Audio encoding device, audio decoding device, audio encoding method, and audio decoding method

• US 20080082321A1
• Filed: 10/01/2007
• Published: 04/03/2008
• Est. Priority Date: 10/02/2006
• Status: Active Grant

• Current Assignee: Casio Computer Company Limited
• Original Assignee: Casio Computer Company Limited
• Inventors: Ide, Hiroyasu
• Granted Patent: US 8,447,597 B2
• Time in Patent Office: Days
• US Class Current: 704/203
• CPC Class Codes: G10L 19/035 Scalar quantisation

Abstract

In an encoding process, a CPU transforms an audio signal from the real-time domain to the frequency domain, and transforms the signal into spectra consisting of MDCT coefficients. The CPU separates the audio signal into several frequency bands, and performs bit shifting in each band such that the MDCT coefficients can be expressed with pre-configured numbers of bits. The CPU re-quantizes the MDCT coefficients at a precision differing for each band, and transmits the values acquired thereby and shift bit numbers as encoded data. Meanwhile, in a decoding process, a CPU receives encoded data and inverse re-quantizes and inverse bit shifts the data, thereby restoring the MDCT coefficients. Furthermore, the CPU transforms the data from frequency domain to the real-time domain by using the inverse MDCT, and restores and outputs the audio signal.

Claims

1. An audio encoding device, comprising:

a storage unit which stores information indicating the frequency width of each band of audio data which is comprised of a plurality of first frequency bands, in association with a predetermined number of bits respectively;

a discrete transformation unit that discretely transforms the audio data from the real-time domain to the frequency domain by acquiring the audio data and calculating quantized values indicating audio strength for each frequency;

a maximum value calculation unit that separates the discretely transformed frequency domain audio data into the plurality of first frequency bands having the frequency widths, and calculates the maximum value of the calculated quantized values for each of the separated first frequency bands;

a determining unit that, for each of the first frequency bands, determines whether or not the number of bits necessary for expressing the maximum values calculated by the maximum value calculation unit exceeds the set number of bits stored in the storage unit;

an adjustment unit that, for each of the first frequency bands, in the case where it is determined by the determining unit that the necessary number of bits exceeds the set number of bits, acquires for a divisor, this divisor (a value expressed as a power of

     2) being the smallest value wherein the necessary number of bits does not exceed the set number of bits when the maximum value is divided thereby, and respectively divides the quantized values within the first frequency bands by this divisor, thereby adjusting the audio strength;

a re-quantizing unit that, for each of a plurality of second frequency bands having a predetermined band width, re-quantizes the quantized values calculated by the discrete transformation unit, or alternatively, the quantized values adjusted by the adjustment unit, at a precision decided in advance; and

an encoding unit that encodes, associates, and outputs the divisors calculated by the adjustment unit and the quantized values calculated by the re-quantizing unit.

2. The audio encoding device according to claim 1, wherein:

the storage unit associates numbers of bits that are large to the degree that the central frequencies of the frequency widths are small.

3. The audio encoding device according to claim 1, wherein:

the maximum value calculation unit separates the audio data into the plurality of first frequency bands such that the logarithms of every band width increase linearly in succession from the low-frequency side.

4. The audio encoding device according to claim 1, wherein:

the re-quantizing unit re-quantizes such that the re-quantization precision increases to the degree that one of the plurality of second frequency bands is on the low-frequency side.

5. The audio encoding device according to claim 1, wherein:

the re-quantizing unit respectively calculates the logarithms of the quantized values adjusted by the adjustment unit, and re-quantizes the logarithms.

6. The audio encoding device according to claim 1, wherein:

the re-quantizing unit separates and re-quantizes the audio data, the separation being into the plurality of second frequency bands having the set band widths, and wherein the logarithms of every band increase linearly in succession from the low-frequency side.

