Patent Description:
Conventionally, there is known a technique for removing quantization noise of data digitized by use of a low number of quantization bits. For example, Patent Reference <NUM> discloses a technique in which digital data quantized by use of a low number of bits is subjected to spectrum conversion to omit a spectrum at or below a predetermined level, and then the data to which inverse Fourier transform is applied is outputted.

<CIT> describes a speech enhancement system for the reduction of background noise that comprises a time-to-frequency transformation unit to transform frames of time-domain samples of audio signals to the frequency domain, background noise reduction means to perform noise reduction in the frequency domain, and a frequency-to-time transformation unit to transform the noise reduced signals back to the time-domain.

In <NPL>, a new approach to shape the coding noise in speech and audio coders is described.

<CIT> describes an apparatus for processing signals.

In Patent Reference <NUM>, there has been a problem where, due to uniform removal of quantization noise that is not annoying in the sense of hearing, an originally existing sound component is also removed together with the quantization noise.

Examples of problems to be solved by the present invention include the example as described above. A main object of the present invention is to provide a signal processing device capable of suitably attenuating quantization noise.

The object is achieved by the present invention in the aspects of a signal processing device, a control method, a program and a storage medium having the features of the independent claims.

Additional features for advantageous embodiments of the present invention are provided in the dependent claims.

In a preferred embodiment of the present invention, a signal processing device includes: an acquisition unit configured to acquire sound data subjected to quantization; and a quantization noise control unit configured to determine a control amount for quantization noise of the sound data, generated due to the quantization, based on the level of the sound data. In this embodiment, focusing on the fact that the sense of hearing for quantization noise varies in accordance with the level of the acquired sound data, the signal processing device determines the control amount for controlling the quantization noise based on the level of the sound data. Thereby, the signal processing device can suitably control quantization noise heard by a user to reduce its influence.

The control amount is an attenuation amount by which a signal level of the sound data is attenuated in a predetermined frequency band, and the quantization noise control unit changes the attenuation amount based on the sound level. With this aspect, the quantization noise control unit can suitably prevent unnecessary removal of an originally existing sound component while attenuating the quantization noise.

In another aspect of the signal processing device, the smaller the sound level is, the larger the quantization noise control unit makes the attenuation amount. As described later in an embodiment, the applicant has obtained knowledge that the smaller the sound level of the sound data is, the easier the quantization noise is to hear and that the smaller the sound level of the sound data is, the smaller the influence, caused by quantization noise attenuation processing, on a sound component that is not noise becomes. Thus, with this aspect, the signal processing device can suitably attenuate the quantization noise while avoiding the influence on the originally existing sound component.

In another aspect of the signal processing device, the quantization noise control unit determines the attenuation amount based on the sound level and a frequency of the sound data. As described later in the embodiment, the applicant has obtained knowledge that the quantization noise tends to be easy to hear when the frequency of the input signal is relatively low. Thus, with this aspect, the signal processing device can more effectively attenuate the quantization noise while avoiding the influence on the originally existing sound component. Preferably, the lower the frequency is, the larger the quantization noise control unit may make the attenuation amount.

The signal processing device further includes a conversion unit configured to convert a time waveform of the sound data to a frequency domain, and the quantization noise control unit attenuates amplitude which is less than a predetermined level, based on the control amount. Therefore, the signal processing device can attenuate the amplitude affected by the quantization noise and suitably reduce the quantization noise.

The conversion unit converts the time waveform of the sound data, cut out with a predetermined time interval, to a frequency domain. When the sound data is cut out in this way, characteristic in the frequency domain spreads due to the influence of the cut-out, and depending on the level of the sound data, the quantization noise is mixed with the original sound component in a specific frequency band. By determining the control amount based on the sound level, the signal processing device can suitably reduce the influence on the originally existing sound component while suitably attenuating the quantization noise in the range to be heard by the user.

