JPH06253388A - Digital audio signal processor and digital network device - Google Patents

Abstract

PURPOSE:To provide a digital signal processor and a digital network device capable of reducing re-quantized noise by a rounding-off processing. CONSTITUTION:The digital filters 5A-5D of plural bands for taking out the signals of the plural bands from digital audio signals are provided. Noise shaping circuits 7A and 7B are provided for the digital filters 5B and 5D for taking out a low band from among the digital filters 5A-5D. Thus, the re-quantized noise is shifted on the outside of the reproducing band of woofer speaker units 10B and 10D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital audio signal processing device and a digital network device used for reproducing voice using a multi-way speaker.

[0002]

2. Description of the Related Art A crossover network circuit is required when reproducing sound using a plurality of speakers such as a woofer and a tweeter. The crossover network circuit is composed of a plurality of filters for extracting a signal in a band corresponding to the characteristics of each speaker from the input audio signal. Among such crossover network circuits, a digital crossover network circuit using a digital filter is known.

FIG. 8 shows an example of a conventional digital crossover network circuit. In FIG. 8, the left and right analog audio signals from the input terminals 51A and 51B are supplied to the A / D converter 52. A
The / D converter 52 converts the analog audio signal into a digital audio signal. At this time, the sampling frequency is Fs and the number of bits is k.

The output of the A / D converter 52 is supplied to the network circuit 53. The network circuit 53 is
It is composed of digital delay circuits 54A to 54D, digital filters 55A to 55D, and rounding circuits 56A to 56D.

The output of the A / D converter 52 is separated into left and right digital audio signals, and the left digital audio signal is digital delay circuits 54A and 5A.
4B and the right digital audio signal is supplied to the digital delay circuits 54C and 54D.
The digital delay circuits 54A and 54B, 54C and 54D include speaker units 60A, 60B and 60, respectively.
A time axis correction between A and 60B is performed.

In the left channel, the output of the digital delay circuit 54A is the digital high-pass filter 5
5A, and the output of the digital delay circuit 54B is supplied to the digital low pass filter 55B. In the right channel, digital delay circuit 54C
Is supplied to the digital high-pass filter 55C, and the output of the digital delay circuit 54D is supplied to the digital low-pass filter 55D.

In the digital high-pass filter 55A, filtering processing is performed for the tweeter speaker unit 60A, and the high frequency component in the audio signal of the left channel is extracted. Digital low pass filter 55B
Then, the filtering process for the speaker unit 60B of the woofer is performed, and the low frequency component in the audio signal of the left channel is extracted. In the digital high-pass filter 55C, filtering processing is performed for the tweeter speaker unit 60C, and a high frequency component in the right channel audio signal is extracted. Speaker unit 6 of woofer with digital low-pass filter 55D
The filtering process for 0D is performed, and the low frequency component in the audio signal of the right channel is extracted. In addition,
The digital filters 55A to 55D operate at the sampling frequency Fs, and the number of output bits is m bits.

The output of the digital high pass filter 55A is supplied to the rounding circuit 56A, and the output of the digital low pass filter 55B is supplied to the rounding circuit 56B. The output of the digital high-pass filter 55C is a rounding circuit 56C.
And the output of the digital low pass filter 55D is supplied to the rounding circuit 56D.

The output of the rounding circuit 56A is supplied to the D / A converter 58A, and the output of the rounding circuit 56B is supplied to the D / A converter 58B. The output of the rounding circuit 56C is D
The output of the rounding circuit 56D is supplied to the D / A converter 58D.

The D / A converters 58A to 58D operate at the sampling frequency Fs, and the number of input bits is n bits. On the other hand, the number of bits of the digital filters 55A to 55D is m bits (m> n) because the number of bits is increased by the filtering process. D / A
Since the number of input bits n of converters 58A to 58D is smaller than the number of output bits m of digital filters 55A to 55D, rounding circuits 56A to 56D perform rounding processing from m bits to n bits.

The outputs of the D / A converters 58A and 58B are supplied to the tweeter speaker unit 60A and the woofer speaker unit 60B via the amplifiers 59A and 59B. D / A converters 58C and 58
The output of D is supplied to the tweeter speaker unit 60C and the woofer speaker unit 60D via the amplifiers 59C and 59D.

[0012]

In the above-mentioned conventional digital crossover network 53, the D / A
Since the number of input bits n of converters 58A to 58D is smaller than the number of output bits m of digital filters 55A to 55D, rounding circuits 56A to 56D are required. However, if the rounding circuits 56A to 56D simply perform rounding processing, there is a problem that requantization noise occurs.

