There is provided a quantizing error reducer for an audio signal, which is constructed to feed a quantizing error back to the input side of a quantizer through a noise filter, wherein the coefficient of the noise filter is set on the basis of information relating to the equiloudness curve, thereby making it possible to reduce the noise in the hearing sense.
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0. 16. A recording medium having recorded thereon digital signal, said recorded digital signal being a quantized digital audio signal, wherein a quantized noise signal included said recorded digital signal is concentrated in the high frequency range.
0. 43. A disc recording medium having quantized signals recorded thereon, said quantized signals being produced by subtracting an error feedback signal from an input digital audio signal and quantizing the result, said error feedback signal being produced by filtering a difference signal indicative of the difference between the input and the output of said quantizer, a filter characteristic of said filtering corresponding to an equi-loudness characteristic.
0. 17. A method for recording digital signals on a record medium comprising the steps of:
receiving an input digital audio signal; subtracting an error feedback signal from said input audio signal to produce a first signal; quantizing said first signal; generating a difference signal indicative of a difference between said first signal and the quantized first signal; filtering said difference signal to produce said error feedback signal; and recording said quantized first signal on a record medium, wherein a filtering characteristic is set on the basis of information relating to an equi-loudness characteristic.
0. 9. An apparatus for transmitting digital signals comprising:
means for receiving an input digital audio signal; means for subtracting an error feedback signal from said input digital audio signals quantizing means for quantizing an output signal from said subtracting means; means for generating a difference signal indicating a difference between said output signal from said subtracting means and an output signal of said quantizing means; a filter for filtering said difference signal to produce said error feedback signal; transmitting said output signal from said quantizing means; and controlling-means for controlling the characteristic of said filter.
0. 12. A method for recording digital signals comprising the steps of:
receiving an input digital audio signal; subtracting an error feedback signal from said input digital audio signal to produce a first signal; quantizing said first signal; generating a difference signal indicating a difference between said first signal and the quantized first signal; filtering said difference signal to produce said error feedback signal; and recording said quantized first signal, wherein a filtering characteristic is set so as to cause the frequency spectrum of a quantization noise signal included in a quantized first signal to be concentrated in the high frequency range.
1. A quantizing noise reducer for an audio signal comprising:
quantizing means for quantizing an input audio signal, subtracter means for subtracting an input signal to said quantizing means from an output signal from said quantizing means, filter means supplied with an output from said subtractor means, means for synthesizing an output from said filter means and said input audio signal, and filter control means for controlling a characteristic of said filter means, and including filter coefficient calculating means and data generating means for generating data relating to an equi-loudness characteristic, said data being supplied to said filter coefficient calculating means.
0. 13. A method for recording digital signals comprising the steps of:
receiving an input digital audio signal; subtracting an error feedback signal from said input digital audio signal to produce a first signal; quantizing said first signal; generating a difference signal indicating a difference between said first signal and the quantized first signal; filtering said difference signal to produce said error feedback signal; and recording said quantized first signal, wherein a filtering characteristic is set so as to cause the frequency spectrum of a quantization noise signal included in said quantized first signal to be coincident with the spectrum of said digital audio signal.
0. 33. A method to transmit digital signals, comprising:
receiving an input digital audio signal; subtracting an error feedback signal from the digital input audio signal to produce a first signal; quantizing the first signal; generating a difference signal indicative of a difference between the first signal and the quantized first signal; filtering the difference signal to produce the error feedback signal; analyzing the frequency spectrum of the input digital signal to produce second information; and transmitting the quantized first signal, wherein the filtering characteristic is controlled according to first information relating to an equi-loudness characteristic and the second information.
0. 40. A method to transmit digital signals, comprising:
receiving an input digital audio signal; subtracting an error feedback signal from the digital input audio signal to produce a first signal; quantizing the first signal; generating a difference signal indicative of a difference between the first signal and the quantized first signal; filtering the difference signal to produce the error feedback signal; and transmitting the quantized first signal, wherein the filtering characteristic is set on the basis of first information relating to an equi-loudness characteristic, and wherein the first information is generated on the basis of an equi-loudness characteristic in which low frequency band is corrected to be flat.
