Patent Grant
8145478
The present invention relates to an apparatus and a method for expanding or widening an audio signal band, and particularly to the apparatus and method for widening the band of an input audio signal by applying digital processing to the input audio signal.
Patent document 1 discloses a conventional audio signal reproduction apparatus which generates high-order harmonic components based on a signal read from a recording medium and adds them to the read signal to realize a natural sound reproduction.
A digital audio signal inputted via an input terminal T1 is subjected to oversampling processing in the low pass filter 171. Then, the harmonic generating circuit 174 including the absolute value circuit 173 and the multiplier 172 generates harmonic signals based on an audio signal generated by the oversampling processing. The high pass filter 175 allows only high frequency components of the generated harmonic signal to pass through the filter 175. The adder 176 adds the output signal from the high pass filter 175 to the audio signal generated by the oversampling processing. The D/A converter 177 converts the added audio signal to an analogue signal, thus generating an audio signal of which band is expanded or widened and outputting the band expanded audio signal via an output terminal T2.
Patent document 1: JP-A-07-93900
As described above, the harmonic signals are generated based on the original audio signal, and the generated harmonic signals are added to the original audio signal, thereby expanding a width of high frequency band. However, the aforementioned conventional audio signal reproduction apparatus has a problem shown below.
(1) The harmonic signal generated by the harmonic generating circuit 174 includes only odd-order harmonic components.
(2) A level of each order of the generated harmonic component is fixed.
A natural musical sound has even-order harmonic components and odd-order harmonic components, and the level of each harmonic component is different for each musical sound. Therefore, an audio signal having characteristic of the aforementioned (1) and (2) has a different structure of harmonics from a natural musical signal and has a tone quality providing discomfort in a listening sense, depending on an input audio signal.
In order to solve the above-described problem, the present invention is provided, and the present invention has an object to provide an apparatus and method for expanding an audio signal band including the higher harmonic components that has a harmonic structure close to that of a natural musical sound, and being capable of reproducing a voice signal with no sense of incompatibility and deterioration in tone quality.
In a first aspect of the invention, an apparatus for expanding a band of an audio signal is provided. The apparatus includes an input section for inputting an audio signal having a predetermined band, a harmonic generator for generating harmonic signals based on the input audio signal, and an adder for adding the harmonic signals generated by the harmonic generator to the input audio signal. The harmonic generator simulates input-output characteristics of a predetermined amplifier or a device included in the amplifier to generate the harmonic signals from the input audio signal.
In a second aspect of the invention, an audio reproduction apparatus is provided. The audio reproduction apparatus includes a signal reproducing unit for reproducing an audio signal from a recording medium storing audio information, the band expanding apparatus according to the invention, for expanding a band of an audio signal reproduced by the signal reproducing unit, and an amplifier for amplifying the audio signal outputted from the band expanding apparatus.
In a third aspect of the invention, a method of expanding a band of an audio signal is provided. The method includes inputting an audio signal having a predetermined band, simulating input-output characteristics of a predetermined amplifier or a device included in the amplifier to generate harmonic signals from the input audio signal, and adding the generated harmonic signals to the input audio signal.
According to the present invention, the harmonic components are generated in a higher frequency range than that of the input digital audio signal, having the same spectral structure as that of the input audio signal. The harmonic components thus generated are added to the input audio signal, thereby expanding or widening the band. Particularly, according to the present invention, input-output characteristics of an audio amplifier and/or a circuit included in the audio amplifier is simulated to generate the harmonic components. Therefore, with simulation for the characteristic of a device regarded as having an excellent tone quality and an amplifier using such a device, it is possible to generate harmonic components equivalent to those generated by such a device and the amplifier using the device. The harmonic components thus generated include even-order and odd-order harmonics, thereby providing a harmonic structure close to that of a natural musical sound. Therefore, it is possible to realize a reproduction signal having natural tone quality without auditory discomfort and deterioration in tone quality, by providing such harmonic components into a final output audio signal.
