JP2000059222A - D/a converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce audio data with high accuracy by providing a means that expands the number of bits of a digital signal and a means that selects the digital signal obtained by a demodulation means or the digital signal obtained by the bit number expansion means to a component of the D/A converter. SOLUTION: A data line changeover device 402 is connected to an output of a bit expansion device 401. One of signals given to the data line changeover device 402 is a signal outputted from the bit expansion device 401 and the other signal is a signal before it is given to the bit expansion device 401. In the case that a bit length of sample data given to the bit expansion device 401 is 16-bit, a microcomputer 9 controls the data line changeover device 402 to connect to a data line A. In the case that a bit length of sample data given to the bit expansion device 401 is 24-bit, the microcomputer 9 controls the data line changeover device 402 to connect to a data line B that bypasses the bit expansion device 401.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital-to-analog converter for converting digital audio data into an analog audio signal.

[0002]

2. Description of the Related Art Music compact disks (hereinafter referred to as CDs)
-DA) is a digital audio disc in which an audio signal is linearly quantized and recorded as 16-bit audio data. A compact disc player (hereinafter, referred to as a CD player) for reproducing this CD-DA is
Normally 16-bit DAC (Digital Analog Converter)
Is a device that converts digital audio data into an analog audio signal and outputs the same.

[0003] In addition to the function of outputting analog audio signals, the CD player has a function of outputting digital audio data reproduced from a CD-DA to a digital audio interface standard (Japan Electronic Machine Industry Association standard, digital audio interface (PC-340) 1987). (Established in September)
Some have a function of converting the signal into a signal of a transmission format based on the data and outputting the signal.

A digital-to-analog converter (hereinafter, referred to as a DA converter) that inputs a signal transmitted in the above-described transmission format, demodulates the signal into digital audio data, and outputs the signal as an audio signal has been put to practical use. .
In a conventional DA converter, a digital audio signal transmitted in the transmission format is a CD-DA
Alternatively, the bit length for reproducing a signal recorded on a DAT (Digital Audio Tape) is assumed to be 16 bits.

[0005] In recent years, as the recording density of digital audio discs has increased, it has become possible to record data having a bit length of 16 bits or more as sample data which is digital audio data.
It has been required to be able to reproduce 4-bit digital audio data. For this reason, a high-precision 24-bit DAC capable of converting 24-bit digital audio data into an analog signal is provided in the DA converter.
(Digital Analog Converter) has been built in.

In order to make full use of the performance of the 24-bit DAC built in the DA converter,
When bit sample data is input, LSB
A method has been performed in which 8-bit prediction data is added to the lower order, converted to 24-bit sample data, and then subjected to DA conversion. For bits lower than the 16-bit LSB, there is a method of using prediction data obtained by calculating from the difference between the sample data and the preceding and following sample data, and the accuracy of the prediction data can be further improved.

[0007]

However, the transmission format based on the digital audio interface standard does not include data such as a flag indicating the bit length of the transmitted sample data. Even if the bit length of the signal to be output is data of 24 bits, the data of the lower 8 bits is ignored and the upper 16 bits are ignored.
The D / A conversion is performed on 24-bit sample data to which prediction data obtained by calculation using only bit data is added. For this reason, even if the lower 8-bit data is sample data of a normal audio signal, the data is replaced with prediction data, and the inherent information is not used. For this reason, even if the DA converter has a 24-bit DAC, there is a problem that the accuracy of digital audio data, which is the original 24-bit data, cannot be obtained. SUMMARY OF THE INVENTION It is an object of the present invention to provide a DA converter capable of resolving the above-described problem and reproducing high-precision audio data.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a digital-to-analog converter for converting a digital signal into an analog signal, a demodulator for demodulating a digitally modulated signal to obtain a digital signal, and a digital signal demodulated by the demodulator. Bit expansion means for expanding the number of bits of the digital signal obtained by the demodulation means or a digital signal obtained by the bit expansion means, and a digital signal selected by the switching means. Digital-to-analog conversion means for converting the signals into signals, control means for controlling the operation of the demodulation means, the bit extension means, and the signal switching means, and operation means for instructing the signal switching means to switch digital signals. It is a digital-to-analog converter.

Further, according to the present invention, in the digital-to-analog conversion apparatus according to the first aspect, the control means receives a signal transmitted from the operation means and instructs the signal switching means to switch a digital signal. Digital-to-analog converter.