Another aspect of the signal processing device further includes a division unit configured to divide the sound data into a plurality of frequency bands, and the quantization noise control unit determines the control amount for the quantization noise with respect to each of the plurality of frequency bands. Thereby, the signal processing device can appropriately determine the control amount with respect to each frequency band. In a preferred example, the signal processing device further includes an overtone generation unit configured to perform overtone generation on the sound data with the quantization noise controlled, and an output unit configured to output the sound data subjected to the overtone generation. With this aspect, the signal processing device can suitably perform up-conversion to a greater standard with higher quality.

In another embodiment of the present invention, a control method to be performed by a signal processing device includes: an acquisition step of acquiring sound data subjected to quantization; and a quantization noise control step of determining a control amount for quantization noise of the sound data, generated due to the quantization, based on the level of the sound data. By performing this control method, the signal processing device can suitably control quantization noise heard by the user to reduce its influence.

In another embodiment of the present invention, a program to be executed by a computer causes the computer to function as acquisition unit configured to acquire sound data subjected to quantization, and quantization noise control unit configured to determine a control amount for quantization noise of the sound data, generated due to the quantization, based on the level of the sound data. By executing this program, the computer can suitably control quantization noise heard by the user to reduce its influence. Preferably, the program is stored in a storage medium.

A preferred embodiment of the present invention will be described below with reference to the drawings. Hereinafter, "high-resolution" means a sound source with a sampling frequency of <NUM> and a bit length of <NUM> bit, or higher precision.

<FIG> shows a configuration of a sound output system <NUM> according to the present embodiment. The sound output system <NUM> is a system for reproducing sound data by upconverting sound data of the CD standard to that of the high-resolution standard, and as shown in <FIG>, the sound output system <NUM> includes an input device <NUM>, a converter <NUM>, and an output device <NUM>.

The input device <NUM> inputs an input signal S1, which is digital data of a CD sound source, into the converter <NUM>. The input device <NUM> may, for example, be an interface device that reads sound data from a recording medium such as a CD, be a communication device that receives sound data transmitted from another device by wire or wirelessly, or be a storage device that stores the input signal S1.

The converter <NUM> upconverts the input signal S1 input from the input device <NUM> to output an output signal S2, which is digital data of the high-resolution standard, to the output device <NUM>. In this case, as described later, the converter <NUM> first upconverts each of a sampling frequency and a bit length to a predetermined high-resolution format. At this time, although the format has been upconverted, the signal is one with the quality of the CD specification due to being a signal containing quantization noise and not yet subjected to high-frequency interpolation. Next, processing for attenuating the quantization noise contained in the input signal S1 (quantization noise attenuation processing) and overtone generation processing are performed to generate the output signal S2 that is sound data with quality exceeding the CD specification. The converter <NUM> is an example of a "signal processing device" in the present invention.

The output device <NUM> is, for example, a speaker or the like and outputs sound based on the output signal S2 output from the converter <NUM>. It is noted that at least one of the input device <NUM> and the output device <NUM> may be configured integrally with the converter <NUM>. The input device <NUM> and the output device <NUM> may be configured integrally.

<FIG> shows a functional block configuration diagram of the converter <NUM>. The converter <NUM> includes a hardware configuration such as a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and functionally includes a time window cut-out block <NUM>, a fast Fourier transform (FFT) block <NUM>, an attenuation amount limitation block <NUM>, a quantization noise attenuation block <NUM>, an overtone generation block <NUM>, an inverse fast Fourier transform (IFFT) block <NUM>, and a time window resynthesis block <NUM>.

Based on any one of various window functions such as a hanning window, the time window cut-out block <NUM> cuts out the time waveform of the sound data from the input signal S1 for each window width with a predetermined time length (for each frame) while applying the window function with overlap. Then, the time window cut-out block <NUM> supplies sound data for each frame to each of the FFT block <NUM> and the attenuation amount limitation block <NUM>.

