JP3089476B2 - Audio signal quantization error reduction device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio signal quantization error reduction device that reduces a quantization error generated by audio signal quantization.

[Summary of the Invention]

The present invention relates to an audio signal quantization error reducing device in which a quantization error is fed back to an input side of a quantizer via a noise filter.
By setting the characteristics to perform the quantization error reduction processing according to the equal loudness curve and the processing to reduce the quantization error of the low frequency component, it is possible to reduce the noise in the auditory sense with a simple configuration. It is an object of the present invention to provide an audio signal quantization error reducing device capable of performing the following.

[Conventional technology]

Currently, digital audio devices that handle digital audio signals include, for example, a so-called compact disk (CD) player or a so-called digital audio tape recorder (DAT). Various unified standards are defined for these digital audio devices. For example, the bit length of a digital audio signal handled by these devices is defined as 16 bits from the unified standards. As a digital audio signal in these digital audio devices, an analog audio signal (sound waveform signal) is, for example, a so-called digital audio signal.
Digital audio signals obtained by encoding using simple linear quantization such as PCM (pulse code modulation) are used.

[Problems to be solved by the invention]

By the way, in recent years, it has been desired that the above-mentioned digital audio equipment can obtain a reproduced sound having a higher audibility than a reproduced sound actually obtained from the unified standard. In order to obtain such a better reproduced sound, for example, a digital audio signal handled by these digital audio devices is set as a signal in which noise components included in the digital audio signal itself are reduced. It is possible.

As the noise component reduction processing of the digital audio signal, for example, a quantization error (quantization noise or quantization distortion) generated by a quantizer at the time of quantization of an audio signal is quantized via a noise filter. There is known a noise shaping process in which a quantization error is reduced by a so-called error feedback that is fed back to an input side of a transformer.

Here, the quantization distortion in the linear quantization such as the PCM described above has a flat frequency characteristic in the entire frequency band of the audio signal. However, the human ear has different sensitivity depending on the frequency of the sound,
It is hard to say that the quantization error reduction processing by the error feedback is necessarily effective for human hearing.

Further, in order to perform a quantization error reduction process that is effective for human hearing, a quantization error reduction device having a very complicated configuration is required (particularly, a high-order noise filter is required).

Therefore, the present invention has been proposed in view of the above-described circumstances, and has a simple configuration, which is capable of effectively reducing a quantization error (quantization noise) on an auditory sense with a simple configuration. It is an object to provide a reduction device.

[Means for solving the problem]

The audio signal quantization error reduction device of the present invention has been proposed to achieve the above object, and is fed back to the input side of the quantizer via a quantization error noise filter generated by the quantizer. In this case, the filter characteristics of the noise filter are subjected to quantization error reduction processing in accordance with the equal loudness curve, and quantization error reduction processing for reducing the quantization error of the reduced component is performed. It is set to the characteristic which is changed.

In the audio signal quantization error reduction method of the present invention,
A digital audio signal is input, an error feedback signal is added to the input digital audio signal, a first signal is generated, the first signal is quantized, and the input digital audio signal is quantized with the quantized first audio signal. A quantization error representing a signal difference is generated, and the quantization error is filtered to generate an error feedback signal. At this time, the filter characteristics in the filtering are set based on the information related to the equal loudness curve, and the low range is flat.

In the audio signal recording method of the present invention, a digital audio signal is input, an error feedback signal is added to the input digital audio signal, a first signal is generated, the first signal is quantized, and the digital signal is input. Generating a quantization error representing a difference between the digital audio signal and the quantized first signal; filtering the quantization error to generate an error feedback signal; and recording the quantized first signal on a recording medium I do. At this time, the filter characteristics in the filtering are set based on information related to the equal loudness curve.

In the audio signal transmission method according to the present invention, a digital audio signal is input, an error feedback signal is added to the input digital audio signal, a first signal is generated, the first signal is quantized, and the digital signal is input. Generating a quantization error representing a difference between the digital audio signal and the quantized first signal, filtering the quantization error to generate an error feedback signal, and transmitting the quantized first signal via a transmission medium; To transmit. At this time, the filter characteristics in the filtering are set based on information related to the equal loudness curve.

