JP2005033245A - Audio signal reproducing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an aliasing noise generated when generating harmonics in an audio signal reproducing apparatus for generating harmonic components from a digital audio signal limited to some band to correct missed frequency components of high frequency. <P>SOLUTION: The audio signal reproducing method for correcting the component of a high sound band exceeding the upper limit of high sound band of reproduction frequency of the digital audio signal has a harmonics generating means for generating harmonics from a digital audio signal, a harmonics adjusting means for adjusting the generated harmonics, and an adding means for adding corrected signals adjusted by the harmonics adjusting means to the digital audio signal. In the method, a means for preventing or reducing the aliasing noise generated when generating the harmonics is provided on the prestage of the harmonics generating means. By doing this, the noise is reduced, and acoustic feeling is improved. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a playback device for digital audio signals, and more particularly to a device for improving playback sound quality in a high sound range.
[0002]
[Prior art]
Conventionally, a digital audio signal processed by converting an audio signal such as a CD, DVD, or MD into a digital signal is limited to a Nyquist frequency by a sampling frequency at the time of A / D conversion. Furthermore, compressed audio such as MP3 and ATRAC3 is limited to frequencies lower than the Nyquist frequency. In this way, a high frequency component missing due to being restricted to a certain band is generated from the original digital audio signal, a predetermined band is selected, and added to the original digital audio signal. 2. Description of the Related Art An audio signal reproduction device that is corrected by reproduction is known (see, for example, Patent Document 1). In addition, there is a signal in which the amount of time delay of the additional signal added to the original digital audio signal is adjusted in various forms (see, for example, Patent Document 2).
[0003]
FIG. 9 is a block diagram showing the overall configuration of a conventional audio signal reproducing apparatus. In FIG. 9, reference numeral 1 denotes a high-frequency additional signal generator that includes a high-frequency component based on the original digital audio signal a, a frequency component equal to or higher than the predetermined frequency, and a frequency component substantially continuous with the predetermined frequency. Means for generating a signal having the same. Reference numeral 2 denotes an adder, which adds the high-frequency additional signal b generated by the high-frequency additional signal generator 1 to the original digital audio signal a and outputs it as a reproduction signal c.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2-311006 (page 2-3, FIG. 1)
[Patent Document 2]
JP-A-9-307385 (page 5-7)
[0005]
[Problems to be solved by the invention]
An audio signal reproducing apparatus that generates and corrects a high frequency component from the original digital audio signal for a high frequency component that is lost by being limited to a certain band by sampling or compression at the time of A / D conversion. In, aliasing noise occurs when the harmonic component to be generated includes a component higher than the Nyquist frequency. FIG. 10 shows a spectrum diagram of a conventional audio signal reproducing apparatus.
[0006]
For example, ATRAC3 is compressed audio that is limited to about 12 kHz for LP4 and about 15 kHz for LP2, but the Nyquist frequency is 22 kHz. The next harmonic component of 24 kHz is generated at 20 kHz, and is no longer a harmonic component of the original digital audio signal but noise.
[0007]
In addition, oversampling of non-compressed audio limited to the Nyquist frequency by sampling at the time of A / D conversion such as a CD, and generating harmonics by anti-aliasing processing, the sampling frequency of oversampling If the order of the generated harmonic is higher than the multiple, the harmonic component to be generated includes a component higher than the Nyquist frequency, and aliasing noise is generated.
[0008]
Therefore, it is not limited to the compressed audio or the order or Nyquist frequency shown in the example, but aliasing noise occurs when the harmonic component generated from the original digital audio signal is higher than the Nyquist frequency. Since the aliasing noise becomes noise rather than a harmonic of the original digital audio signal, it is required to reduce the aliasing noise.
[0009]
The present invention provides an audio signal reproducing apparatus that generates and corrects a harmonic component from the original digital audio signal for a high-frequency component that has been lost due to being limited to a certain band. The present invention has been made for the purpose of providing an audio signal reproduction device that improves the audibility by reducing the noise.