The FFT block <NUM> performs fast Fourier transform on the sound data with the predetermined time length, outputted by the time window cut-out block <NUM>, and outputs amplitude and a phase for each frequency. In this case, information on the amplitude is supplied to the quantization noise attenuation block <NUM>, and information on the phase is supplied to the overtone generation block <NUM>.

The attenuation amount limitation block <NUM> determines the maximum attenuation amount with respect to the amplitude in a frequency domain to be attenuated in the quantization noise attenuation block <NUM>, based on the magnitude of the signal level of sound data supplied from the time window cut-out block <NUM>. The attenuation amount limitation block <NUM> has an RMS value calculation block <NUM> and a maximum attenuation amount calculation block <NUM>. The RMS value calculation block <NUM> calculates a root mean square (RMS) value for each frame of the sound data supplied from the time window cut-out block <NUM>. In this case, the RMS value calculated by the RMS value calculation block <NUM> corresponds to the magnitude of an average signal level of the sound data for each frame. Based on the RMS value calculated by the RMS value calculation block <NUM>, the maximum attenuation amount calculation block <NUM> calculates the maximum attenuation amount corresponding to the upper limit of the attenuation amount, which is used in the quantization noise attenuation block <NUM>, for the amplitude in the frequency domain. A detail of the calculation method for the maximum attenuation amount will be described later with reference to <FIG>.

It is noted that the RMS value calculation block <NUM> may calculate a calculated value other than the RMS value so long as being the value indicating the signal level of the sound data.

The quantization noise attenuation block <NUM> adjusts the amplitude in the frequency domain of a target frame based on the maximum attenuation amount determined by the attenuation amount limitation block <NUM>, to thereby attenuate the quantization noise. In the present embodiment, the quantization noise attenuation block <NUM> estimates level less than -<NUM> dB, which is the minimum level reproducible in the CD standard, as one generated due to quantization noise, and therefore attenuates the signal level at the frequency whose level is less than - <NUM> dB. At this time, the quantization noise attenuation block <NUM> adjusts the signal level at the frequency whose level is less than -<NUM> dB so as not to exceed the maximum attenuation amount determined by the attenuation amount limitation block <NUM>. A specific example of the adjustment processing will be described later with reference to <FIG>. It is noted that -<NUM> dB is an example of a "predetermined level" in the present invention. Further, the frequency whose level is less than -<NUM> dB is an example of a "predetermined frequency band" in the present invention.

On the basis of the information indicative of the amplitude for each frequency outputted by the quantization noise attenuation block <NUM>, and the information indicative of the phase outputted by the FFT block <NUM>, the overtone generation block <NUM> performs the overtone generation processing to generate an overtone for high-frequency interpolation. Thereby, the overtone generation block <NUM> performs pseudo up-sampling on the CD sound source. Any known overtone generation technique may be applied to the overtone generation processing.

The IFFT block <NUM> performs inverse fast Fourier transform on the sound data in the frequency domain subjected to the overtone generation processing, to convert the sound data for each frame from the frequency domain to a time domain. The time window resynthesis block <NUM> overlaps and adds the sound data of each frame outputted by the IFFT block <NUM> to generate the smoothly connected output signal S2. Then, the time window resynthesis block <NUM> supplies the generated output signal S2 to the output device <NUM>.

In the configuration shown in <FIG>, the time window cut-out block <NUM> is an "acquisition unit" in the present invention, and the FFT block <NUM> is a "conversion unit" in the present invention. Further, the attenuation amount limitation block <NUM> and the quantization noise attenuation block <NUM> are a "quantization noise control unit" in the present invention, the overtone generation block <NUM> is "overtone generation unit" in the present invention, and the time window resynthesis block <NUM> an "output unit" in the present invention. Further, the CPU and the like of the converter <NUM> which constitute each block are an example of a "computer" for executing a program in the present invention.

Here, a specific description will be given of a method for setting of the maximum attenuation amount by the maximum attenuation amount calculation block <NUM>.