Hereinafter, preferred embodiments of the present invention will be explained with reference to the appended drawings. It is noted that in the appended drawings, the same reference signs and numerals are assigned to the same constituent elements.
Embodiment 1
The audio signal band expanding apparatus 100a having the above-described configuration will be explained.
In
As shown in
For example, as shown in
The digital low pass filter 32 has (a) a passing band of frequency of 0 to 0.45fs, (b) a rejection band of frequency of 0.45fs to fs, and (c) an attenuation quantity of 60 dB or more with a frequency of fs or more. The filter 32 allows predetermined low frequency components of the input digital audio signal to pass through itself. The digital low pass filter 32 limits the band so as to remove a return noise generated by the aforementioned oversampling processing. The filter 32 allows only a substantially effective band (frequency of 0 to 0.45fs) of the input digital audio signal to pass through itself, and outputs the signal of the passing band to the adder 2 and the harmonic generating circuit 3.
The harmonic generating circuit 3 is a nonlinear processing circuit having nonlinear input-output characteristics. The harmonic generating circuit 3 applies nonlinear processing to the input digital audio signal, thereby distorting the digital audio signal to generate the harmonic signal and outputting the generated harmonic signal to the digital band-pass filter 4. A concrete realizing method is as follows. That is, the method includes simulating the amplifier with small signal parameters of devices included in the amplifier, that is, calculating the output signal of an audio amplifier by a DSP (Digital Signal Processor), by software processing by a processor, or by hardware processing in a digital circuit, to generate the harmonic signal of the input signal.
The harmonic generating circuit 3 simulates the input-output characteristics of a vacuum tube amplifier as shown in
A current and voltage of each device is expressed as follows, with a voltage signal vin applied to the input of the vacuum tube amplifier shown in
When 12AX7 by RCA Corporation is used in the triode 21, a constant example is shown below.
[Example Constants]
Conceptually, the output of the oversampling type low pass filter 1 is inputted as vin in
Actually, by using the aforementioned formula and constant, vout is calculated by software or hardware, to thereby generate the harmonics.
The level of the signal inputted in the harmonic generating circuit 3 can be changed by the level adjuster 15. Changing of level of a signal inputted to the harmonic generating circuit 3 causes a spectral structure of the harmonic signal outputted from the harmonic generating circuit 3 to be changed. Specifically, the larger the signal level inputted to the harmonic generating circuit 3 is, the larger the higher harmonic level becomes, while the smaller the inputted signal level is, the smaller the higher harmonic level becomes. This is because the harmonic generating circuit 3 simulates the audio amplifier having the nonlinear characteristic and the spectral structure of harmonics for the output signal is changed based on the level of the input signal. For example, the audio amplifier has the nonlinear characteristic in that the audio amplifier can not output a signal having the amplitude of not less than a power supply voltage. In addition, the devices included in the audio amplifier also have nonlinear characteristic which particularly become prominent in the vicinity of the power supply voltage. Therefore, when an amplitude level of the input signal is made larger, the nonlinearity strongly appears in the output signal. Such a phenomenon causes the harmonic level to change. Change of the signal level inputted to the harmonic generating circuit 3 causes the spectral structure of the harmonics to change, thus making it possible to control the tone quality.
As shown in
(1) Cutoff frequency fLC of a low frequency region=approximately fs/4.
(2) Cutoff characteristic of the low frequency region shows attenuation quantity of 80 dB or more at a frequency of fs/4. The attenuation quantity is laid in the vicinity of S/N ratio based on quantization numbers of the original sound. For example, when the quantization number of the original sound is 16 bits, a theoretical S/N ratio is 98 dB, and therefore the attenuation quantity is preferably in a range from 80 to 100 dB or more. Gradual cutoff characteristic of the low frequency region produces a soft tone quality, while steep cutoff characteristic of the low frequency region produces a sharp tone quality. In the latter case, effect of band expanding can be achieved without deteriorating trend of a tone quality of the original sound. Accordingly, the cutoff characteristic of the low frequency region of the digital low pass filter 7 may be made switchable from an external controller so as to be selectively changed between the aforementioned two characteristics according to an instruction signal by a user.