[0010]
[Means for Solving the Problems]
In order to solve this problem, the invention according to claim 1 of the present invention includes a harmonic generation unit that generates a harmonic from a digital audio signal, a harmonic adjustment unit that adjusts the generated harmonic, and a harmonic. Harmonic generation in an audio signal reproduction method having an addition means for adding a correction signal adjusted by an adjustment means to a digital audio signal, and correcting a high frequency component exceeding a high frequency upper limit of a digital audio signal reproduction frequency. It is characterized by comprising means for preventing or reducing aliasing noise generated at the time of harmonic generation before the means, and correction of the high frequency range component exceeding the high frequency upper limit of the reproduction frequency of the digital audio signal, This is done by adjusting harmonics that prevent or reduce aliasing noise and adding them to the digital audio signal. It has the effect that.
[0011]
The invention according to claim 2 is characterized in that the harmonic adjustment means is means for extracting a desired frequency band, and correction of a high-frequency component exceeding a high-frequency upper limit of a reproduction frequency of a digital audio signal is performed. Is performed by taking out a desired frequency band from the harmonics in which aliasing noise is prevented or reduced and adding it to the digital audio signal, thereby improving the audibility.
[0012]
The invention according to claim 3 is characterized in that it is a means for taking out a weighted frequency band by the harmonic adjustment means, and correcting a high-frequency component exceeding a high-frequency upper limit of a reproduction frequency of a digital audio signal. Is performed by taking out a desired frequency band weighted from harmonics that prevent or reduce aliasing noise and adding it to the digital audio signal, thereby improving the audibility.
[0013]
According to a fourth aspect of the present invention, the harmonic adjustment means includes means for extracting a desired frequency band, and level tracking means for detecting the level of the digital audio signal and adjusting the level of the harmonic or the desired frequency band. The digital audio signal is obtained by correcting the high frequency range component exceeding the upper limit of the high frequency range of the playback frequency of the digital audio signal and extracting the desired frequency band from the harmonics that prevent or reduce aliasing noise. Using the detected level, the level of a harmonic or a desired frequency band is adjusted and added to the digital audio signal, thereby improving the audibility.
[0014]
According to a fifth aspect of the present invention, there is provided means for extracting a desired frequency band weighted by the harmonic adjustment means, and a level for adjusting the level of the harmonic or weighted desired frequency band by detecting the level of the digital audio signal. It is characterized by having tracking means, and the desired frequency weighted by higher harmonics that prevent or reduce aliasing noise in the correction of the high frequency range component that exceeds the upper limit of the high frequency range of the playback frequency of the digital audio signal The band is taken out, and the level detected from the digital audio signal is used to adjust the level of the desired frequency band that is harmonic or weighted and added to the digital audio signal, thereby improving the audibility. .
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS.
[0016]
(Embodiment 1)
FIG. 1 shows a block diagram of an audio signal reproducing apparatus according to Embodiment 1 of the present invention. In FIG. 1, a is a digital audio signal, which is an input of an audio signal reproduction device, and is a digital signal limited to a certain band by sampling or compression at the time of A / D conversion. Reference numeral 5 denotes a low-pass filter (hereinafter abbreviated as LPF) as a means for preventing or reducing aliasing noise, and reduces aliasing during harmonic generation. Reference numeral 3 denotes a half-wave rectifier as harmonic generation means, which generates even-order harmonics of the input signal. However, the output of the half-wave rectifier 3 includes a fundamental wave and a DC component in addition to the harmonics. 4 is a harmonic adjustment means, and the output of the half-wave rectifier 3 is higher than the frequency of the digital audio signal a, and a high-frequency parameter suitable for hearing is added such as having a frequency component substantially continuous with the predetermined frequency. Adjust to signal b. An adder 2 adds the digital audio signal a which is an input of the audio signal reproducing apparatus and the high frequency additional signal b, and outputs the result as a reproduced signal.