0. 42. A method to transmit digital signals, comprising:
receiving an input digital audio signal; subtracting an error feedback signal from the digital input audio signal to produce a first signal; quantizing the first signal; generating a difference signal indicative of a difference between the first signal and the quantized first signal; filtering the difference signal to produce the error feedback signal; and transmitting the quantized first signal, wherein the filtering characteristic is set on the basis of first information relating to an equi-loudness characteristic, and wherein the first information is generated on the basis of an equi-loudness characteristic in which low frequency band is corrected to be heightened.
0. 14. A method for recording digital signals comprising the steps of:
receiving an input digital audio signal; subtracting an error feedback signal from said input digital audio signal to produce a first signal; quantizing said first signal; generating a difference signal indicating a difference between said first signal and the quantized first signal; filtering said difference signal to produce said error feedback signal; and transmitting said quantized first signal, wherein a filtering characteristic is set so as to cause the frequency spectrum of a quantization noise signal included in said quantized first signal to be coincident with the spectrum of said input digital audio signal and to be concentrated in the high frequency range.
0. 32. A method to transmit digital signals, comprising:
receiving an input digital audio signal; subtracting an error feedback signal from the digital input audio signal to produce a first signal; quantizing the first signal; generating a difference signal indicative of a difference between the first signal and the quantized first signal; filtering the difference signal to produce the error feedback signal; and transmitting the quantized first signal, wherein the filtering characteristic is set on the basis of first information relating to an equi-loudness characteristic, and wherein the filter characteristic is set on the basis of a masking effect of an ordinary audio signal that includes many medium and low frequency band components.
0. 35. A method to transmit digital signals, comprising:
receiving an input digital audio signal; subtracting an error feedback signal from the digital input audio signal to produce a first signal; quantizing the first signal; generating a difference signal indicative of a difference between the first signal and the quantized first signal; filtering the difference signal to produce the error feedback signal; analyzing the frequency spectrum of the input digital signal to produce second information; detecting the amplitude of the input digital audio signal; and transmitting the quantized first signal, wherein the filtering characteristic is controlled according to first information relating to an equi-loudness characteristic, the second information, and the detected amplitude.
0. 6. An apparatus for transmitting digital signals comprising:
means for receiving an input digital audio signal; means for subtracting an error feedback signal from said input digital audio signal; quantizing means for quantizing an output signal from said subtracting means; means for generating a difference signal indicating a difference between said output signal from said subtracting means and an output signal of said quantizing means; a filter for filtering said difference signal to produce said error feedback signal; and means for transmitting said output signal from said quantizing means, wherein said filter has a characteristic causing the spectrum of a quantization noise signal component included in said output signal from said quantizing means to be concentrated in the high frequency range.
0. 7. An apparatus for transmitting digital signals comprising:
means for receiving an input digital audio signal; means for subtracting an error feedback signal from said input digital audio signal; quantizing means for quantizing an output signal from said subtracting means; means for generating a difference signal indicating a difference between said output signal from said subtracting means and an output signal of said quantizing means; a filter for filtering said difference signal to produce said error feedback signal; and transmitting said output signal from said quantizing means, wherein said filter has a characteristic causing the spectrum of a quantization noise signal component included in said output signal from said quantizing means to be coincident with the spectrum of said input digital audio signal.
0. 8. An apparatus for transmitting digital signals comprising:
means for receiving an input digital audio signal; means for subtracting an error feedback signal from said input digital audio signal; quantizing means for quantizing an output signal from said subtracting means; means for generating a difference signal indicating a difference between said output signal from said subtracting means and an output signal of said quantizing means; a filter for filtering said difference signal to produce said error feedback signal; and transmitting said output signal from said quantizing means, wherein said filter has a characteristic causing the spectrum of a quantization noise signal component included in said output signal from said quantizing means to be coincident with the spectrum of said input digital audio signal and to be concentrated in the high frequency range.