According to another aspect of the present invention, there is provided a digital-to-analog converter for converting a digital signal into an analog signal, a demodulating means for demodulating the digitally modulated signal to obtain a digital signal, and a digital signal demodulating means for demodulating the digital signal. Bit expansion means for expanding, a signal switching means for selecting any of a digital signal obtained by the demodulation means and a digital signal obtained by the bit expansion means, and converting the digital data selected by the switching means into an analog signal. Digital-to-analog conversion means, control means for controlling the operation of the demodulation means, the bit extension means, and the signal switching means, and whether or not there is a significant signal in the lower bit signal of the digital signal output from the demodulation means And identification means for identifying the digital signal. Upon identifying that there is a digital-to-analog converter for controlling said signal switching means to select the digital signal obtained from said demodulating means.

According to a third aspect of the present invention, in the digital-to-analog converter according to the third aspect, the control means determines whether or not there is a significant signal in a lower bit signal of the digital signal output from the demodulation means. A digital-to-analog converter that controls the signal switching means so as to select a digital signal obtained from the demodulation means when it is determined that there is a significant signal.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a transmission format of a digital audio interface standard used in an embodiment of the present invention will be described. FIG. 2 is a diagram illustrating a subframe format of a digital audio interface used in the DA converter according to one embodiment of the present invention.

The subframe format of the digital audio interface is a transmission format in which 32 time slots constitute one subframe. Time slots 0 to 3 are signal portions including a synchronization signal called a preamble. Time slots 4-7
Is a 4-bit extension signal portion used to extend the bit length of an audio sample called an "occasion". Time slots 8 to 27 are 20-bit signal sections for transferring sample data.

In the case of a CD-DA, the sample data is 1
Since it is 6 bits, 16 to time slots 12 to 27
The bit sample data is transferred. At this time,
Time slot 27 is the MSB of the sample data,
Time slot 12 is an LSB. The time slots 8 to 11 and 4 to 7 are 0.

The time slot 28 is a data portion indicating whether or not the value of sample data called a validity flag is correct. The time slot 29 is a data part called user data in which information other than the sample data is recorded. The time slot 30 is a sampling frequency of sample data called a channel status and a data portion of a time code. The time slot 31 is a parity bit for detecting an error in data in the subframe format.

The DA converter receives a signal of a transmission format determined by the digital audio interface standard, demodulates a sample data signal and a signal other than the sample data, and reproduces an audio signal and information on music to be reproduced. get.

Next, an embodiment of the present invention will be described. FIG. 1 is a configuration diagram illustrating a configuration of a DA converter according to an embodiment of the present invention. The input unit 1 has an optical input A101, an optical input B102, a coaxial input C103, and a coaxial input D104 as a plurality of digital input terminals.

The input switching unit 2 is a switching unit for switching a plurality of input signals from the digital input terminal unit 1, and is switched by operating an input switching button 10b provided on the operation unit 10. The input signal selected by the input switch 2 has the transmission format shown in FIG. 2 and is input to the digital audio interface demodulator 3 and demodulated as audio data.

Here, it is assumed that the bit length of the transmitted sample data is 16 bits. In the bit extender 401, the prediction data obtained by calculating from the sample data and the values of a plurality of sample data before and after the sample data is added to 17 to 24 bits, which are the lower 8 bits of 24 bits. An embodiment of the means for obtaining the prediction data described here will be described later.

The 24-bit output data output from the bit expander 401 is a DIF (Digital Filter) 5
Is over-sampled, and then input to a 24-bit DA converter 6 to be converted into an analog signal. Thereafter, the signal passes through an LPF (low-pass filter) 7 for removing out-of-band frequencies, is amplified by an amplifier 8, and is output to the outside as an audio signal.

Next, it is assumed that the bit length of the transmitted sample data is 24 bits. In the related art, it is not possible to determine whether the input transmission data is 16 bits or 24 bits, so the bit expander of the DA converter always performs the bit expansion operation using only the upper 16 bits. Therefore, the input data of the lower 8 bits is unnecessary, so that the operation is performed only with the upper 16 bits excluding the lower 8 bits, and the prediction data of the operation result is added to the lower 8 bits 17 to 24 bits and the subsequent circuit. Input to the digital filter.

In the embodiment shown in FIG.
1 shows a configuration in which a data line switching unit 402 is provided at the output of No. 01. One of the signals connected to the data line switch 401 is a signal output from the bit expander 401, and the other is a signal before being input to the bit expander 401. When the bit length of the sample data input to the bit extender 401 is 16 bits, the microcomputer 9 controls the data line switch 402 so as to be connected to the data line indicated by A in FIG. When the bit length of the sample data input to the bit expander 401 is 24 bits, the microcomputer 9 sets the data line switch 402
Is controlled to be connected to the data line indicated by B in FIG.