[0017]
The operation of the audio signal reproducing apparatus configured as described above will be described below. For the sake of explanation, the digital audio a is assumed to be a compressed digital audio signal of ATRAC3 (LP4) as an example, and is assumed to have a sampling frequency fs = 44 kHz (hence the Nyquist frequency fn = 22 kHz) and a high frequency upper limit frequency fmax = 12 kHz. In addition, the harmonics generated by the half-wave rectifier 3 generate even-order harmonics in an infinite series. However, in order to avoid complicated explanation, the description will be made mainly assuming that secondary harmonics are generated. Actually, the spectrum of the harmonics generated by the half-wave rectifier 3 is reduced to the second order as the order becomes higher with the second harmonic being the maximum. Therefore, it can be considered that the fourth and higher harmonics are sufficiently smaller than the second harmonic. Further, the LPF 5 has a Nyquist frequency fn and the half-wave rectifier 3 mainly generates a second harmonic, and as an example, has a steep attenuation characteristic in which the cutoff frequency fc is one half of the Nyquist frequency fn. Assume that it is an LPF.
[0018]
First, the digital audio signal a which is an original signal is input to the LPF 5 and a frequency of 1/2 fn or higher is removed, and a signal in a frequency band of 1/2 fn or lower is output. The output of the LPF 5 is input to the half-wave rectifier 3 to generate a harmonic for a signal in a frequency band of 1/2 fn or less (however, the output includes a fundamental wave and a DC component in addition to the harmonic).
[0019]
The harmonics (including fundamental wave and DC component) output from the half-wave rectifier 3 are input to the harmonic adjustment means 4 to remove the frequency band overlapping with the original signal (digital audio signal a), level adjustment, etc. And output as a high-frequency additional signal b. The high frequency additional signal b and the digital audio signal a are added by the adder 2 and output as a reproduction signal c. Also, an image diagram of the spectrum of the intermediate output is shown in FIG.
[0020]
As described above, according to the present embodiment, by using the LPF 5 as means for preventing or reducing aliasing noise, the input to the half-wave rectifier 3 is limited to a frequency band equal to or less than ½ fn. Aliasing noise is prevented for the secondary harmonic generated by the wave rectifier 3, and since the harmonic generated by the half-wave rectifier 3 is mainly the secondary harmonic, aliasing can be significantly reduced. Therefore, by using the LPF 5 as a means for preventing or reducing aliasing noise, a high frequency component that has been lost by being limited to a predetermined band by compression or the like is used as a source of the digital audio signal a limited to the predetermined band. In addition, harmonic components exceeding the upper limit of the high sound range with reduced aliasing noise can be generated, and correction to a more natural sound can be performed.
[0021]
(Embodiment 2)
Next, FIG. 2 shows a block diagram of an audio signal reproducing apparatus according to the second embodiment. In FIG. 2, components having the same reference numerals as those in FIG. Reference numeral 6 denotes a bandpass filter (hereinafter abbreviated as BPF) as harmonic adjustment means, which extracts a frequency band equal to or higher than the frequency of the digital audio signal a from the harmonic generated by the half-wave rectifier 3 and outputs it as a high-frequency additional signal b. .
[0022]
The operation of the audio signal reproducing apparatus configured as described above will be described below. For the sake of explanation, the digital audio a and the LPF 5 use the same example as in the first embodiment. That is, the digital audio a is, for example, a compressed digital audio signal of ATRAC3 (LP4), and the LPF 5 has, for example, a steep attenuation characteristic in which the cutoff frequency fc is half of the Nyquist frequency fn. To do. Moreover, the harmonics generated by the half-wave rectifier 3 will be described assuming that they generate mainly second-order harmonics, as in the first embodiment.
[0023]
The difference from the first embodiment is that the harmonics (including the fundamental wave and DC component) output from the half-wave rectifier 3 are first input to the BPF 6 and the frequency band overlapping the original signal (digital audio signal a) is removed. And output as a high frequency additional signal b. The high frequency additional signal b and the digital audio signal a are added by the adder 2 and output as a reproduction signal c. An image of the spectrum of the intermediate output is shown in FIG.