0. 31. An Apparatus for recording a digital signal on a disc record medium, comprising:
first means for receiving an input digital audio signal; means connected to said first means for subtracting an error feedback signal from said input digital audio signal to produce a first signal; quantizer means connected to receive said first signal and to produce a quantized first signal; means connected to receive said first signal and said quantized signal to produce a difference signal indicative of a difference between said first signal and said quantized first signal; a filter means connected to receive said difference signal, said filter means having a filter characteristic set on the basis of an equi-loudness characteristic; means for connecting an output of said filter means as said error feedback signal; and means for recording said quantized first signal on said disc recording medium.
2. A quantizing noise reducer according to
3. A quantizing noise reducer according to
4. A quantizing noise reducer according to
5. A quantizing noise reducer according to
0. 10. An apparatus according to
0. 11. An apparatus according to
0. 15. A method according to
controlling said filtering characteristic so as to cause that the frequency spectrum of a quantization noise signal included in said quantized first signal to be coincident with the detected frequency spectrum of said input digital audio signal.
0. 18. A method according to
0. 19. A method according to
0. 20. A method according to
analyzing the frequency spectrum of said input digital audio signal to produce second information; and controlling said filtering characteristic according to said first and second information.
0. 21. A method according to
0. 22. A method according to
0. 23. A method according to
detecting the amplitude of said input digital audio signal, wherein said filtering characteristic is controlled on the basis of the detected amplitude.
0. 24. A method according to
0. 25. A method according to
0. 26. A method according to
0. 27. A method according to
0. 28. A method according to
0. 29. A method according to
0. 30. A method according to
0. 34. A method according to
0. 36. A method according to
0. 37. A method according to
0. 38. A method according to
0. 39. A method according to
0. 41. A method according to
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A first embodiment to which this invention is applied will now be described with reference to the attached drawings.
Initially referring to
In the quantizing error reducer of the embodiment shown in
Namely, in the quantizing error reducer of this embodiment, by an adder 12 for subtracting an input to the quantizer 11 from an output from the quantizer 11 to thereby provide a quantizing error generated in quantization at the quantizer 11, a noise filter 13 for applying a filtering processing to an output from the adder 12 to output it, in which a filter characteristic is set by the filter coefficient which will be described later, and an adder 10 for adding an output from the noise filter 13 to an input to the quantizer 11, so called an error feedback circuit is constituted. By this error feedback circuit, the quantizing error reducing effect (so called noise shaping processing) is carried out. Further, the quantizing error reducer of this embodiment includes an equi-loudness curve generation circuit 15 for generating data of an equi-loudness curve RC of
Here, the equi-loudness curve RC is a curve corresponding to the hearing sense characteristic of the human being. This curve RC is obtained by connecting, by curve segments, sound pressures of a sound at respective frequencies which can be heard at the same loudness as that of a pure sound of, e.g., 1 KHz, and is also called an equi-sensitivity curve of loudness. In the equiloudness curve RC, as shown in
Information (information of an allowable noise spectrum) relating to the equi-loudness curve RC (or its approximate curve) is outputted from the equi-loudness curve generation circuit 15, and is then sent to the filter coefficient calculation circuit 14. Accordingly, at the filter coefficient calculation circuit 14, a filter coefficient is calculated on the basis of the information relating to the equi-loudness curve RC. The filter coefficient thus calculated is further sent to the noise filter 13. In this way, by carrying out a noise shaping of an audio signal by the error feedback circuit using the noise filter 13 having a filter characteristic based on the information relating to the equi-loudness curve RC, the dynamic range in the hearing sense can be improved. Namely, by carrying out a noise shaping using an allowable noise spectrum (allowable noise level) obtained by taking the equi-loudness curve RC into account, noise shaping more effective in view of the hearing sense is conducted, thus making it possible to improve the dynamic range in the hearing sense of a reproduced sound.