<FIG> is a graph schematically showing the relationship between the dB value converted from the RMS value calculated by the RMS value calculation block <NUM> and the maximum attenuation amount determined by the maximum attenuation amount calculation block <NUM>. In the example of <FIG>, the maximum attenuation amount is defined in the level range from <NUM> dB to about -<NUM> dB. It is noted that about -<NUM> dB corresponds to the minimum value of the level reproducible in the CD standard.

As shown in <FIG>, the maximum attenuation amount calculation block <NUM> increases the maximum attenuation amount as the level approaches -<NUM> dB, that is, as the level gets smaller. In this case, for example, with reference to a formula, a table, or the like corresponding to <FIG> stored in advance, the maximum attenuation amount calculation block <NUM> determines the maximum attenuation amount from the RMS value calculated by the RMS value calculation block <NUM> or the dB value converted from the RMS value. Thereby, the smaller the input level of the input signal S1 is, the larger the attenuation amount can be made, and the quantization noise can be attenuated effectively while suitably reducing the attenuation of the originally required input signal. This effect will be described in more detail in the [Effect] section.

It is noted that the relation between the maximum attenuation amount and the level is not limited to the relation represented by the graph shown in <FIG> and that it may only be necessary to satisfy a relation in which the maximum attenuation amount increases with decreasing level. For example, the relation may be a relation indicated by a linear graph.

Next, a specific example of the quantization noise attenuation processing by the quantization noise attenuation block <NUM> will be described.

<FIG> shows a frequency characteristic of the sound data cut out by the time window cut-out block <NUM>. In <FIG>, a solid line indicates a characteristic before the quantization noise attenuation processing , and a dashed line indicates a characteristic after the quantization noise attenuation processing by quantization noise attenuation block <NUM>.

As shown in <FIG>, the quantization noise attenuation block <NUM> attenuates the signal level at the frequency whose level is less than -<NUM> dB, which is the minimum level reproducible in the CD standard, so as not to exceed the maximum attenuation amount determined by the attenuation amount limitation block <NUM>. Here, as an example, in a target frequency band, the quantization noise attenuation block <NUM> calculates the difference between -<NUM> dB and the signal level of the characteristic as the attenuation amount. Then, when the calculated attenuation amount exceeds the maximum attenuation amount, the quantization noise attenuation block <NUM> takes the maximum attenuation amount as the attenuation amount to be applied. Thereby, the quantization noise attenuation block <NUM> can suitably attenuate the quantization noise while preventing the characteristic from being discontinuous.

Next, the effect of the present embodiment will be supplemented with reference to <FIG>.

<FIG> shows a signal waveform obtained by quantizing sound data of a sine wave in accordance with the high-resolution standard (<NUM>-bit quantization ), and <FIG> shows a signal waveform obtained by quantizing sound data of a sine wave in accordance with the CD standard (<NUM>-bit quantization ). Further, <FIG> shows the frequency characteristic of the sound data in <FIG> shows the frequency characteristic of the sound data in <FIG>.

As shown in <FIG>, in the case of the CD standard, small level sound or the like has become a stepped signal. Thus, as shown in <FIG>, in the case of the CD standard, even in an audible frequency band of <NUM> or less, quantization noise, which hardly appears in the quantization in the high-resolution standard (<NUM> bits here), is generated in large amount. On the other hand, in the case of the high-resolution standard, a smooth signal waveform has been formed due to the number of quantization bits being high (cf. <FIG>), and the quantization noise has hardly been generated (cf.

<FIG> is a graph quantitatively representing the sense of hearing for quantization noise generated in the CD standard when the combination of the frequency and the level of the original sound data, which is a sine wave, is changed. In <FIG>, quantization noise, generated with respect to each of any combinations of the frequency and level of the original sound data that is a sine wave, is calculated and multiplied by the hearing-sense characteristic derived from a loudness curve or the like, to quantitatively obtain and visualize the sense of hearing for the quantization noise. <FIG> shows that the darker the color of the region, the easier the quantization noise is to hear (i.e., the easier the difference between the high-resolution standard and the CD standard is to perceive) in the region. It is noted that the applicant has conducted a listening experiment to obtain the same result as the tendency of the graph of <FIG>.