An audio signal quantization error reducing apparatus according to the present invention is connected to a first adding means for adding an error feedback signal to an input digital audio signal to generate a first signal, and a first adding means, Quantizing means for quantizing the first signal; and a second means connected to the quantizing means for generating a quantization error representing a difference between the input digital audio signal and the quantized first signal. An adder is provided, and a filter is connected to the second adder and filters the quantization error to generate an error feedback signal. The filtering means is set based on the information related to the equal loudness curve, and has a filter characteristic in which the low band is flat.

According to the audio signal recording apparatus of the present invention, an error feedback signal is added to an input digital audio signal to obtain a first audio signal.
A first addition means for generating a signal of the first type, a quantization means connected to the first addition means for quantizing the first signal, and a digital audio signal connected to the quantization means for A second adding means for generating a quantization error representing a difference between the obtained first signal, a filtering means connected to the second adding means for filtering the quantization error to generate an error feedback signal, Recording means for recording the converted first signal on a recording medium. The filtering means is set based on information related to the equal loudness curve, and has a filter characteristic in which the low band is flat.

An audio signal transmitting apparatus according to the present invention adds an error feedback signal to an input digital audio signal to generate a first digital audio signal.
A first addition means for generating a signal of the first type, a quantization means connected to the first addition means for quantizing the first signal, and a digital audio signal connected to the quantization means for Second adding means for generating a quantization error representing a difference between the obtained first signals, filtering means connected to the second adding means for filtering the quantization error to generate an error feedback signal, and filtering. Transmission means connected to the transmission means for transmitting the quantized first signal via a transmission medium. The filtering means is set based on the information related to the equal loudness curve, and has a filter characteristic in which the low band is flat.

[Action]

According to the present invention, the equal loudness curve is in accordance with human auditory characteristics, and by performing quantization error reduction processing in accordance with the equal loudness curve, noise in a frequency band that is easily audible can be reduced. Will be able to
The configuration of the noise filter is simplified by setting the filter characteristics so as to reduce the quantization error of the low-frequency component.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a block diagram of a configuration of the audio signal quantization error reducing device of the present embodiment.

1, the quantization error generated by the quantizer 11 is transmitted through the noise filter 13 to the quantizer 11.
The filter characteristic of the noise filter 13 is changed to a quantization error reduction process according to a so-called equal loudness curve RC as shown in FIG. Is performed, and a quantization error reduction process for reducing the quantization error of the low-frequency component is performed. The filter characteristic is set to a reduced flat filter characteristic as shown by a curve MR in FIG. . here,
A digital audio signal obtained by sampling at an arbitrary sampling frequency is supplied to an input terminal 1 of FIG. 1, and the digital audio signal is requantized by a quantizer 11 and output from an output terminal 2. It is output.

That is, the present embodiment device is an adder that obtains a quantization error generated at the time of quantization in the quantizer 11 by subtracting the input to the quantizer 11 from the output of the quantizer 11.
12, a noise filter 13 having a filter characteristic set by a filter coefficient, which will be described later, which performs filtering processing on the output of the adder 12, and adds the output of the noise filter 13 to the input side of the quantizer 11. A so-called error feedback circuit including the adder 10 is provided. This error feedback circuit performs quantization error reduction processing (so-called noise shaping processing). Further, the apparatus according to the present embodiment includes an equal loudness curve generating circuit 15 for generating information related to the equal loudness curve RC in FIG. 2 according to human auditory characteristics, and a noise for reducing noise of a reduction component described later. A low-frequency correction control signal circuit 16 that generates a low-frequency correction control signal for performing a shaping process, and an output of the low-frequency correction control signal generation circuit 16 and an output of the equal loudness curve generation circuit 15 And a filter coefficient calculation circuit 14 for calculating a filter coefficient of the noise filter 13 based on the above.

Here, the equal loudness curve RC is a curve corresponding to human auditory characteristics, for example, a curve obtained by connecting sound pressures of sounds at each frequency that sounds as loud as a pure tone of 1k,
This is also called a loudness iso-sensitivity curve.
As shown in FIG. 2, this equal loudness curve RC has a sound pressure near 1k where the human hearing is sharp near 4k.
Even if it is 8dB to 10dB lower, it sounds almost the same size as 1k, and conversely,
For example, it shows that it is harder to hear about 20dB at 10k than at around 4k. Therefore, this equal loudness curve RC
By performing the noise shaping of the audio signal by the error feedback circuit based on the information related to the above, it is possible to increase the audible dynamic range. That is, by performing noise shaping using an allowable noise spectrum (acceptable noise level) obtained in consideration of the equal loudness curve RC, more effective noise shaping can be performed on the audibility, and the audibility of the reproduced sound can be improved. The above dynamic range can be increased. Information (information on allowable noise spectrum) related to the equal loudness curve RC (or an approximate curve thereof) is output from the equal loudness curve generation circuit 15 and sent to the filter coefficient calculation circuit.