Further, in this embodiment, in determination of the filter characteristic of the noise filter 13, the so called masking effect is taken into consideration. Here, the masking effect is the phenomenon that a signal is masked by another signal by the characteristic in the hearing sense of the human being, so sound cannot be heard. As this masking effect, there are the masking effect with respect to a signal on the time base, and the masking effect with respect to a signal on the frequency base (or the same time masking, the temporal masking). Even if there is a noise in the portion subject to masking, that noise cannot be heard by the masking effect. For this reason, when an approach is employed to carry out a quantizing error reducing processing in which the masking effect is taken into consideration, the dynamic range in the hearing sense can be improved. In order to determine a filter characteristic in which such a masking effect is taken into consideration, a filter coefficient in which, e.g., the masking effect in the direction of the frequency base is taken into consideration is set in advance at the filter coefficient calculation circuit 14 of this embodiment. For example, in order to cope with an ordinary audio sound including many medium and low frequency band components, a fixed filter coefficient in which the masking effect in the low frequency band is taken into consideration is set. Alternatively, in order to have an ability of coping with the masking effect corresponding to a spectrum of an input audio signal, an approach may be employed to generate an adaptive filter coefficient corresponding to the spectrum.
Thus, the filter coefficient from the filter coefficient calculation circuit 14 is provided under the state where the equi-loudness curve RC and the masking effect are taken into consideration. Accordingly, the filter characteristic of the noise filter 13 is set on the basis of the fixed or adaptive filter coefficient in which the masking effect is taken into consideration and the filter coefficient relating to the equi-loudness curve RC.
Namely, the noise filter 13 at this time serves as a filter having a filter characteristic as indicated by the curve MR obtained from the masking effect and the equi-loudness curve as shown in FIG. 3. By allowing the noise filter 13 to have the filter characteristic indicated by the curve MR of
A third embodiment utilizing the masking effect will now be described with reference to FIG. 6. Also in
The quantizing noise reducer of this embodiment of
Namely, in the quantizing error reducer of this embodiment, by the adder 12 for subtracting an input to the quantizer 11 from an output from the quantizer 11 to thereby provide a quantizing error generated in quantization at the quantizer 11, the noise filter 13 for applying filtering processing to an output from the adder 12 to output it, and the adder 10 for adding an output from the noise filter 13 to an input to the quantizer 11, the so called error-feedback circuit is constituted. Here, the filter characteristic of the noise filter 13 is determined as follows. In actual terms, an approach is employed to calculate, by the filter coefficient calculation circuit 14, a filter coefficient based on information of an allowable noise spectrum which will be described of the allowable noise spectrum calculation circuit 18 to send this filter coefficient information to the noise filter 13. Accordingly, in the above-mentioned error feedback circuit, a quantizing error reducing processing (so called noise shaping processing) based on the allowable noise spectrum which will be described is carried out. A signal thus processed is then outputted from the output terminal 2.
Meanwhile, in carrying out the quantizing error reducing processing (noise shaping processing) of an audio signal by using the above-mentioned error feedback circuit, by carrying out a processing in which so called masking of the input signal spectrum is taken into account, the dynamic range in the hearing sense can be improved. As the noise shaping in which the masking is taken into consideration, there may be enumerated, e.g., a noise shaping corresponding to a spectrum of an input audio signal in which the pattern of a signal spectrum is fixed to some extent, i.e., a noise shaping using an allowable noise spectrum obtained in consideration of so called masking which will be described later of an input audio signal spectrum. Alternatively, there is enumerated a noise shaping using an allowable noise spectrum adaptive with respect to changes in the spectrum of an input audio signal obtained in consideration of the masking of the spectrum, or the like. Here, the masking is the phenomenon that a signal is masked by another signal by the characteristic in the hearing sense of the human being, so sound is not heard. As the masking effect, there are the masking effect with respect to a signal on the time base and the masking effect with respect to a signal on the frequency base (or the same time masking, temporal masking). Even if there is a noise at the portion subjected to masking, that noise is difficult to be heard by the masking effect. For example, as the same time masking effect, as shown in
Further, when an approach is employed to band-divide an input signal at the critical band by making use of the hearing sense characteristic of the human being to carry out noise shaping, every band, by using an allowable noise spectrum in which the masking as described above is taken into consideration, noise shaping more effective in the hearing sense can be carried out. By carrying out such a noise shaping, the dynamic range in the hearing sense of a reproduced sound can be improved.