[0024]
As described above, this embodiment can provide the same operations and effects as those of the first embodiment. In this embodiment, the BPF 6 is configured at the subsequent stage of the half-wave rectifier 3, but instead of the half-wave rectifier 3, a full-wave rectifier that does not generate a fundamental wave and a harmonic generation unit configured to cut a DC component are used. And an HPF that removes a frequency of 1/2 fmax or less, for example, before the harmonic generation means, and the harmonics output from the harmonic generation means overlap the original signal (digital audio signal a). If the adjustment is made so as not to output, an effect similar to that of BPF 6 can be obtained without using BPF 6. Therefore, the position of the harmonic adjustment means is not limited to the subsequent stage of the harmonic generation means.
[0025]
(Embodiment 3)
Embodiment 3 of the present invention will be described. FIG. 2 is also a block diagram of the audio signal reproduction device according to the third embodiment. The difference from the second embodiment is that
The BPF 6 not only extracts a frequency band higher than the frequency of the digital audio signal a from the harmonics generated by the half-wave rectifier 3, but also outputs it as a high-frequency additional signal b weighted by 1 / f. . For example, if the configuration of the BPF 6 is an HPF and LPF configuration, the HPF is a filter with a steep cutoff frequency fmax, and the LPF is a filter with a gentle slope cutoff frequency fmax. The above frequency band can be extracted.
[0026]
The operation of the audio signal reproducing apparatus configured as described above will be described below. For the sake of explanation, digital audio a and LPF 5 use the same example as in the first or second embodiment. Further, the harmonics generated by the half-wave rectifier 3 will be described as generating mainly second-order harmonics, as in the first or second embodiment.
[0027]
The difference from the second embodiment is that, as described above, the harmonics (including the fundamental wave and the DC component) output from the half-wave rectifier 3 are input to the BPF 6 and overlap the original signal (digital audio signal a). The band is removed, and further weighted and output as a high-frequency additional signal b. The high frequency additional signal b and the digital audio signal a are added by the adder 2 and output as a reproduction signal c. An image diagram of the spectrum of the intermediate output is shown in FIG.
[0028]
As described above, this embodiment can provide the same operations and effects as those of the first or second embodiment.
[0029]
(Embodiment 4)
FIG. 5 is a block diagram of an audio signal reproducing apparatus according to the fourth embodiment. In FIG. 5, components having the same reference numerals as those in FIG. The BPF 6 may be weighted or not. A level detector 7 detects the amplitude level of the input signal. A level adjuster 8 adjusts the level of the other input signal according to the input from the level detector 7 and outputs it.
[0030]
The operation of the audio signal reproducing apparatus configured as described above will be described below. For the sake of explanation, the digital audio a and the anti-aliasing filter 5 use the same examples as in the first, second, or third embodiment. Further, the harmonics generated by the half-wave rectifier 3 will be described assuming that second harmonics are mainly generated as in the first to third embodiments.
[0031]
The difference from Embodiment 2 or 3 is that the harmonics (including fundamental wave and DC component) output from the half-wave rectifier are input to BPF 6 and the frequency band overlapping the original signal (digital audio signal a) is removed. The harmonics are input to level adjustment 8. On the other hand, the level detection 7 detects the level from the digital audio signal a and inputs it to the level adjustment 8.
[0032]
The harmonic whose level is adjusted by the level adjustment 8 is output as the high-frequency additional signal b. The high frequency additional signal b and the digital audio signal a are added by the adder 2 and output as a reproduction signal c. An image diagram of the spectrum of the intermediate output is shown in FIG. FIG. 8 shows an image diagram of a spectrum when a frequency band weighted by BPF is taken out.
[0033]
In the present embodiment, the level of the output of the BPF 6 is adjusted by the level adjuster 8. However, as shown in FIG. 6, after adjusting the level of the output of the half-wave rectifier, the necessary frequency band is extracted by the BPF 6. Changing the value does not change the effect.
