The present disclosure relates to a coding technique and a decoding technique for improving the audio quality of audio signals, such as speech signals and music signals.

BACKGROUND OF THE INVENTION

A coding technique for compressing audio signals at a low bit rate is a technique essential to realize the effective use of radio waves and so on in mobile communication. Meanwhile, there has recently been an increasing desire to improve audio quality in telephone communication, and implementation of telephone communication services that produce a greater sensation of presence is anticipated. To implement such services, audio signals having a wide frequency band at a high bit rate have to be coded. However, this approach conflicts with the effective use of radio waves and frequency bands.

Now, an audio signal coding technique adopted by Standard G.719 (ITU-T Standard G.719, 2008), for example, is studied.

In Standard G.719, upon coding an audio signal, a frequency transform is performed on the audio signal, and predetermined bits are allocated to a spectrum obtained as a result of the frequency transform. Specifically, the spectrum is divided into sub-bands having predetermined frequency bandwidths, and a unit (a unit having a needed number of bits) used in quantization based on lattice vector quantization is allocated to each of the sub-bands in decreasing order of energy as follows.

(1) One unit is allocated to a sub-band having the largest energy among all of the sub-bands.

One bit is allocated per spectrum. Therefore, if the number of spectral samples in a sub-band is eight, for example, one unit contains eight bits (note that the maximum number of bits that can be allocated per spectrum is nine bits, and therefore, if the number of spectral samples in a sub-frame is eight, up to 72 bits can be allocated).

(2) The quantized sub-band energy of the sub-band to which one unit has been allocated is decreased by two levels (6 dB). If the number of bits allocated to the sub-band to which one unit has been allocated exceeds the maximum value (nine bits), the sub-band is excluded from quantization in the succeeding loops.

Back to (1) above, the same process is repeated.

FIG.6illustrates the sub-band energy of each sub-band. The horizontal axis represents the frequency, and the vertical axis represents the amplitude on a logarithmic scale. In the figure, the sub-band energy of each sub-band is represented by a horizontal line instead of a point. The length of each horizontal line represents the frequency bandwidth of each sub-band.

FIG.7andFIG.8are diagrams illustrating examples of the results of bit allocation to each sub-band in a case of using a coding method specified in Standard G.719. In the figures, the horizontal axis represents the frequency, and the vertical axis represents the allocated number of bits.FIG.7illustrates a case of a bit rate of 128 kbit/s, andFIG.8illustrates a case of a bit rate of 64 kbit/s.

In the case of 128 kbit/s, an abundant bit budget is available for allocation, and therefore, nine bits, which is the maximum value, can be allocated to a large number of sub-bands (spectra), and the quality of audio signals can be maintained at a high level.

In contrast, in the case of 64 kbit/s, no sub-band is allocated nine bits, which is the maximum value, but every sub-band is allocated some bits. Accordingly, it is considered that degradation in the quality of audio signals can be suppressed and the effective use of radio waves and frequency bands can be realized.

However, the effective use of radio waves and frequency bands needs to be further promoted. Here, in a case of coding an audio signal having a sampling frequency of about 32 kHz at a low bit rate of 20 kbps/s or less by using the above-described method adopted by Standard G.719, it is not possible to reserve a unit (a number of bits) used in quantization of all sub-bands, which is a problem.

FIG.9is a diagram illustrating an example of the result of bit allocation to each sub-band in a case of using the coding method specified in Standard G.719 at 20 kbit/s. As illustrated, bit allocation fails not only in a high-frequency range but also, depending on the situation, in a low-frequency range, which is essential for hearing. Consequently, coding of spectra in the corresponding sub-bands is not possible, resulting in significant degradation in the quality of audio signals.

To solve such a problem, a method for dynamically changing a bit allocation method may be employed (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-534328).

However, the bit allocation method is changed while a single coding method (quantization method) is used without changing the coding method (quantization method), and therefore, this approach to degradation in the quality of audio signals has a limited effect.

SUMMARY

According to an embodiment, an audio signal coding apparatus may have: a time-frequency transformer that generates a spectrum having performing a transform on an input audio signal into a frequency domain, divides the spectrum into a plurality of sub-bands, which are predetermined frequency bands, and outputs sub-band spectral samples; a sub-band energy quantizer that acquires, for each of the plurality of sub-bands, a quantized sub-band energy; a tonality calculator that analyzes a tonality of the sub-band spectral samples and output an analysis result; a bit allocator that selects a second sub-band on which quantization is performed by a second quantizer from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy, and determine a first number of bits to be allocated to a first sub-band, among the plurality of sub-bands, on which quantization is performed by a first quantizer; and a multiplexer that multiplexes information output from the first quantizer and from the second quantizer, the quantized sub-band energy, and the analysis result of the tonality, and outputs the multiplexed information, wherein the first quantizer codes a sub-band spectral sample among the sub-band spectral samples that is included in the first sub-band by a first coding method using the first number of bits, and the second quantizer codes a sub-band spectral sample among the sub-band spectral samples that is included in the second sub-band by a second coding method to acquire the coded information output from the second quantizer, wherein the second coding method is configured for calculating lag information for the second subband.

According to another embodiment, an audio signal decoding apparatus for decoding coded information may have: a demultiplexer that demultiplexes the coded information into first coded information, second coded information, quantized sub-band energy acquired by quantizing energy of each sub-band among the plurality of sub-bands, and an analysis result for a tonality calculated for each sub-band among the plurality of sub-bands; a bit allocator that selects the second sub-band on which decoding is performed by a second decoder from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy, and determines a first number of bits to be allocated to a first sub-band, among the plurality of sub-bands, on which decoding is performed by a first decoder; and a frequency-time transformer that generates and outputs an output audio signal by performing a transform on a spectrum output from the second decoder into a time domain, wherein the first decoder generates a first decoded spectrum by decoding the first coded information using the first number of bits, and the second decoder generates a second decoded spectrum using decoding the second coded information, and generates a reconstructed spectrum by combining the second decoded spectrum and the first decoded spectrum.