In view of this, in the frequency analysis circuit 17, an approach is employed to divide the above-mentioned audio signal into so called critical bands by making use of the hearing sense characteristic of the human being to carry out a frequency analysis every critical band. As the division at the above-mentioned critical band at this time, e.g., an approach may be employed to transform, e.g., by Fast Fourier Transformation (FFT), an input audio signal into the components on the frequency base thereafter to divide (band-divide) the amplitude term Am (m=0 to 1024) of the FFT coefficient into, e.g., groups Gn of 25 bands (n represents the number of respective bands, n=0 to 24) at the above-mentioned critical band having a broader bandwidth in the higher frequency band in which the hearing sense characteristic of the human being is taken into consideration. Further, as the frequency analysis every respective critical bands, there may be carried out such an analysis to determine a bark spectrum (spectrum of sum total) Bn obtained by taking a sum total (the sum total of peak, average or energy of the amplitude term Am) of respective amplitude terms Am every band, e.g., by the following equation (1):
where n is 0 to 24, and Cn is the number of elements in the n-th band, i.e., the amplitude term (the number of points), and Pn is a peak value in each band. Bark spectra Bn of respective bands are, e.g., as shown in FIG. 8. It is to be noted that, in the example of
From the equi-loudness curve generation circuit 15, information of the equi-loudness curve RC is generated and outputted. Namely, by carrying out a noise shaping using an allowable noise spectrum obtained in consideration of the equi-loudness curve RC, noise shaping more effective in the hearing sense is conducted. Thus, the dynamic range in the hearing sense of a reproduced sound can be improved. Information of the equi-loudness curve RC (or its approximate curve) is outputted from the equi-loudness curve generation circuit 15, and is sent to the allowable noise spectrum calculation circuit 18.
Accordingly, in the allowable noise spectrum calculation circuit 18, the allowable noise spectrum is calculated on the basis of output information from the above-described equi-loudness curve generation circuit 15 and output information from the frequency analysis circuit 17. At this time, from the bark spectrum Bn every critical band at the frequency analysis circuit 17, by carrying out convolution (convoluting a predetermined weight function) in consideration of the influence between bands by using the following equation (2), the bark spectrum Sn convoluted every band is calculated.
where Hn is the coefficient of convolution. By this convolution, the sum total of the portions indicated by the dotted lines in
For example, when N is assumed to be 38, K is permitted to be equal to 1. There results less degradation of sound quality at this time. Namely, as shown in
In the allowable noise spectrum calculation circuit 18, the allowable noise spectrum is calculated on the basis of synthetic information obtained by synthesizing output information from the frequency analysis circuit 17 obtained as described above and output information from the previously described equi-loudness curve generation circuit 15.
Here, there are instances where the allowable noise level at the allowable noise spectrum based on the equi-loudness curve RC may be less than the allowable noise level where the masking effect is exerted by the level of an input audio signal. Namely, for example, in the case where the level of an input audio signal is high, the level of the allowable noise spectrum based on the equi-loudness curve may be also masked at the same time by the allowable noise level where the masking effect by the input audio signal is exerted.