(3) Cutoff frequency of fHC of a high frequency region=approximately fs/2.
(4) Cutoff characteristic of the high frequency region is −6 dB/oct (see
The 1/f characteristic filter 8 is so-called low pass filter of 1/f characteristic, provided with attenuation characteristic having an inclination of −6 dB/oct in the frequency band B2 ranging from fs/2 to p*fs/2, which is higher than the frequency band B1 ranging from 0 to fs/2, as shown in
As described above, the band-pass filter 4 filters the digital signal inputted from the harmonic generating circuit 3. A digital signal which is filtered and provided for band-expanding is outputted to the adder 2 via the variable amplifier 5.
The adder 2 adds the digital signal for band-expanding is expanded from the variable amplifier 5 to the low-pass filtered digital audio signal from the oversampling type low pass filter 1. Then, the digital audio signal resulting from the addition including the digital signal for band-expanding and digital audio signal of the original sound is outputted via the output terminal T2.
The variable amplifier 5 is a level control circuit. The variable amplifier 5 changes a level (amplitude level) of the input signal with an amplification factor based on a control signal and outputs the signal with the level changed to the adder 2. Note that the amplification factor can be set to a value at which both positive and negative amplification processes are possible. That is, the variable amplifier 5 can amplify the input signal and control a normal/reverse control of attenuation and phase. The variable amplifier 5 is used for relatively adjusting a level of the digital audio signal from the oversampling type low pass filter 1 and a level of the digital signal for band-expanding from the band-pass filter 4. Preferably, in the adder 2, the adjustment is made so that levels of these two signals are identical at the frequency of fs/2, for example, that is, so that continuity of the spectrum is maintained. In addition, the adjustment may be changed according to a listener's taste.
As described above, according to this embodiment, the band is expanded in such a way that the harmonic signals having the spectral structure similar to that of the musical sound are generated in a frequency band higher than an original frequency band of the input audio signal. Then, the frequency band of the harmonic signals is limited by the band-pass filter 4 and the level of the harmonic signal is controlled. Finally, the harmonic signals are added to the input audio signal, thereby expanding the band of the input audio signal. The higher harmonics thus generated includes the even-order harmonics which provide good tone quality and are considered to be comfortable in listening.
Particularly, according to this embodiment, the harmonic generating circuit 3 simulates the input-output characteristics of the audio amplifier or the devices composing the audio amplifier to generate the harmonics. The simulating generates harmonics equivalent to those generated by the amplifier or the devices composing the amplifier. For example, simulating the characteristic of the amplifier evaluated to have a good tone quality or the devices composing the amplifier allows the harmonics having better tone quality and being comfortable to the listener to be generated. Generally, there is an organoleptic evaluation in that the vacuum tube amplifier has a more preferable tone quality than that of the amplifier including semiconductor devices, or there is a difference in tone quality between these amplifiers. Therefore, the band can be widened maintaining tone quality characteristic of the vacuum tube amplifier by making a simulation so as to include tone quality characteristic (input-output characteristics) of the vacuum tube amplifier and generating the harmonics.
In addition, change of a setting of parameters for simulation allows the level of each order of the harmonic component to be easily changed. A difference in devices (such as vacuum tube) or a difference in circuit configuration (configuration of output stage is single or push-pull) produces a difference in tone quality. Also, the difference in tone quality can be reflected on the characteristic of the generated harmonics by suitably setting the parameters, thus achieving the band expanding using the characteristic of the devices or the circuit configuration.