Incidentally, normally, the D / A converters 58A-5
8D is externally attached, and its input bit number and sampling frequency are specified. Therefore, it is difficult to use the conventional oversampling technique.

Therefore, an object of the present invention is to provide a digital signal processing device and a digital network device capable of reducing requantization noise due to rounding processing.

[0015]

SUMMARY OF THE INVENTION According to the present invention, a low-pass component is extracted from a digital audio signal having a sampling frequency Fs by a digital low-pass filter, the output of the digital low-pass filter is supplied to a noise shaping circuit, and sampling is performed from the noise shaping circuit. A digital audio signal processing device characterized in that a digital audio signal is output at a frequency Fs.

According to the present invention, a digital network having a multi-band digital filter for extracting a multi-band signal from a digital audio signal, converting the output of the digital filter into an analog audio signal and supplying the analog audio signal to each speaker unit. In the device,
A digital network device characterized in that a noise shaping circuit is provided for a digital filter for extracting a low frequency band among digital filters of a plurality of bands.

[0017]

When the outputs of the m-bit digital low-pass filters 5B and 5D are given to the n-bit D / A converters 8B and 8D (m> n), noise shaping is performed on the outputs of the digital low-pass filters 5B and 5D. Circuits 7A and 7B are provided. Due to the noise shaping 7A and 7B, requantization noise is generated by the woofer 10B.
And out of the 10D playback band to reduce passband requantization noise.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention. In FIG. 1, left and right analog audio signals from the input terminals 1A and 1B are supplied to the A / D converter 2. The analog audio signal is converted into a digital audio signal by the A / D converter 2.
At this time, the sampling frequency is Fs and the number of bits is k.

The output of the A / D converter 2 is supplied to the network circuit 3. The network circuit 3 includes digital delay circuits 4A to 4D and a digital filter 5A.
.About.5D, rounding circuits 6A and 6B, and noise shaping circuits 7A and 7B.

The output of the A / D converter 2 is separated into left and right digital audio signals, and the left digital audio signal is supplied to the delay circuits 4A and 4B.
The right digital audio signal is supplied to the digital delay circuits 4C and 4D. The digital delay circuits 4A and 4B, 4C and 4D perform time axis correction between the speaker units 10A and 10B, 10C and 10D.

In the left channel, the output of the digital delay circuit 4A is the digital high pass filter 5A.
And the output of the digital delay circuit 4B is supplied to the digital low-pass filter 5B. In the right channel, the output of the digital delay circuit 4C is supplied to the digital high pass filter 5C, and the output of the digital delay circuit 4D is supplied to the digital low pass filter 5D.

The digital high-pass filter 5A performs a filtering process for the tweeter speaker unit 10A, and extracts the high frequency component in the left channel audio signal. With the digital low pass filter 5B,
The filtering process for the speaker unit 10B of the woofer is performed, and the low frequency component in the audio signal of the left channel is extracted. The digital high-pass filter 5C performs a filtering process for the tweeter speaker unit 10C, and extracts the high frequency component in the right channel audio signal. The digital low-pass filter 5D performs the filtering process for the speaker unit 10D of the woofer, and the low frequency component in the audio signal of the right channel is taken out. The digital filters 5A to 5D operate at the sampling frequency Fs, and the number of output bits is m bits.

The output of the digital high pass filter 5A is supplied to the rounding circuit 6A, and the output of the digital low pass filter 5B is supplied to the noise shaping circuit 7A. The output of the digital high-pass filter 5C is supplied to the rounding circuit 6B, and the output of the digital low-pass filter 5D is supplied to the noise shaping circuit 7B.

The output of the rounding circuit 6A is the D / A converter 8
Is supplied to A, and the output of the noise shaping circuit 7A is D
It is supplied to the / A converter 8B. The output of the rounding circuit 6B is supplied to the D / A converter 8C, and the output of the noise shaping circuit 7B is supplied to the D / A converter 8D.

The D / A converters 8A to 8D operate at the sampling frequency Fs, and the number of input bits is
It is n bits. On the other hand, the digital filter 5
The number of bits of A to 5D is m, and the number of input bits of n of the D / A converters 8A to 8D is n.
It is less than the output bit number m of A to 5D. Therefore, the rounding circuits 6A and 6B perform rounding processing from m bits to n bits. The noise shaping circuits 7A and 7B are first-order noise shaping circuits and reduce requantization noise in the conversion processing from m bits to n bits.