In view of the above, in this embodiment, an approach is employed to detect the level of the input audio signal at the level detector 16 to vary, on the basis of the level detected output, the synthetic ratio between output information from the equi-loudness curve generation circuit 15 and output information from the frequency analysis circuit 17. Here, synthesis of output information of the equi-loudness curve generation circuit 15 and the frequency analysis circuit 17 is carried out, e.g., every frequency band. In this case, the level detection at the level detector 16 is carried out every band. Accordingly, the synthetic ratio can be changed every band on the basis of level detected outputs every band. Namely, with respect to synthetic information for determining an allowable noise spectrum in the allowable noise spectrum calculation circuit 18, for example, in the case where the level in a low frequency band of an input audio signal is high and the masking effect in the low frequency band is great, synthetic information is prepared at a synthetic ratio such that an allowable noise spectrum having a high level in the low frequency band and a low level in the high frequency band is provided. In contrast, for example, in the case where the level in a high frequency band is high and the masking effect in the high frequency band is great, synthetic information is prepared at a synthetic ratio such that an allowable noise spectrum having a high level in the high frequency band and a low level in the low frequency band is provided. Information of the allowable noise spectrum thus provided is sent to the filter coefficient calculation circuit 14. Thus, a filter coefficient corresponding to the allowable noise spectrum is outputted from the filter coefficient calculation circuit 14, and is then sent to the noise filter 13.
Since the approach described above is employed, the filter characteristic of the noise filter 13 is caused to be in correspondence with the filter coefficient based on an allowable noise spectrum obtained by varying a synthetic ratio every band in dependency upon the level of an input audio signal. Here, for example, in the case where the level of the input audio signal is flat, the filter characteristic of the noise filter 13 is assumed to be indicated by the curve MR of
Namely, in the quantizing noise reducer of this embodiment, when the level of an input audio signal is small, the filter characteristic of the noise filter 13 is caused to be the characteristic as indicated by the equi-loudness curve RC to carry out noise shaping. Further, in order to allow the quantizing level not to be conspicuous by the level of the input audio signal according as the signal level becomes large, the characteristic of the noise filter 13 is caused to be flat in correspondence with the signal level of the input audio signal. In addition, when the signal level is small, the characteristic as indicated by the equi-loudness curve RC is caused to become closer to a flat characteristic in correspondence with the signal level by the noise filter 13 to change it to a noise shaping characteristic (masking characteristic, etc.) caused to be in correspondence with the signal characteristic. Namely, when the signal level is small, the characteristic of the noise filter 13 is caused to be a filter characteristic as indicated by the equi-loudness curve RC, while when the signal level is large, the characteristic of the noise filter 13 is caused to be the filter characteristic in which the masking effect is taken into consideration.
In the curve MR showing the filter characteristic when the level of the input audio signal of
The actual example of the system configuration where the quantizing error reducer of this embodiment is used as the encoder/decoder system in, e.g., so called compact disk (CD) is shown in FIG. 14. In
Further, the actual example of the system configuration using e.g., a medium for recording data by using 10 bits different from the above-mentioned CD is shown in FIG. 15. In this case, an analog signal inputted and delivered to the input terminal 41 is converted to, e.g., a 16 bit digital data at for an A/D converter 42. The signal thus obtained is sent to a 10 bit correspondence encoder 43 including therein the quantizing error reducer of this embodiment. At the encoder 43, the digital data undergoes the quantizing error reducing processing and is encoded into 10 bit data. The data thus processed is recorded onto the medium. The data recorded on the medium is converted to an analog signal at a reproducing circuit 44 and a D/A converter 45 of the existing player, and is outputted from the output terminal 46. Also in this case, a reproduced signal obtained has a high dynamic range.
The actual example where the quantizing error reducer of this embodiment is used in the D/A conversion system for carrying out oversampling is shown in FIG. 16. In this case, an analog signal inputted and delivered to the input terminal 51 is converted to, e.g., digital data of 20 bits at an A/D converter 52 for carrying out over-sampling, and is sent to a quantizing error reducer 53 of this embodiment through a transmission path. At this quantizing error reducer 53, that digital data undergoes the quantizing error reducing processing. The digital data thus processed is converted to an analog signal through a D/A converter 54, and is outputted from the output terminal 55. Thus, oversampling is permitted to be carried out and the resolution of the D/A converter is permitted to be lowered. Thus, the D/A converter 54 having a high linearlity can be easily prepared.
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