Therefore, the filter coefficient calculation circuit 14 calculates the filter coefficient of the noise filter 13 based on the equal loudness curve RC. That is, the filter coefficient is set based on information such that the response greatly changes depending on the frequency as in the equal loudness curve RC. However, in order to realize the filter characteristics based on such filter coefficients in the noise filter 13, it is necessary to increase the order of the filter. There is a risk that the scale will increase. Therefore, in order to prevent the filter from having a higher order, it is necessary to make the filter characteristics flat with little change.

For this reason, in the apparatus of this embodiment, by taking into account the so-called masking effect of the auditory characteristics described later and the characteristics of general audio signals, it is possible to obtain an effective audibility without increasing the order of the filter. Noise shaping can be performed, and the perceived dynamic range of the reproduced sound can be increased.

Here, the masking refers to a phenomenon in which a certain signal masks another signal and becomes inaudible due to human auditory characteristics.
There are a masking effect for signals on the time axis and a masking effect for signals on the frequency axis (or simultaneous masking and temporal masking). Therefore, as shown in FIG. 3, for example, when there is an audio signal S having a certain frequency characteristic, noise below the masking curve MC cannot be heard due to the masking curve MC based on the audio signal S. That is, the level below the masking curve MC is an acceptable noise level. Note that FIG. 3 also shows the above equal loudness curve RC for reference.

Further, the general audio signal S has a middle
Although the low-frequency signal component is increased, the above-mentioned masking effect is generally lower in the lower frequency range than in the high frequency range. That is, when noise shaping of the normal audio signal S is performed, it is effective to reduce noise in a low frequency range rather than a high frequency range.

As described above, in the device of the present embodiment, the filter characteristic of the noise filter 13 is changed to the general audio signal S.
In consideration of the characteristics of the low loudness noise and the masking effect,
In consideration of the RC and the masking curve MC, the characteristic is such that the allowable noise level in the high range becomes high. That is, in the device of the present embodiment, the filter characteristics of the noise filter 13 are changed by the equal loudness curve RC and the masking curve M.
By taking into account the properties of C and the general audio signal S, as shown in FIG. 4, a filter characteristic such as a curve MR having a flat and low low-frequency portion and a high high-frequency portion is obtained. I have. By making the low-frequency portion flat in this way, the curve MR has little change as a whole. Therefore, the filter characteristic of the curve MR can be realized by a filter having a low order. Also this curve
If the noise filter 13 having the MR characteristic is used, as described above, the equal loudness curve RC and the masking curve MC are taken into account, so that effective noise shaping can be performed on the audibility, and the dynamic range on the audibility of the reproduced sound can be obtained. Can be raised.

In the present embodiment, the filter coefficient calculation circuit 14 is used in order to realize a flat filter characteristic such as the curve MR for reducing the low-frequency noise while considering the equal loudness curve RC and the masking curve MC. The output of the equal loudness curve generation circuit 15 (equal loudness curve RC)
), The output (low-frequency correction control signal) from the low-frequency correction control signal generation circuit 16 is also sent.

That is, the low-frequency correction control signal generated by the low-frequency correction control signal generation circuit 16 is generated in consideration of the masking effect, and the filter coefficient calculation circuit 14 generates the equal loudness curve. By calculating together with RC, the filter characteristic of the noise filter 13 becomes the curve MR in FIG.

FIG. 5 shows a specific example of an encoder and a decoder in a system in which the apparatus of the present embodiment uses, for example, a so-called compact disk (CD) as a (transmission) medium. In FIG. 5, an input terminal 31 is supplied with an analog audio signal. This analog audio signal is A / D
After being converted into a 20-bit digital audio signal by the converter 32, the quantization error reduction device of the present embodiment is built in.
The data is sent to the encoder 33 corresponding to 20 bits. This encoder
The audio signal quantized to 3316 bits and subjected to the quantization error reduction processing is recorded on the CD. The data recorded on this CD is used as the playback circuit of an existing CD player.
The signal is converted into an analog audio signal by the D / A converter 34 and output from the output terminal 36. That is, since the data recorded on the CD has a reduced quantization error by the apparatus of the present embodiment, the sound obtained by reproducing the CD has a high perceptual dynamic range. .