The outputs of the D / A converters 8A and 8B are
It is supplied to the tweeter speaker unit 10A and the woofer speaker unit 10B via the amplifiers 9A and 9B. The outputs of the D / A converters 8C and 8D are
It is supplied to the tweeter speaker unit 10C and the woofer speaker unit 10D via the amplifiers 9C and 9D.

FIG. 2 shows an example of the noise shaping circuits 7A and 7B. In FIG. 2, a digital audio signal is supplied to the input terminal 21. This digital audio signal is supplied to the adding circuit 22.
The output of the addition circuit 22 is supplied to the quantization circuit 23 and the subtraction circuit 24. The output of the quantization circuit 23 is output from the output terminal 25 and is also supplied to the subtraction circuit 24. The output of the subtraction circuit 24 is supplied to the addition circuit 22 via the delay circuit 26.

In such noise shaping circuits 7A and 7B, assuming that the input signal is x, the output signal is y, and the quantization error by the quantization circuit 23 is N, the transfer function is y = x + (1-Z ^{− 1} ) · N, and the quantization error can be shifted out of band.

In FIG. 3, the sampling frequency Fs is (Fs
= 44.1 kHz), the characteristics of the quantization noise at the output y are shown. In FIG. 3, the horizontal axis represents frequency and the vertical axis represents level. As shown in FIG. 3, when such a noise shaping circuit is used, the quantization noise is shifted to a high band, and the quantization noise increases at about 7 kHz or higher. However, in general, the frequency characteristic of the woofer is already attenuated at 7 kHz, and the noise increased at about 7 kHz or more is caused by the speaker unit 1 of the woofer.
It is not played back in 0B and 10D. On the other hand, the quantization noise is significantly reduced below 1 kHz, which is the reproduction band of a general woofer.

As described above, when the noise shaping circuit is provided, the requantization noise shifts to the high frequency range.
A noise shaping circuit is effective for the path of the woofer to the speaker unit.

FIG. 4 shows a low-pass filter having a cut-off frequency of 800 kHz, which is simply rounded. FIG. 5 is an enlargement of this characteristic centered on an amplitude of 0 dB. As shown in FIG. 5, it can be seen that ripples occur when the rounding process is simply performed.

On the other hand, FIG. 6 shows the characteristics when the first-order noise shaping is performed, and FIG. 7 shows the characteristics enlarged with an amplitude of 0 dB as the center. As shown in FIG. 7, it can be seen that the lip is reduced by the primary noise shaping circuit.

[0033]

According to the present invention, when the outputs of the m-bit digital low-pass filters 5B and 5D are given to the n-bit D / A converters 8B and 8D (m> n), the digital low-pass filters 5B and 5D are provided. Noise shaping circuits 7A and 7B are provided for the output of. By the noise shaping 7A and 7B, the requantization noise is shifted out of the reproduction band of the woofers 10B and 10D, and the requantization noise in the pass band can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of an example of a noise shaping circuit according to an embodiment of the present invention.

FIG. 3 is a frequency characteristic diagram used for explaining one embodiment of the present invention.

FIG. 4 is a frequency characteristic diagram used for explaining one embodiment of the present invention.

FIG. 5 is a frequency characteristic diagram used for explaining one embodiment of the present invention.

FIG. 6 is a frequency characteristic diagram used for explaining one embodiment of the present invention.

FIG. 7 is a frequency characteristic diagram used for explaining one embodiment of the present invention.

FIG. 8 is a block diagram of a conventional digital crossover network.

[Explanation of symbols]

 2 A / D converter 3 Network circuit 5A-5D Digital filter 6A, 6D Rounding circuit 7A, 7B Noise shaping circuit 8A-8D D / A converter 10A, 10C Tweeter 10B, 10D Woofer

Claims (2)

[Claims] 1. A digital low-pass filter extracts low-frequency components of a digital audio signal having a sampling frequency Fs, supplies the output of the digital low-pass filter to a noise shaping circuit, and the noise shaping circuit outputs the sampling frequency Fs.
A digital audio signal processing device, characterized in that a digital audio signal is output by s. 2. A digital network device comprising a multi-band digital filter for extracting a multi-band signal from a digital audio signal, converting the output of the digital filter into an analog audio signal, and supplying the analog audio signal to each speaker unit. 2. A digital network device according to claim 1, wherein a noise shaping circuit is provided for a digital filter for extracting a low frequency band among the digital filters for a plurality of bands.