JPH03201716A - Noise shaping circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a so-called noise shaping circuit that changes the noise spectrum of an input signal.

[Summary of the invention]

The present invention provides a noise shaping circuit in which quantization noise is fed back to a quantizer via a noise filter, in which the noise filter has an IIR type filter configuration, so that noise shaping with bandpass characteristics can be performed. This is what I did. Further, the noise filter has a plurality of IIR type filters and has a bandpass characteristic that passes each critical band of the input signal, thereby making it possible to perform noise shaping for each critical band. Also, the quantization noise is fed back through the noise filter of that stage and is supplied to the next stage circuit, and the output of the quantizer of each stage except the first stage is outputted through a filter with desired filter characteristics. In a noise shaping circuit in which the output of each stage is made to bottom out, the noise filter of each stage is an IIR type filter and has a bandpass characteristic that passes each critical band of the input signal. The present invention provides a noise shaping circuit that can set bandpass characteristics for each critical band and perform noise shaping for each critical band.

[Conventional technology]

For example, high-efficiency encoding that compresses and encodes the amount of information such as an audio signal can be roughly divided into encoding in the time wI domain and encoding in the frequency domain.

Furthermore, this time-domain or frequency-domain encoding can be performed using a method of encoding with a predetermined number of quantization bits or a method of encoding with a number of quantization bits that is adaptively changed based on the input audio signal. It can be divided into two methods. Coding in the time domain includes so-called predictive coding, adaptive predictive coding, and differential quantization (DPCM).
method, adaptive differential PCM (adaptive DPCM or ADPCM)
) method, delta modulation (ΔM or DM) method, adaptive delta modulation method, variable length coding method, etc. In addition, as coding in the frequency domain, there is so-called band division coding (sub-band coding: 5BC), in which a time-axis audio signal is divided into multiple frequency bands and coded.
So-called adaptive transform coding (ATC), which converts a time axis signal into a frequency axis signal (orthogonal transformation), divides it into multiple frequency bands, and adaptively encodes each band, or the S
By combining BC and so-called adaptive predictive coding (APC), the time axis signal is divided into bands, each band signal is converted to baseband (low frequency), and then multi-order linear predictive analysis (L
Examples include so-called adaptive bit allocation (APC-AB), which performs predictive coding using PG).

Furthermore, when encoding an audio signal or the like, so-called noise shaping may be applied to change the spectrum of quantization noise to substantially reduce noise in the audible band. This noise shaping has generally been performed by using a so-called FIR (finite impulse response or acyclic) type digital filter.

[Problem to be solved by the invention]

By the way, when encoding an audio signal, the audio signal is encoded in multiple frequency bands (for example, 25 bands) in a so-called critical band, in which the higher the frequency range, the wider the bandwidth, taking into account the characteristics of human hearing. ) and may be encoded for each band. When an audio signal is divided into such critical bands, noise shaping is performed for each band, for example.

However, when attempting to perform noise shaping by frequency-dividing an audio signal in a critical band as described above, it is not possible to obtain steep bandpass characteristics with the above-mentioned FIR type filter. It was difficult to perform noise shaping on the audio signal for each band divided into bands.

Therefore, the present invention was proposed in view of the above-mentioned circumstances, and an object of the present invention is to provide a noise shaping circuit that can perform noise shaping of a frequency-divided signal in a critical band. It is.

[Means to solve the problem]

The noise shaping circuit of the present invention has been proposed in order to achieve the above-mentioned object, and is designed to feed back the quantization noise generated in the quantizer to the input side of the quantizer via a noise filter. In the noise shaping circuit described above, the noise filter has an IIR (infinite impulse response or cyclic) type filter configuration.

Further, the noise filter is composed of a plurality of IIR type filters, and each IIR type filter has a bandpass characteristic that allows the critical band of the input signal to pass through.

Furthermore, the noise shaping circuit of the present invention feeds back the quantization noise generated in the quantizer to the input side of the quantizer via the noise filter of that stage, and also supplies it to the next stage circuit as human power. In a noise shaping circuit in which the output of the quantizer in each stage except the first stage is output through a filter having desired filter characteristics, and the outputs of each stage are combined and output, the noise filter in each stage is
In addition to having a type filter configuration, the filter has a bandpass characteristic that allows each critical band of the input signal to pass.