7. The audio encoding device according to claim 1, wherein:

the re-quantizing unit separates the audio data into at least three of the second frequency bands, being a low-frequency band, a mid-frequency band, and a high-frequency band, and the separation being such that the boundary frequency between the mid-frequency band and the high-frequency band is half the upper limit frequency of the audio data.

8. The audio encoding device according to claim 1, wherein:

the encoding unit, for each of a plurality of third frequency bands comprised of one or a plurality of the first frequency bands, calculates a total sum value of squared values of the quantized values contained in the band, decides a priority in descending order from the largest calculated total sum, and entropy encodes the quantized values within the plurality of third frequency bands in descending order by the priority.

9. The audio encoding device according to claim 8, wherein:

the encoding unit calculates the value of each total sum value multiplied by a predetermined value that increases to the degree that the total sum value is on the low-frequency side, and decides priorities in descending order from the largest calculated total sum.

10. The audio encoding device according to claim 1, wherein:

the discrete transformation unit calculates the quantized values by using the MDCT

(Modified Discrete Cosine Transform).

11. An audio decoding device, comprising:

a receiving unit that receives encoded data, the data being encoded by a predetermined encoding method and consisting of;

quantized audio data, the signal strength thereof expressed by quantized values adjusted for each of a plurality of first frequency bands; and

numerical parameters, expressed as powers of 2, associated with each band of a plurality of set second frequency bands configured for the audio data;

a decoding unit that, by using a decoding method corresponding to the set encoding method, decodes the encoded data into the quantized audio data and the numerical parameters

a strength restoration unit that, for each of the plurality of second frequency bands, multiplies the decoded quantized values within the band by the decoded numerical parameter associated with that band, thereby restoring the audio strength of the audio data; and

a discrete inverse transformation unit that, for each of the plurality of first frequency bands, transforms the strength-restored audio data from the frequency domain to the time domain.

12. An audio encoding method executed in an audio encoding device having a storage unit which stores information indicating the frequency width of each band of audio data which is comprised of a plurality of first frequency bands, in association with a predetermined number of bits respectively, the audio encoding method comprising the steps of:

acquiring the audio data and discretely transforming it from the time domain to the frequency domain by calculating quantized values indicating audio strength for each frequency;

separating the discretely transformed frequency domain audio data into the plurality of first frequency bands having the frequency widths, and calculating a maximum value of the calculated quantized values for each of the separated first frequency bands;

for each of the first frequency bands, determining whether or not the number of bits necessary for expressing the maximum values calculated in the maximum value calculation step exceeds the set number of bits stored in the storage unit;

for each of the first frequency bands, in the case where it is determined in the determination step that the necessary number of bits exceeds the set number of bits, adjusting the audio strength by solving for a divisor, this divisor (a value expressed as a power of

     2) being the smallest value wherein the necessary number of bits does not exceed the set number of bits when the maximum value is divided thereby, and dividing the quantized values within the first frequency bands by this divisor;

for each of a plurality of second frequency bands having a predetermined band width, re-quantizing the quantized values calculated in the discrete transformation step, or alternatively, the quantized values adjusted in the adjustment step, at a precision decided in advance; and

encoding, associating, and outputting the divisors calculated in the adjustment step and quantized values calculated in the re-quantizing step.

13. An audio decoding method, comprising the steps of:

receiving encoded data, the data being encoded by a predetermined encoding method and consisting of;

quantized audio data, the signal strength thereof expressed by quantized values adjusted for each of a plurality of first frequency bands; and

numerical parameters, expressed as powers of 2, associated with each band of a plurality of set second frequency bands configured for the audio data;

by using a decoding method corresponding to the set encoding method, decoding the encoded data into the quantized audio data and the numerical parameters;

for each of the plurality of second frequency bands, restoring the audio strength of the audio data by multiplying the decoded quantized values within the band by the decoded numerical parameter associated with that band; and

for each of the plurality of first frequency bands, discretely inverse transforming the strength-restored audio data from the frequency domain to the time domain.