According to the above-described embodiment, the harmonic generating circuit 3 simulates the vacuum tube amplifier using a triode to generate the harmonics. However, a target to be simulated may be an arbitrary circuit or device. The effect in tone quality which is similar to the effect due to the harmonics generated by the simulated circuit or device can be obtained.
In addition, in the above-described embodiment, the band-pass filter 4 limits the band of the output of the harmonic generating circuit 3, and thereafter the variable amplifier 5 changes the level. However, the level may be changed in first and thereafter the band may be limited. The similar advantage can be obtained.
In addition, according to the above-described embodiment, the harmonic generating circuit 3 models the characteristic (input-output characteristics) caused by the audio amplifier or device included in the audio amplifier to generate the harmonics. However, other audio equipment (such as a speaker or a cartridge) may be modeled for simulation. In this case also, the band expanding effect can be similarly obtained. For example, the harmonic generating circuit 3 can also be realized by applying impulse response of certain audio equipment (such as a speaker or a cartridge) to the digital filter.
Further, although the 1/f characteristic filter is used in the above-described embodiment, instead of it a 1/f2 characteristic filter having attenuation characteristic shown in
In the above-described embodiment, explanation is made to a preferable specification of the band-pass filter 4 for an input digital audio signal which is not compressed and originates from the CD or the like. When the input digital audio signal is a digital signal (hereinafter referred to as “MD signal”) from a MD (Mini Disc), or a digital audio signal (hereinafter referred to as “AAC signal”) compressed and encoded by AAC (Advanced Audio Coding) used in an audio signal of MPEG-4, it is preferable to set the cutoff frequency fs/2 of the low and high frequency regions of the band-pass filter 4 at an upper limit frequency of a reproduction band of these compressed audio signals. Sampling frequencies fs of an MD signal and an AAC signal are, for example, set at 44.1 kHz or 48 kHz, and the sampling frequency fs for a half rate signal of the AAC signal is set at 22.05 kHz or 24 kHz. In the former case, the upper limit frequency of the reproduction band is approximately set at 10 kHz or 18 kHz. In the latter case, the upper limit frequency of the reproduction band is set at approximately 5 kHz or 9 kHz.
Embodiment 2
The harmonic generating circuit 3 generates the harmonics based on the output signal of the oversampling type low pass filter 1. The variable amplifier 5 changes the level of the harmonics to be added by the adder 2.
The noise signal generating circuit 8 generates a random noise. The random noise has the frequency band from 0 to p*fs/2, is not correlated to the input audio signal, and has a random amplitude level with respect to the time axis. Here, “fs” is a sampling frequency of the audio signal inputted from the input terminal T1, and “p” is the oversampling rate of the oversampling type low pass filter 1.
Each PN sequences generating circuit 60-n has an independent initial value. Each PN sequences generating circuit 60-n generates, for example, a pseudo noise signal which is an M-series noise signal having a uniform random amplitude level, and outputs it to the adder 61. Subsequently, the adder 61 sums up the plurality of (N) pseudo noise signals outputted from the plurality of PN sequences generating circuits 60-1 to 60-N, and outputs the sum of the pseudo noise signals to the subtractor 64. Meanwhile, the DC offset removing constant signal generator 63 generates a DC offset removing constant signal which is a sum of time average values of the pseudo noise signals from the plurality of (N) PN sequences generating circuits 60-1 to 60-N, and outputs the generated signal to the subtractor 64. Then, the subtractor 64 subtracts the DC offset removing constant signal from the sum of the pseudo noise signals, thereby generating and outputting a dither signal having no DC offset.