According to the graph of <FIG>, it is understood that there is a tendency that the lower the level and frequency of the input signal are, the easier the quantization noise is to hear, and particularly in a level region of about -<NUM> dB or less as well as in a low-frequency band of about <NUM> or less, the quantization noise is easy to hear. It is thus estimated that in the case of the CD standard, in the low-level region and the low-frequency band, the sound quality has been degraded due to the quantization noise. It is conceivable therefrom that the lower the input level of the input signal S1 is, or the lower the frequency of the input signal S1 is, the higher the need to attenuate the quantization noise becomes. Furthermore, it is conceivable that the lower the input level of the input signal S1 and the frequency of the input signal S1, the higher the need to attenuate the quantization noise.

Here, a description will be given of the influence at the time when in the quantization noise attenuation processing, the maximum attenuation amount is not provided and the amplitude is uniformly attenuated regardless of the magnitude of the input level of the input signal S1.

<FIG> shows the frequency characteristic of the input signal S1 with a relatively large input level, and <FIG> shows a characteristic after the Fourier transform has been performed by applying a window function to the input signal S1 that has the frequency characteristic shown in <FIG> (i.e., before the quantization noise attenuation processing). Further, <FIG> shows a characteristic after the quantization noise attenuation processing has been performed to uniformly attenuate the frequency less than -<NUM> dB with respect to the characteristic of <FIG>, and <FIG> shows the frequency characteristic of the output signal S2, generated from the characteristic shown in <FIG>.

As shown in <FIG>, when the input level of the input signal S1 is relatively large, by applying the window function, the range of the characteristic portion to be the peak spreads. Then, as shown in <FIG>, when the quantization noise attenuation processing is performed to uniformly attenuate the frequency less than -<NUM> dB without depending on the input level of the input signal S1, the spread portion described above also attenuates with the quantization noise. The spread portion is originally required information to correctly turn the main signal contained in the input signal, except for the quantization noise, back to the output signal S2 in the IFFT block <NUM> and the time window resynthesis block <NUM>. Hence, as shown in <FIG>, noise caused by the attenuation of the spread portion described above is generated in the output signal S2.

As thus described, in the case where the quantization noise attenuation processing is performed without providing the maximum attenuation amount when the input level of the input signal S1 is relatively large, there is a possibility that the originally required signal also attenuates due to the quantization noise attenuation processing, resulting in the degradation of the sound quality.

<FIG> shows the frequency characteristic of the input signal S1 with a relatively small input level, and <FIG> shows a characteristic after the Fourier transform has been performed by applying a window function to the input signal S1 that has the frequency characteristic shown in <FIG> (i.e., before the quantization noise attenuation processing). Further, <FIG> shows a characteristic after the quantization noise attenuation processing has been performed to uniformly attenuate the frequency less than -<NUM> dB with respect to the characteristic of <FIG>, and <FIG> shows the frequency characteristic of the output signal S2, generated from the characteristic shown in <FIG>.

As shown in <FIG>, when the input level of the input signal S1 is relatively small, even in a case where the Fourier transform is performed by applying a window function, the range of the characteristic portion to be the peak does not spread to the low-level region. Therefore, in this case, as shown in <FIG>, even when the quantization noise attenuation processing for attenuating the frequency less than -<NUM> dB is performed, the attenuation range of the spread portion described above is small. Thus, in this case, as shown in <FIG>, noise caused by the quantization noise attenuation processing is hardly generated in the output signal S2.

As thus described, in the case where the quantization noise attenuation processing is performed when the input level of the input signal S1 is relatively small, it is possible to suitably attenuate the quantization noise without attenuating the originally required input signal. Further, as described with reference to <FIG>, the smaller the input level of the input signal S1 is, the easier the quantization noise is to hear, and thus the higher the need to attenuate the quantization noise becomes. Considering the above, in the present embodiment, the smaller the input level of the input signal S1 is, the larger the maximum attenuation amount which the converter <NUM> sets becomes. Thereby, the converter <NUM> can effectively attenuate the quantization noise while suitably reducing the attenuation of the originally required input signal.