[Effect]

According to the present invention, since an IIR type filter having a bandpass characteristic that passes a critical band is used, it is possible to adjust the spectrum of quantization noise for each critical band. Therefore, noise shaping can be performed in the critical band.

〔Example〕

Embodiments to which the present invention is applied will be described below with reference to the drawings.

FIG. 1 shows a schematic configuration of an encoding device equipped with a noise shaping circuit according to an embodiment of the present invention. The signal is quantized by a quantizer 34 through a quantizer 33, and then outputted as an encoded output from an output terminal 32. By the circuit
Noise shaping processing of the quantization noise generated by the quantizer 34 is performed.

That is, the noise shaping circuit of this embodiment includes the adder 33, the addition 1i35, and the noise filter 36, and the adder 35 converts the quantized output of the quantizer 34 into from the quantizer 34
Quantization noise obtained by subtracting the input signal supplied to the adder 33 is fed back to the adder 33 as a subtraction signal via the noise filter 36, thereby performing noise shaping processing using so-called error feedback. be. At this time, the noise filter 36 is II
It has an R (infinite impulse response or cyclic) type filter configuration.

Right! The IIR type noise filter 36 is an at least second-order IIR type filter, and specifically, it can be realized with a configuration as shown in FIG. The input terminal 41 of the noise filter shown in FIG. 2 is connected to the adder 3 shown in FIG.
The quantization noise from 5 is supplied. The noise filter includes an adder 43, 44, 51, a delay device 47.48 with a predetermined time (one sample) delay (z-'), and a multiplication coefficient a.
, b.

It is composed of multipliers 45, 46 and 49, 50 of c and d. In the noise filter, the output via the delay device 47 and multiplier 45 and the output via the delay device 48 and multiplier 46 are added by the adder 44, and further, the adder 43 adds the quantum is added to the noise. Further, the output of the delay device 47 is passed through the multiplier 49 to the delay device 48.
The outputs are transmitted to the adder 51 via the multiplier 50, and after being added by the adder 51, are outputted from the output terminal 42 as the output of the noise filter.

Here, if the filter characteristic of the noise filter in FIG. 2 (that is, the noise filter 36 in FIG. 1) is P, then
The signal component obtained by noise shaping is (1
) can be shown by the formula.

1- (az-'+bz-") That is, the filter characteristic P of the noise filter is (az-'+b z-"). Therefore, each multiplication coefficient in the multiplier 49.50 is as follows. This can be expressed as c-ad-b in formula (2). The filter characteristics of the second-order IIR type filter shown in FIG. 2 can have a steep bandpass characteristic with a center frequency r as shown in FIG.

Further, in the noise shaping circuit of the present invention, the noise filter 36 is composed of a plurality of IIR type filters, and each IIR type filter has a bandpass characteristic that passes the critical band of the input signal. can do. That is, II in this case
The number of R-type filters is the same as the number of critical bands, and the passband of each IIR-type filter is set to have a width corresponding to the critical band width.

Furthermore, the noise shaping circuit of the present invention may be
It can be applied to an encoding device as shown in the figure.

In FIG. 4, an input signal such as a digital audio signal supplied to the input terminal l is divided by the above-mentioned critical band, and the signal for each band is sent to the quantizer in each stage corresponding to each band. 14+ to 141 is quantized and output as encoded output from output terminal 2. During encoding, so-called error feedback eliminates quantization noise generated in the quantizers at each stage. It performs noise shaping processing.

In the noise shaping circuit constituting the error feedback circuit used in the apparatus shown in FIG. 4, the quantizers 141 to 14. , the respective quantization noises generated in the respective noise filters 12+~ of each stage are
12. ．． via the quantizer 14. 141, respectively, and is supplied as an input to the next stage circuit, and is fed back to the input side of each of the quantizers 141-14.
Filters 15! having desired filter characteristics for each output of the filters 15! ~15. ．． 163-16□..., 20. and the output of each stage is sent to each adder 171-1.
7. The noise filters 12t to 12. of each stage are combined and output by adding the noise filters 12t to 12. l,
The above-mentioned noise filter 12 in each stage is configured to have at least a second-order IIR type filter as shown in FIG.
1-12. By setting the multiplication coefficient of each multiplier to a value corresponding to each band width of the critical band, it is possible to have a bandpass characteristic that allows each critical band of the input signal to pass. In addition, when the above critical band is, for example, 25 bands from DC (O7) to 20 kHz (i.e., n=25
), the number of stages in the above circuit is 25.