FIG. 6 shows a specific configuration of a system using a medium for recording an audio signal in, for example, 10 bits, unlike the above-mentioned CD. In this case, the analog audio signal input via the input terminal 41 is converted into, for example, a 16-bit digital audio signal by the A / D converter 42, and the analog audio signal is a 10-bit digital signal in which the quantization error reduction device of the present embodiment is built. This is sent to the encoder 43. The audio signal quantized to 10 bits by the encoder 43 and subjected to the quantization error reduction processing is recorded on the medium. The digital audio signal recorded on this medium is converted into an analog audio signal by a reproduction circuit 44 and a D / A converter 45 of an existing reproduction machine and output from an output terminal 46. In this case, similarly, the obtained reproduction signal has a high dynamic range.

FIG. 7 shows a specific example of using the apparatus of this embodiment in a D / A conversion system that performs oversampling. In this case, the analog audio signal input through the input terminal 51 is converted into, for example, a 20-bit digital audio signal by the A / D converter 52 that performs oversampling, and the quantization error of the present embodiment is transmitted via the transmission path. It is sent to the reduction device 53. The digital audio signal subjected to the quantization error reduction processing by the quantization error reduction device 53 is converted into an analog audio signal by the D / A converter 54 and output from the output terminal 55. As a result, oversampling can be performed, and the resolution of D / A conversion can be reduced, which makes it easier to create the D / A converter 54 in a direction that increases linearity.

As described above, according to the present embodiment, in the audio signal quantization error reduction device in which the quantization error is fed back to the input side of the quantizer 11 through the noise filter 13, the filter of the noise filter 13 The noise shaping process is performed according to the equal loudness curve RC as shown in FIG. 2 according to the human auditory characteristics, and the noise shaping process is performed to reduce the quantization error of the low frequency component. By setting the filter characteristic to a low band flat as shown by a curve MR in FIG. 4,
With a simple configuration (low order of the filter), it is possible to reduce audible noise. Therefore, the quantization error reduction device of the present embodiment is used, for example, for digital audio equipment (for example, a so-called compact disc,
When applied to a digital audio tape recorder or the like, it is possible to obtain a reproduced sound having a higher dynamic range in terms of audibility than a dynamic range actually obtained from the unified standard. For example, unified standards (CD, DAT
In the case of this, it is possible to raise the perceived dynamic range of the reproduced sound of this audio signal while maintaining the 16-bit slot word length standard (without changing the playback side and maintaining compatibility). it can.

〔The invention's effect〕

In the audio signal quantization error reduction apparatus according to the present invention, the filter characteristics of the noise filter are changed such that the quantization error reduction processing according to the equal loudness curve and the processing for reducing the quantization error of the low frequency component are performed. By setting to, the audible noise can be reduced with a simple configuration, and therefore the audible noise can be reduced and the audible dynamic range can be increased.

For this reason, if the audio signal quantization error reduction device of the present invention is applied to, for example, digital audio equipment of a standardized standard, a dynamic range that is higher in audibility than a dynamic range actually obtained from the standardized standard is obtained. A playback sound is obtained. For example, it is possible to increase the perceptual dynamic range of the reproduced sound of this audio signal while maintaining the unified standard (without changing the reproduction side and maintaining compatibility).

[Brief description of the drawings]

FIG. 1 is a block diagram of a schematic configuration of an audio signal quantization error reducing device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram showing an equal loudness curve, and FIG. 3 is a diagram for explaining a masking effect by an audio signal. FIG. 4 is a characteristic diagram showing filter characteristics, FIG. 5 is a block diagram showing a specific example in which the apparatus of the present embodiment is applied to an encoder / decoder system of a compact disk, and FIG. FIG. 7 is a block diagram showing a specific example applied to a bit system, and FIG. 7 is a block diagram showing a specific example in which the device of this embodiment is applied to a D / A conversion system for performing oversampling. 10, 12 adder 11 quantizer 13 noise filter 14 filter coefficient calculation circuit 15 equal loudness curve generation circuit 16 low-frequency correction control signal generation circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03M 7/38

Claims (13)