In an example of
(1) bit positions of the 8-bit PN sequences noise signal taken out from the 32-bit counter 71 may be mutually differentiated. That is, the PN sequences generating circuit 60-1 takes out the 8-bit PN sequences noise signal from the least significant 8 bits, and the PN sequences generating circuit 60-2 takes out the PN sequences noise signal from 8 bits just above the least significant 8 bits. In other PN sequences generating circuit 60-n also, the PN sequences noise signal is similarly taken out;
(2) the bit position of the 32-bit counter 71 for taking out one bit data inputted to the exclusive OR gate 72 may be mutually differentiated in each PN sequences generating circuits 60-n; or
(3) at least two of the example shown in
Then, by adding mutually independent plurality of PN sequences noise signals, the PN sequences noise signal having a probability density with respect to the amplification level can be generated, as shown in
The level detector 11 detects a level fluctuation of the original audio signal which is subjected to the oversampling processing. Gains of the variable amplifier 5 and the variable amplifier 9 are changed according to a detection result of the level detector 11. As shown in
The noise signal generated by the noise signal generating circuit 8 is inputted to the variable amplifier 9 which changes the level of the noise signal. Meanwhile, the output signal of the harmonic generating circuit 3 is inputted to the variable amplifier 5 which changes the level of the output signal. Output signals from the variable amplifier 5 and the variable amplifier 9 are added by the adder 10. Note that each gain of the variable amplifier 5 and the variable amplifier 9 is changed according to the detection result of the level detector 11. The band-pass filter 4 limits the band of the added signal, thus generating the signal for expanding the band. Then, the adder 2 adds the signal for band-expanding to the output of the oversampling type low pass filter 1 to generate the audio signal of which band is expanded.
As described above, in this embodiment also, the band is expanded by simulating the amplifier or the device composing the amplifier. The similar advantage to that of Embodiment 1 is achieved, in which the even-order harmonics with good tone quality and comfortable in listening can be generated.
Further, in this embodiment, the noise signal generating circuit 8 generates a signal for band-expanding which is not correlated to the input signal, and based on this signal the band-expanded signal is generated. Therefore, it is possible to realize band expanding of the audio signal with no incompatible sense in listening and with further less deterioration in the tone quality, compared to a case of expanding the band only by using the harmonics generated from the input signal, as shown in Embodiment 1.
In the audio signal band expanding apparatus 100b of this embodiment, the level adjuster 15 may be inserted before the harmonic generating circuit 3.
Embodiment 3
The quantization noise generating circuit 12 applies a first-order delta sigma modulation (called Δ-Σ modulation, or also called sigma/delta (Σ-Δ) modulation) to the digital audio signal from the oversampling type low pass filter 1 to generate the quantization noise. Thus, a simulated wide band random noise signal correlated to the input signal is generated.
The digital audio signal from the oversampling type low pass filter 1 is inputted to the subtractor 81. The subtractor 81 subtracts the digital audio signal sent from the delay circuit 84, from the digital audio signal sent from the oversampling type low pass filter 1 to output the digital audio signal as the subtraction result to the quantizer 82 and the subtractor 83. The quantizer 82 re-quantizes the inputted digital audio signal to output a delta-sigma modulation signal which is the re-quantized digital audio signal to the subtractor 83. The subtractor 83 subtracts the delta-sigma modulation signal sent from the quantizer 82, from the digital audio signal sent from the subtractor 81 to output a quantization noise signal which is the digital audio signal as the subtraction result (generated at the time of quantization). The quantization noise signal is outputted to the subtractor 81 via the delay circuit 84.
Turning to
Meanwhile, the harmonic generating circuit 3 generates the harmonic signals based on the digital audio signal sent from the oversampling type low pass filter 1.
The level detector 11 detects a level fluctuation of the original audio signal which is subjected to the oversampling processing, and changes the gain of the variable amplifier 5 or 9 by its detection result.
The adder 10 sums up the harmonic signals which are generated by the harmonic generating circuit 3 and amplified by the variable amplifier 5 and the noise signal which is generated by the quantization noise generating circuit 12 and amplified by the variable amplifier 5. The digital band-pass filter 4 limits the band of the output signal of the adder 10 to generate a signal for band-expanding. The adder 2 adds the signal for band-expanding to the inputted digital audio signal. Thus, the band expanding is achieved.