According to another embodiment, a terminal apparatus may have: the inventive audio signal coding apparatus; and an antenna that transmits the coded information.

According to another embodiment, a terminal apparatus may have: an antenna that receives and outputs to the demultiplexer the coded information; and the inventive audio signal decoding apparatus.

According to another embodiment, an audio signal coding method may have the steps of: generating a spectrum by performing a transform on an input audio signal into a frequency domain; dividing the spectrum into a plurality of sub-bands, which are predetermined frequency bands, and outputting sub-band spectral samples; acquiring, for each of the plurality of sub-bands, quantized sub-band energy; analyzing a tonality of the sub-band spectral samples and outputting an analysis result; selecting a second sub-band from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy; determining a first number of bits to be allocated to a first sub-band among the plurality of sub-bands; generating first coded information by coding a sub-band spectral sample among the sub-band spectral samples that is included in the first sub-band by a first coding method using the first number of bits; generating second coded information by coding a sub-band spectral sample among the sub-band spectral samples that is included in the second sub-band by using a second coding method, wherein the second coding method is configured for calculating lag information for the second subband; and multiplexing together and outputting the first coded information and the second coded information.

According to another embodiment, an audio signal decoding method for decoding coded information may have the steps of: demultiplexing the coded information into first coded information, second coded information, quantized sub-band energies for each sub-band among a plurality of sub-bands, and an analysis result for a tonality calculated for each sub-band among the plurality of sub-bands; selecting a second sub-band from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy; determining a first number of bits to be allocated to a first sub-band among the plurality of sub-bands; generating a first decoded spectrum by decoding the first coded information using the first number of bits; generating a second decoded spectrum using decoding the second coded information, and generating a reconstructed spectrum by combining the second decoded spectrum and the first decoded spectrum; and generating and outputting an output audio signal by performing a transform on the reconstructed spectrum into a time domain.

Another embodiment may have a non-transitory digital storage medium having a computer program stored thereon to perform the audio signal coding method having the steps of: generating a spectrum by performing a transform on an input audio signal into a frequency domain; dividing the spectrum into a plurality of sub-bands, which are predetermined frequency bands, and outputting sub-band spectral samples; acquiring, for each of the plurality of sub-bands, quantized sub-band energy; analyzing a tonality of the sub-band spectral samples and outputting an analysis result; selecting a second sub-band from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy; determining a first number of bits to be allocated to a first sub-band among the plurality of sub-bands; generating first coded information by coding a sub-band spectral sample among the sub-band spectral samples that is included in the first sub-band by a first coding method using the first number of bits; generating second coded information by coding a sub-band spectral sample among the sub-band spectral samples that is included in the second sub-band by using a second coding method, wherein the second coding method is configured for calculating lag information for the second subband; and multiplexing together and outputting the first coded information and the second coded information; when said computer program is run by a computer.

Another embodiment may have a non-transitory digital storage medium having a computer program stored thereon to perform the audio signal decoding method for decoding coded information, the audio signal decoding method having the steps of: demultiplexing the coded information into first coded information, second coded information, quantized sub-band energies for each sub-band among a plurality of sub-bands, and an analysis result for a tonality calculated for each sub-band among the plurality of sub-bands; selecting a second sub-band from among the plurality of sub-bands on the basis of the analysis result for the tonality and the quantized sub-band energy; determining a first number of bits to be allocated to a first sub-band among the plurality of sub-bands; generating a first decoded spectrum by decoding the first coded information using the first number of bits; generating a second decoded spectrum using decoding the second coded information, and generating a reconstructed spectrum by combining the second decoded spectrum and the first decoded spectrum; and generating and outputting an output audio signal by performing a transform on the reconstructed spectrum into a time domain; when said computer program is run by a computer.

One non-limiting and exemplary embodiment provides a coding technique and a decoding technique for realizing high-quality audio signals while reducing the overall bit rate.

In one general aspect, the techniques disclosed here feature an audio signal coding apparatus including a time-frequency transformer, a sub-band energy quantizer, a tonality calculator, a bit allocator, and a multiplexer. The time-frequency transformer generates a spectrum by performing a transform on an input audio signal into a frequency domain, divides the spectrum into sub-bands, which are predetermined frequency bands, and outputs sub-band spectra. The sub-band energy quantizer obtains, for each of the sub-bands, quantized sub-band energy. The tonality calculator analyzes tonality of the sub-band spectra and outputs an analysis result. The bit allocator selects a second sub-band on which quantization is performed by a second quantizer from among the sub-bands on the basis of the analysis result of the tonality and the quantized sub-band energy, and determines a first number of bits to be allocated to a first sub-band, among the sub-bands, on which quantization is performed by a first quantizer. The multiplexer multiplexes into information coded information output from the first quantizer and from the second quantizer, the quantized sub-band energy, and the analysis result of the tonality, and outputs the multiplexed information. The first quantizer codes a sub-band spectrum among the sub-band spectra that is included in the first sub-band by first coding method using the first number of bits, and the second quantizer codes a sub-band spectrum among the sub-band spectra that is included in the second sub-band by using a second coding method.

With the coding apparatus, decoding apparatus, and so on according to the present disclosure, it is possible to code and decode high-quality audio signals while reducing the overall bit rate.

It should be noted that general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages.