As described above, the time window cut-out block <NUM> of the converter <NUM> acquires the input signal S1 that is the quantized sound data, and cuts out sound data for each predetermined time interval. Then, the attenuation amount limitation block <NUM> determines the maximum attenuation amount based on the level of the sound data for each cut-out frame. Thereafter, on the basis of the maximum attenuation amount determined by the attenuation amount limitation block <NUM>, the quantization noise attenuation block <NUM> determines the attenuation amount for the amplitude in the frequency domain of the input signal S1 (i.e. the control amount for the quantization noise). Thereby, the converter <NUM> can effectively attenuate the quantization noise in an audible range while suitably reducing the attenuation of the originally required input signal.

Next, modifications preferred for the present embodiment will be described. The following modifications may be applied to the above embodiment in any combination.

In the above embodiment, as an example, the maximum attenuation amount calculation block <NUM> has determined the maximum attenuation amount with respect to each frame based on the RMS value corresponding to the average input level for each frame. As another example, the maximum attenuation amount calculation block <NUM> determines the maximum attenuation amount for each frame by further considering the frequency for each frame in addition to the RMS value described above.

<FIG> shows the block configuration of the converter <NUM> according to the present modification. As shown in <FIG>, the attenuation amount limitation block <NUM> of the converter <NUM> has a frequency gravity-center calculation block <NUM> in addition to the RMS value calculation block <NUM> and the maximum attenuation amount calculation block <NUM>. Here, the frequency gravity-center calculation block <NUM> calculates the center of gravity of the frequency (i.e. spectral center of gravity) based on the frequency spectrum obtained by the FFT block <NUM> which performs the Fourier transform on the input signal S1 cut out for each frame by the time window cut-out block <NUM>. Then, the frequency gravity-center calculation block <NUM> supplies the information on the calculated spectral center of gravity to the maximum attenuation amount calculation block <NUM>.

The maximum attenuation amount calculation block <NUM> determines the maximum attenuation amount based on the RMS value obtained from the RMS value calculation block <NUM> and the spectral center of gravity obtained from the frequency gravity-center calculation block <NUM>. In this case, for example, with reference to a table or a formula stored in the memory of the converter <NUM> in advance, the maximum attenuation amount calculation block <NUM> sets the maximum attenuation amount to be higher with the RMS value being lower, and the maximum attenuation amount calculation block <NUM> sets the maximum attenuation amount to be higher with the spectral center of gravity of the frequency being lower.

As described with reference to <FIG>, there is a tendency that the lower the frequency of the input signal is, the easier the quantization noise is to hear. Therefore, according to the present modification, the lower the spectral center of gravity of the frequency is, the higher the maximum attenuation amount which the converter <NUM> sets becomes, so that the converter <NUM> can effectively attenuate the audible quantization noise and suitably improve the sound quality. In the above modification, the center of the spectrum has been calculated for each frame. Instead, the center of the spectrum may be calculated at intervals of a predetermined time length or, for example, the center of gravity corresponding to one song may be calculated as the center of the spectrum.

In the embodiment, the example of upconverting the input signal S1 of the CD standard to the output signal S2 of the high-resolution standard has been shown, but the example to which the present invention is applicable is not limited thereto.

For example, the converter <NUM> may convert the input signal S1 of the sound source such as MPEG-<NUM> Audio Layer-<NUM> (MP3) to the output signal S2 having a specification of the CD standard or the high-resolution standard level. In this case, after decoding the input signal S1, the converter <NUM> performs the processing of each processing block shown in <FIG> and the like, thereby performing the attenuation of quantization noise, the overtone generation, and the like. In this case, the quantization noise attenuation block <NUM> of the converter <NUM> estimates, as a component generated due to the quantization noise, a component with level less than the minimum level reproducible in the standard employed to the input signal S1 (-<NUM> dB in the embodiment). Thus, the converter <NUM> attenuates the signal level at the frequency whose level is equal to or less than the above minimum level. As thus described, the present invention is suitably applied to various processing for up-conversion to a standard with a higher number of quantization bits.