Hence, according to this embodiment, regarding the noise signal, the random signal which is generated based on the digital audio signal of the original sound is used for a signal for band-expanding. Thus, in addition to an effect of Embodiment 1, a specific advantage allowing a listener to hear sound more naturally can be obtained compared to when using the signal for band-expanding including only the harmonics.
Note that in this embodiment, the first order delta-sigma modulation type quantizer is used. However, the present invention is not limited thereto, and a multiple order delta-sigma modulation type quantizer may also be used.
Also, in this embodiment, the delta-sigma modulation type quantizer is used. However, the present invention is not limited thereto, and a sigma-delta modulation type quantizer that performs sigma-delta modulation to the input audio signal may also be used.
In addition, in this embodiment, the delta-sigma modulation type quantizer is used. However, the present invention is not limited thereto, and an error signal, which is generated at the time of expanding the input audio signal after compressing it, may be outputted from the quantization noise generating circuit 12.
In addition, in the audio signal band expanding apparatus 100c, the level adjuster 15 may be inserted before the harmonic generating circuit 3.
In the above-described embodiments 1 to 3, the audio signal band expanding apparatus is constituted of the digital signal processing circuit of hardware. However, the present invention is not limited thereto. For example, the function of each processing part of the audio signal band expanding apparatus shown in
In addition, the recording medium storing an audio signal is not limited to a CD, but may be other kind of recording medium (DVD (Digital Versatile Disk), and so on).
Embodiment 4
In the audio reproduction apparatus 120, the signal reproduction unit 101 reads audio information from the CD 200, and reproduces the digital audio signal. The band expander 100 has the same configuration and function as the audio signal band expanding apparatus described in any one of Embodiments 1 to 3, which can expand the band of the digital audio signal reproduced by the signal reproduction unit 101. The digital audio signal with the expanded band is converted to an analogue audio signal by the D/A converter 103 with a predetermined high frequency band being cut by the low pass filter 105, and is finally outputted as the audio signal.
The audio signal outputted from the audio reproduction apparatus 120 is amplified by the analogue power amplifier 150 and is inputted to the speaker. Thus, the voice is outputted from the speaker 160.
The digital power amplifier 104 amplifies the digital audio signal with the band expanded by the band expander 100, and converts the digital audio signal to the analogue audio signal. The high frequency band region of the audio signal amplified by the digital power amplifier 104 is cut by the low pass filter 105, and the audio signal with the cut high frequency band region is outputted from the speaker 160.
According to the audio reproduction system of this embodiment, the band of the audio signal is expanded by adding a harmonic signals to the original band of the digital audio signal reproduced from the recording medium such as a CD. Thus, a natural tone quality in listening for human beings can be reproduced. In addition, by simulating the input-output characteristics having excellent performance and generating the higher harmonic components for expanding the band, the tone quality comfortable in listening by the human beings can be reproduced.
Specific embodiments of the present invention have been explained, but it is apparent for a person skilled in the art that other many modified examples, amendment, and other usage are possible. Therefore, the present invention is not limited to specific disclosure here, and can be limited only within the scope of the appended claims. Note that this application relates to Japanese Patent Application No. 2005-167956 (filed on Jun. 8, 2005), and contents thereof are incorporated herein by reference.
Industrial Applicability
According to the present invention, a high band components generated based on an original audio signal is added to the original audio signal to generate an audio signal with the expanded band, thus realizing a natural tone quality in listening. Therefore, the present invention is useful for an apparatus for reproducing an audio signal which does not include signal components of a predetermined or more frequency band such as a reproduction signal from a compact disk.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-167956 | Jun 2005 | JP | national |
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|---|---|---|---|---|
| PCT/JP2006/309561 | 5/12/2006 | WO | 00 | 12/6/2007 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2006/132054 | 12/14/2006 | WO | A |
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