That is, FIG. 4 shows a configuration that can perform noise shaping processing for each band of the critical band, and each stage has the same configuration as in FIG. 1 described above, that is, each adder 11. -11. , 13°~139, quantizer 14+
~14, and has an encoding configuration including a noise shaping circuit composed of noise filters 12+~1211. In addition, each noise filter 12. 127 has at least a second-order IIR type filter configuration.

For this reason, each of these noise filters 12. ~12, l
For example, the bandpass characteristics as shown in FIG. 5, that is, the center frequencies 11 to r of each band of the critical band
7, a steep bandpass characteristic can be obtained. Furthermore, the outputs of the quantizers 14□ to 14.1 in each stage except the first stage are filters having desired filter characteristics.]5. ~15
−． 163-16.1. ..., is sent to 207. Here, the filters 15g to 15, 1.1 of each of these stages
63-16,1. The desired filter characteristics of . Therefore, each stage of the noise filter 12. -1211, respectively, are P, ~P, l, for example, the filter characteristics of the second stage filter IL are the first stage noise filter 121
This is the inverse characteristic of the filter characteristic P1, that is, the characteristic of IP+. Also, for example, the third stage filter 15. Then I
The filter 163 has a characteristic of 1-P. Similarly, the n-th filter 1
5. For l, the characteristic is 1-P, and for filter 16, l-P! Characteristics of... filter 20. Then I P
-1 filter characteristic.

For this reason, if the quantization characteristics of the quantizers 14+ to 14 are set to Q1 to Q7, respectively, and taking the first and second stages as an example, the first stage's quantization F
il 4. of the quantization noise signal component in the output of (I
PI) Q+, that is, and the inflow quantization noise signal bullet Q from the first stage of the second stage, the filter 15□
The outflow to (l Pl)Q+, that is, will cancel each other out. Similarly, in each of the other stages, for example, the quantization noise signal bullet being outputted from the m-th stage, and the inflow quantization noise signal bullet Q from the m-th stage at the m+1 stage.
An offset is made with the outflow to the output of 1. From this, the characteristics of the entire error feedback circuit are determined only by the n-th stage filter characteristics.

Therefore, according to the noise shaping circuit of the embodiment described above, the noise filter is configured to have at least a second-order I
The filter has an IR type filter configuration, and the filter coefficients of this noise filter can be set freely.For example, if the filter coefficients are set corresponding to each band width of the critical band, it will be possible to set the filter coefficients corresponding to each band of the critical band. Adjustment of the noise spectrum, that is, noise shaping becomes possible.

For this reason, for example, this embodiment can be used to record conventional digital audio with a fixed slot word length (for example, so-called compact disc: CD, digital audio).
If used for noise shaping processing of a signal from an audio tape recorder (DAT, etc.), it becomes possible to increase the perceptual dynamic range of the reproduced sound of the digital audio signal.

Note that when performing noise shaping on an input signal such as an audio signal, the perceptual dynamic range can be increased by performing noise shaping that takes into account so-called masking of the input signal spectrum. For example, the perceptual dynamic range can be increased. By performing noise shaping using a noise filter whose filter coefficients are fixed according to the spectrum of the input signal which is fixed to some extent, or by performing adaptive noise shaping to the change in the spectrum when the spectrum of the input signal changes. , it is possible to increase the perceptual dynamic range. here,
The above masking refers to a phenomenon in which one signal masks another signal and becomes inaudible due to the characteristics of human hearing.This masking effect includes a masking effect on signals on the time axis and a masking effect on signals on the frequency axis. There is a masking effect on the signal. That is, due to the masking effect, even if there is noise in the masked part, this noise cannot be heard. For this reason, as mentioned above, when an audio signal is frequency-divided into critical bands, by performing noise shaping according to the level (spectrum) of each critical band, the perceptual dynamic range of the reproduced sound can be improved. can be raised.