(57) [Claims] 1. An audio signal quantization error reducing apparatus wherein a quantization error generated by a quantizer is fed back to an input side of the quantizer via a noise filter. Characterized in that the quantization error reduction processing according to the equal loudness curve is performed, and the quantization error reduction processing is performed to reduce the quantization error of the low frequency component. Signal quantization error reduction device. 2. A digital audio signal is input, an error feedback signal is added to the input digital audio signal to generate a first signal, and the first signal is quantized. Generating a quantization error representing a difference between the signal and the quantized first signal; filtering the quantization error to generate the error feedback signal; a filter characteristic in the filtering is related to an equal loudness curve A method for reducing quantization error of an audio signal, wherein the method is set based on the obtained information and the low frequency band is flat. 3. The filter according to claim 2, wherein the filter characteristics in the filtering are set based on information related to the equal loudness curve, and a frequency of 4 kHz or less is flat. Method for reducing quantization error of audio signal. 4. A digital audio signal is input, an error feedback signal is added to the input digital audio signal to generate a first signal, and the first signal is quantized. Generating a quantization error representing a difference between the signal and the quantized first signal; filtering the quantization error to generate the error feedback signal; and storing the quantized first signal in a recording medium. Wherein the filter characteristics in the filtering are set based on information related to the equal loudness curve, and the low frequency band is flat. 5. The filter characteristic according to claim 4, wherein the filter characteristic in the filtering is set based on information relating to the equal loudness curve, and the frequency of 4 kHz or less is flat. Audio signal recording method. 6. A digital audio signal is input, an error feedback signal is added to the input digital audio signal to generate a first signal, and the first signal is quantized. Generating a quantization error representing a difference between the signal and the quantized first signal; filtering the quantization error to generate the error feedback signal; and transmitting the quantized first signal to a transmission medium. And a filter characteristic in the filtering is set based on information related to the equal loudness curve, and a low frequency band of the filter characteristic is flat. 7. The filter according to claim 6, wherein the filter characteristics in the filtering are set based on information relating to the equal loudness curve, and the frequency of 4 kHz or less is flat. Audio signal transmission method. 8. A first addition means for adding an error feedback signal to an input digital audio signal to generate a first signal; and a first addition means connected to the first addition means for quantizing the first signal. Quantizing means for quantizing; second adding means connected to the quantizing means for generating a quantization error representing a difference between the input digital audio signal and the quantized first signal; Filtering means for generating the error feedback signal by filtering the quantization error, wherein the filtering means is set based on information related to an equal loudness curve, and An audio signal quantization error reducing device, characterized by having a filter characteristic in which a low band is flat. 9. The filter according to claim 8, wherein said filtering means is set based on information relating to an equal loudness curve, and has a filter characteristic in which a frequency of 4 kHz or less is flat. Item 10. An audio signal quantization error reducing device according to the above. 10. A first adder for adding an error feedback signal to an input digital audio signal to generate a first signal, and connected to the first adder, for quantizing the first signal. Quantizing means for quantizing; second adding means connected to the quantizing means for generating a quantization error representing a difference between the input digital audio signal and the quantized first signal; A filtering means connected to a second adding means for generating the error feedback signal by filtering the quantization error, and a recording means for recording the quantized first signal on a recording medium; The audio signal is set based on information related to the equal loudness curve, and has a filter characteristic whose low band is flat, Recording device. 11. The filtering means is set based on information relating to an equal loudness curve,
The audio signal recording device according to claim 10, wherein the device has a filter characteristic whose frequency is 4 kHz or less. 12. A first adding means for adding an error feedback signal to an input digital audio signal to generate a first signal; and a first adding means connected to the first adding means for quantizing the first signal. Quantizing means for quantizing; second adding means connected to the quantizing means for generating a quantization error representing a difference between the input digital audio signal and the quantized first signal; A filtering means connected to a second adding means for filtering the quantization error to generate the error feedback signal; and a filtering means connected to the filtering means for transmitting the quantized first signal via a transmission medium. Transmitting means for transmitting, wherein the filtering means is set based on information related to the equal loudness curve, and has a filter characteristic whose low frequency band is flat. To, the transmission apparatus of an audio signal, characterized in that. 13. The filtering means is set based on information related to an equal loudness curve,
The audio signal transmission device according to claim 12, wherein the device has a filter characteristic whose frequency is 4 kHz or less.