Next, the means for determining whether the bit length of the sample data input to the bit extender 401 is 16 bits or 24 bits and controlling the data line switch 402 will be described. FIG. 3 is a diagram showing a DA according to an embodiment of the present invention.
FIG. 4 is a configuration diagram showing a configuration in which a data line is switched in accordance with an instruction from an operation unit in the conversion device. FIG. 4 is a configuration diagram showing a configuration in which a data line is switched by judging from the contents of sample data in a DA converter according to another embodiment of the present invention. FIG. 5 is a diagram illustrating the relationship between the waveform of an original audio signal and sample data quantized by 16 bits.

The difference between the waveform of the audio signal, which is an analog signal before AD conversion, and the sample data quantized by 16 bits is called a quantization error and has a maximum size of 1/2 LSB. In order to reduce the quantization error, a prediction waveform predicted from the magnitude of the preceding and succeeding sampling data and a difference between the magnitudes of the sampling data before and after the LSB of 16 bits is added to the lower bits, and data (prediction data) obtained from the prediction waveform is added. I do.

The bit extender 401 as a means for obtaining prediction data used in this embodiment will be described with reference to FIGS. 3, 4 and 5. FIG.

The sample data F1, F2,
F3 is 16-bit quantized data. The difference between the sample data F2 and F3 is multiplied by the coefficient of α1 to the difference between the sample data F1 and F2 (F1−F2).
(F3-F2) is multiplied by a coefficient of α2. The sample data F2 is multiplied by a coefficient 1, and F2 and △ (F1-F
2) Find the sum of × α1 and △ (F3-F2) × α2.

By calculating the product of the 16-bit data of F1 and F3 and the coefficient α having a finite number of digits, the number of digits of the numerical value resulting from the operation becomes a number exceeding 16 bits. Where α
By selecting an appropriate value of the significant figure as the value of and calculating, 24-bit data is obtained. However, the 8-bit data from 17 bits to 24 bits is prediction data obtained from finite data, and includes an error when compared with the original audio signal. The arithmetic precision can be improved by appropriately selecting the significant figures of.

In FIG. 5, the error between the sample data F2 and the original audio signal is indicated by △ 2. By adding the prediction data obtained above to the 8 bits lower than the LSB of the 16-bit data, Δ2 can be corrected.

Next, the means for controlling the data line switch will be described. First, the configuration of the present embodiment shown in FIG. 3 will be described.

When it is known in advance that the signal input from the digital input terminal unit 1 transmits 16-bit sample data, the output from the digital audio interface demodulator 3 input to the bit expander 401 Is 16 bits, the data line switch is connected to the data line indicated by A by operating the bit extension ON / OFF switch 10a provided in the operation unit 10 and setting the input to 16 bits. , And the signal output from the bit expander 401 is input to DIF5.

Next, when it is known in advance that the signal input from the digital input terminal 1 transmits a 24-bit signal, the digital audio interface demodulator 3 input to the bit expander 401 Is output in 24 bits, the data line switcher is connected to the B data line by operating the bit extension ON / OFF switch 10a provided in the operation unit 10 to set it as a 24-bit input. Switch as follows. Therefore, the signal output from the bit expander 401 is not input to the DIF 5, but the output of the digital audio interface demodulator 3 is input.

As described above, when the output from the digital audio interface demodulator 3 is 24 bits by the bit extension ON / OFF switch 10a provided in the operation unit 10, the error calculated by the bit extension unit 401 is included. By switching from the prediction data and using the 24 bits of the input data as they are, highly accurate DA conversion can be performed.

In the embodiment shown in FIG. 3, whether the input data is 16 bits or 24 bits is switched by the operation unit. In the other embodiment shown in FIG. A lower bit presence / absence discriminator 12 for monitoring 17 to 24 bits, which are lower 8 bits of input data, is provided.

The lower bit presence / absence discriminator 12 determines whether or not the audio data has information of sample data from time slot 4 to time slot 11, that is, whether the sample data has a bit length of 16 bits or a bit length larger than 16 bits. This is an identification unit for identification.

In the transmission format defined by the digital audio interface standard, information such as a flag for discriminating whether the sample data is 16 bits or 24 bits is not transferred. It is possible to identify the bit length from. According to the digital audio interface standard, when the sample data is 16 bits, 4 bits from time slot 8 to time slot 11 are 0. Time slots 4 to 7 used for data extension are 0. Therefore, by monitoring demodulated data corresponding to signals from time slot 4 to time slot 11, it is possible to identify how many bits the transmitted signal is.