[0034]
As described above, according to the present embodiment, by using the LPF 5 as a means for preventing or reducing aliasing noise, by limiting the input to the half-wave rectifier 3 to a frequency band of 1/2 fn or less, Aliasing noise is prevented with respect to the secondary harmonic generated by the half-wave rectifier 3, and since the harmonic generated by the half-wave rectifier 3 is mainly the secondary harmonic, aliasing can be significantly reduced. Therefore, by using the LPF 5 as a means for preventing or reducing aliasing noise, a high frequency component that has been lost by being limited to a predetermined band by compression or the like is used as a source of the digital audio signal a limited to the predetermined band. In addition, harmonic components exceeding the upper limit of the high sound range with reduced aliasing noise can be generated, and correction to a more natural sound can be performed.
[0035]
【The invention's effect】
As described above, according to the present invention, a high frequency component that has been lost due to compression or the like being limited to a predetermined band is generated and corrected based on a digital audio signal limited to the predetermined band. In the above, the means for preventing or reducing aliasing noise is used in the preceding stage of the harmonic generator to prevent and reduce aliasing noise generated when generating harmonics from the digital audio signal. Thereby, an advantageous effect of reproducing the sound more naturally is obtained by correcting the reproduction signal with the harmonic component exceeding the upper limit of the higher harmonic range having no aliasing noise.
[Brief description of the drawings]
FIG. 1 is a block diagram of an audio signal reproducing apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram of an audio signal reproducing apparatus according to second and third embodiments of the present invention. Fig. 4 is a spectrum diagram of an audio signal reproducing device according to the second embodiment. Fig. 4 is a spectrum diagram of an audio signal reproducing device according to the third embodiment of the present invention. Fig. 5 is an audio signal reproducing device according to the fourth embodiment of the present invention. FIG. 6 is a block diagram of a second audio signal reproducing device according to the fourth embodiment of the present invention. FIG. 7 is a spectrum diagram (BPF weighting) of the audio signal reproducing device according to the fourth embodiment of the present invention. Nothing)
FIG. 8 is a spectrum diagram of the audio signal reproducing device according to the fourth embodiment of the present invention (with BPF weighting).
FIG. 9 is a block diagram of a conventional audio signal reproducing apparatus. FIG. 10 is a spectrum diagram of a conventional audio signal reproducing apparatus.
1 High-frequency additional signal generator 2 Adder 3 Half-wave rectifier 4 Harmonic adjustment means 5 LPF
6 BPF
7 level detector 8 level adjuster a digital audio signal b high frequency additional signal c reproduction signal

Claims (5)

Harmonic generation means for generating harmonics from the digital audio signal; harmonic adjustment means for adjusting the generated harmonics; and addition means for adding the correction signal adjusted by the harmonic adjustment means to the digital audio signal. An audio signal reproduction method for correcting a high-frequency component that exceeds a high-frequency upper limit of a digital audio signal reproduction frequency to prevent aliasing noise generated at the time of harmonic generation before the harmonic generation unit or An audio signal reproducing apparatus comprising a means for reducing. 2. An audio signal reproducing apparatus according to claim 1, wherein said harmonic adjustment means is means for extracting a desired frequency band. 2. An audio signal reproducing apparatus according to claim 1, wherein said harmonic adjustment means is means for extracting a weighted frequency band. The harmonic adjustment means further comprises means for extracting a desired frequency band of the harmonic, and level tracking means for detecting the level of the digital audio signal and adjusting the level of the harmonic or the desired frequency band. The audio signal reproducing apparatus according to claim 1. A means for adjusting the level of the harmonics or the weighted desired frequency band by detecting the level of the digital audio signal by means for taking out the desired frequency band by the harmonic adjustment means weighting the harmonics; 2. The audio signal reproducing apparatus according to claim 1, further comprising a follower.

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