak frequencies in the replicated high frequency subband spectrum; and
a second adjuster that uses, as a reference, the highest frequency of the spectral peaks in the synthesized low frequency spectrum for adjusting the plurality of spectral peak frequencies so that the spacing between the plurality of spectral peak frequencies is equal to the estimated harmonic frequency.
5. An audio signal encoding apparatus, comprising:
a low-band encoder that encodes a low frequency audio signal and outputs synthesized low frequency spectrum, and generates low-band parameters and outputs the parameters;
a subband splitter that transforms a high frequency audio signal to a frequency spectrum and split a high frequency spectrum into a plurality of high frequency subbands, the high frequency spectrum being obtained using time-frequency transformed an input audio signal;
a searcher that identifies the most correlated portion from the synthesized low frequency spectrum for each of the high frequency subbands and outputs the identification result as index information;
a scale factor estimator that estimates an energy scale factor between each of the high frequency subbands and the most correlated portion identified from the synthesized low frequency spectrum and outputs the scale factor as scale factor information;
a harmonic frequency estimator that estimates and outputs a harmonic frequency of the synthesized low frequency spectrum and a harmonic frequency of the transformed input audio signal; and
a multiplexer that multiplexes the low-band parameter, the index information, and the scale factor information.
6. An audio signal decoding method, comprising:
demultiplexing low-band encoding parameters, index information, and scale factor information from encoded information transmitted from an encoding apparatus that encodes an audio signal;
decoding the low-band encoding parameters to obtain a synthesized low frequency spectrum;
replicating a high frequency subband spectrum based on the index information using the synthesized low frequency spectrum;
adjusting an amplitude of the replicated high frequency subband spectrum using the scale factor information,
estimating a frequency of a harmonic component in the synthesized low frequency spectrum;
adjusting a frequency of a harmonic component in the high frequency subband spectrum using the estimated harmonic frequency spectrum; and
generating an output signal using the synthesized low frequency spectrum and the high frequency subband spectrum.
7. An audio signal encoding method, comprising:
encoding a low frequency audio signal and outputs synthesized low frequency spectrum, and generates low-band parameters and outputs the parameters;
transforming a high frequency audio signal to a frequency spectrum;
splitting a high frequency spectrum into a plurality of high frequency subbands, the high frequency spectrum being obtained using time-frequency transformed an input audio signal;
identifying the most correlated portion from the synthesized low frequency spectrum for each of the high frequency subbands;
outputting the identification result as index information;
estimating an energy scale factor between each of the high frequency subbands and the most correlated portion identified from the synthesized low frequency spectrum and outputs the scale factor as scale factor information;
estimating a harmonic frequency of the synthesized low frequency spectrum and a harmonic frequency of the transformed input audio signal; and
multiplexing the low-band parameter, the index information, and the scale factor information.