In the embodiment, the example of the setting the level configured to determine the quantization noise to - <NUM> dB has been shown, but the example to which the present invention is applicable is not limited thereto, and the level configured to determine the quantization noise may be adjusted in accordance with the conditions of the time window and frequency conversion.

In the embodiment, the example has been shown in the description of the attenuation amount limitation block <NUM> of <FIG> where the maximum attenuation amount with respect to the amplitude in the frequency domain, attenuated in the quantization noise attenuation block <NUM>, is determined based on the magnitude of the signal levels in all the frequency bands of the sound data supplied from the time window cut-out block <NUM>. However, the determination method for the maximum attenuation amount is not limited thereto.

For example, it is also possible that at the time of performing the quantization noise attenuation processing, the maximum attenuation amount calculation block <NUM> divides the sound data supplied from the time window cut-out block <NUM> into a band larger than a certain frequency and a band equal to or less than the frequency, and from the RMS value or the dB value converted from the RMS value in each band, the maximum attenuation amount calculation block <NUM> determines the maximum attenuation amount to be applied in each of the divided band.

Specifically, in a signal indicating a characteristic as shown in <FIG>, the frequency band is divided into a frequency band surrounded by a dashed line <NUM> and a frequency band not surrounded thereby. In the present example, the frequency band is divided into a band larger than <NUM> and a band of <NUM> or less. At this time, the RMS value in the band larger than <NUM> is smaller than the RMS value in the band of <NUM> or less. Therefore, the maximum attenuation amount in the band larger than <NUM> is larger than the maximum attenuation amount in the band of <NUM> or less.

Claim 1:
A signal processing device (<NUM>) comprising:
an acquisition unit (<NUM>) configured to acquire sound data (S1) subjected to quantization, and configured to cut out a time waveform from the sound data (S1) for each window width with a predetermined time length, frame, while applying a window function with overlap, and to provide the sound data for each frame to each of a conversion unit (<NUM>) and an quantization noise control unit (<NUM>, <NUM>); and
the conversion unit (<NUM>) configured to perform fast Fourier transform, FFT, on the sound data (S1) with the predetermined time length received from the acquisition unit (<NUM>), and to output for each frequency information on an amplitude to the quantization noise control unit (<NUM>, <NUM>) and an information on the phase to an overtone generation unit (<NUM>);
the quantization noise control unit (<NUM>, <NUM>) configured to determine a control amount for quantization noise of the sound data (S1), generated due to the quantization, based on the level of the sound data, the quantization noise control unit (<NUM>, <NUM>) configured to:
determine a maximum attenuation amount with respect to the amplitude in a frequency domain to be attenuated based on the magnitude of the signal level of sound data received from the acquisition unit (<NUM>);
adjust the amplitude by an attenuation amount corresponding to the control amount, in a frequency band that a sound level of the sound data is less than a predetermined threshold so as not to exceed the maximum attenuation amount to attenuate the quantization noise;
the overtone generation unit (<NUM>) configured to generate an overtone for high-frequency interpolation based on information on the amplitude for each frequency received from the quantization noise control unit (<NUM>, <NUM>), and information on the phase received from the conversion unit (<NUM>),
an inverse fast Fourier transform, IFFT, block (<NUM>) configured to perform inverse fast Fourier transform on the sound data in the frequency domain subjected to the overtone generation processing, to convert the sound data for each frame from the frequency domain to a time domain, and
an output unit (<NUM>) configured to provide an output signal (S2) by overlapping and adding sound data of each frame outputted by the IFFT block (<NUM>).