The embodiment shown in FIG. 4 is provided with a discriminator capable of discriminating whether the lower 8 bits are all 0s or 1s. For example, it can be realized by the sum of the signals of the lower 8 bits, that is, a circuit constituted by an OR circuit. For example, if the bit length of the sample data is 16 bits, the output of the OR circuit remains 0, but if it is 24 bits, the output of the OR circuit becomes 1 because any of the lower 8 bits has changed. Or 0 and 1 at random. The microcomputer 9 may control the data line switch by judging the output state of the OR circuit, or directly input the signal of each of the lower 8 bits to the microcomputer 9 and check the signal on a program to check the bit. The data line switch may be controlled so as to identify the length.

In FIG. 3, although not shown, the bit extension ON / OFF of the operation unit for controlling the data line switch 402 is described.
A signal generated by operating the OFF switch 10a may be input to the microcomputer 9, and the microcomputer 9 may control data line switching. In FIG. 4, although not shown, the low-order bit data presence / absence discriminator 12 that controls the data line switch 402 can be realized by a program of the microcomputer 9.
May be controlled by

In the above embodiment, 16-bit data and 24
Although the bit data has been described, the present invention can be applied to any data having a bit length of 16 bits and 17 bits to 24 bits.

In the above embodiment, the DA converter has been described. However, a digital device such as a CD player, a DVD player, or a DAT having the function of the DA converter can be similarly used.

[0040]

According to the present invention, when the audio sample of the digital audio interface input to the DA converter is transferred as 24-bit data, the data of the input audio sample is used as it is, and the original digital audio data is used. It is possible to obtain the accuracy of the data, and the audio samples of the digital audio interface input to the DA converter are 16
When the data is transferred by bit data, the lower 8 bits, ie, 17 bits, are added with prediction data to 24 bits.
By using bit data, 16 bits or more of D
A conversion accuracy can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration of a DA converter according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a subframe format of a digital audio interface.

FIG. 3 is a configuration diagram showing a configuration in which a data line is switched by an instruction from an operation unit in the DA converter according to one embodiment of the present invention;

FIG. 4 is a configuration diagram showing a configuration in which a data line is switched by determining from the contents of sample data in a DA converter according to another embodiment of the present invention.

FIG. 5 is a view for explaining a waveform of an original audio signal and audio data quantized to 16 bits.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Digital input terminal part 2 Input switch 3 Digital audio interface demodulator 4 Data switching part 5 DIF (Digital Filter) 6 DA converter 7 LPF (Low Pass Filter) 8 Amplifier 9 Microcomputer 10 Operation part 10a Bit expansion ON / OFF button 10b Input switch button 11 Display unit 12 Lower bit presence / absence discriminator 101 Optical input A 102 Optical input B 103 Coaxial input C 104 Coaxial input D 401 Bit extender 402 Data line switch

Claims (4)

[Claims] 1. A digital-to-analog converter for converting a digital signal into an analog signal, a demodulation means for demodulating a digital modulation signal to obtain a digital signal, and a bit extension for expanding the number of bits of the digital signal demodulated by the demodulation means. Means, signal switching means for selecting one of a digital signal obtained by the demodulation means and a digital signal obtained by the bit expansion means, and digital / analog conversion means for converting the digital data selected by the switching means into an analog signal. Digital-to-analog conversion, comprising: a demodulation unit, a bit extension unit, a control unit for controlling operations of the signal switching unit, and an operation unit for instructing the signal switching unit to switch a digital signal. apparatus. 2. The digital-to-analog converter according to claim 1, wherein the control unit receives a signal transmitted from the operation unit and issues an instruction to switch the digital signal of the signal switching unit. Digital-to-analog converter. 3. A digital-to-analog conversion device for converting a digital signal into an analog signal, a demodulation means for demodulating a digital modulation signal to obtain a digital signal, and a bit extension for expanding the number of bits of the digital signal demodulated by said demodulation means. Means, signal switching means for selecting one of a digital signal obtained by the demodulation means and a digital signal obtained by the bit expansion means, and digital / analog conversion means for converting the digital data selected by the switching means into an analog signal. Control means for controlling the operation of the demodulation means, the bit extension means, and the signal switching means; and identification for identifying whether or not there is a significant signal in a lower bit signal of the digital signal output from the demodulation means. Means for identifying the digital signal as having a significant signal. A digital-to-analog converter that controls the signal switching means to select a digital signal obtained from the demodulation means. 4. The digital-to-analog converter according to claim 3, wherein the control means identifies whether or not there is a significant signal in a lower bit signal of the digital signal output from the demodulation means. A digital-to-analog conversion device, wherein when it is determined that there is a signal, the signal switching unit is controlled to select a digital signal obtained from the demodulation unit.