Here, for example, a configuration for performing noise shaping that adaptively changes the noise spectrum according to the input signal spectrum can be realized by a configuration as specifically shown in FIG. 6.

In FIG. 6, a digital audio signal is supplied to an input side 61, an adder 63, a quantizer 64,
The adder 65, the noise filter 66, etc. perform the same encoding and noise shaping as described above, and then output from the output terminal 62.

By setting the encoding and noise shaping configuration to the configuration shown in FIG. 4 described above, it is possible to obtain, for example, bandpass characteristics for each critical band. At this time, as mentioned above,
In order to perform adaptive noise shaping, it is necessary to adaptively change each filter coefficient in each noise filter according to the spectrum of the input signal for each band.

Therefore, the audio signal via the input side 61 is sent to a spectrum analysis circuit 67 that performs spectrum analysis of the audio signal. The spectrum analysis circuit 67
Then, spectrum analysis of the human audio signal for each critical band is performed, and the output for each band is sent to the S/N calculation circuit 68.
sent to.

The S/N calculation circuit 68 calculates the S/N value for each band. This S/N value is obtained by taking into consideration the masking effect, and each filter coefficient of each of the noise filters according to this S/N value is obtained from the filter coefficient calculation circuit 69. There is.

In this way, the filter coefficients adaptively determined according to the spectrum of the audio signal for each critical band are sent to the noise filter 66, that is, each noise filter shown in FIG. It becomes possible to control the noise spectrum according to the noise, and adaptive noise shaping becomes possible.

Therefore, by performing adaptive noise shaping as described above, it is possible to further increase the perceptual dynamic range.

〔Effect of the invention〕

In the noise shaping circuit of the present invention, the noise filter has an IIR type filter configuration, thereby making it possible to perform noise shaping with bandpass characteristics. In addition, the noise filter has a plurality of IIR filter configurations and has bandpass characteristics that allow each critical band of the input signal to pass through, thereby making it possible to perform noise shaping for each critical band. Also, the quantization noise is fed back through the noise filter of that stage and is supplied to the next stage circuit, and the output of the quantizer of each stage except the first stage is outputted through a filter with desired filter characteristics. ,
In a noise shaping circuit in which the output of each stage is set to the bottom, each stage's noise filter is an IIR type filter and has a bandpass characteristic that passes each critical band of the input signal. Bandpass characteristics can be set for each critical band, and noise shaping can be performed for each critical band.

Therefore, for example, the reproduced sound of a digital audio signal can have an audible dynamic range that is greater than the dynamic range determined by the slot word length.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing a schematic configuration of an encoding device equipped with a noise shaping circuit according to an embodiment of the present invention, FIG. 2 is a block circuit diagram showing a specific configuration of a noise filter, and FIG. Figure 4 is a block circuit diagram showing the configuration for performing noise shaping processing for each band in the critical band of the present invention, and Figure 5 is a diagram showing the characteristics of the IIR filter for each band in the critical band. FIG. 6 is a characteristic diagram showing bandpass characteristics, and a block circuit diagram showing a configuration for performing noise shaping according to the spectrum. 121-12. ．． 36.66...Noise filters 141 to 14. , 34.64...
...Quantizer 15! ~15-, 16x~16. l
, 20. ...Filter 45.46.49.50
・・・・・・Multiplier 47.4810000
10. delay device

Claims (3)

[Claims] (1) In a noise shaping circuit in which quantization noise generated in a quantizer is fed back to the input side of the quantizer via a noise filter, the noise filter has an IIR type filter configuration. Features a noise shaping circuit. (2) The noise filter according to claim (1), wherein the noise filter is composed of a plurality of IIR type filters, and each IIR type filter has a bandpass characteristic that passes a critical band of the input signal. shaping circuit. (3) The quantization noise generated in the quantizer is fed back to the input side of the quantizer through the noise filter of that stage, and is also supplied as an input to the next stage circuit, so that the quantization noise generated in each stage except the first stage is In a noise shaping circuit in which the output of the quantizer is output through a filter having desired filter characteristics, and the outputs of each stage are combined and output, the noise filter of each stage is configured as an IIR type filter. Additionally, a noise shaping circuit is characterized in that it has a bandpass characteristic that allows each critical band of an input signal to pass through.