JPS63211921A - Dc elimination circuit for d/a conversion output - Google Patents

Abstract

PURPOSE:To prevent the deterioration in high frequency characteristic of an audio signal effectively by realizing a negative feedback loop of a DC component by a digital circuit. CONSTITUTION:The DC component included in an analog output of an output terminal OUT is extracted by an analog low-pass filter 21, converted into a digital DC component by an A/D converter 24 and fed back negatively to a subtraction input terminal of a subtractor 26 via a resampling circuit 25. A digital audio signal whose DC component is eliminated is outputted from the subtractor 26 as the digital DC component fed back negatively is subtracted, supplied to a D/A converter 11 and converted into a corresponding analog audio signal. Thus, even when the high frequency distortion component is generated in the capacitor in the analog low-pass filter, it is smoothed by rounding resulting from quantization at the time of A/D conversion and not fed back.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a DC removal circuit for a D/A conversion output installed in a digital audio device or the like.

2. Description of the Related Art Conventionally, in digital audio equipment and the like, a negative DC feedback loop is formed by an analog circuit as shown in FIG. 2 in order to remove the DC component from the analog output of a D/A conversion circuit.

That is, the digital audio signal supplied to the input terminal IN is converted into a D/A conversion converter analog audio signal, and is passed through a deglitch circuit 2, a low-pass filter 3,
The signal is supplied to the output terminal 0tJT via the subtracter 4 and the output amplifier 5. On the other hand, the DC component contained in this analog output is extracted by a low-pass filter 6, and is negatively fed back to the subtraction input terminal of the subtracter 4 via a level adjuster 7. The subtracter 4 outputs an analog audio signal from which the DC component has been removed by subtracting the negative feedback DC component.

This analog output is passed through the output amplifier 5 to the output terminal OU.
supplied to T.

Problems to be Solved by the Invention In the D/A conversion output DC removal circuit shown in FIG.
A capacitor is inevitably included inside the capacitor, but the high-frequency characteristics of this capacitor generally change in a complicated manner due to the influence of parasitic inductance. There is a problem in that the change in the high-frequency characteristics of the capacitor causes deterioration such as distortion in the high-frequency characteristics of the analog audio signal.

Instead of forming a DC feedback loop as shown in FIG. 1, it is also conceivable to simply insert a DC blocking capacitor in series between the output amplifier and the output terminal. This method also has the problem that the high-frequency characteristics of the audio signal deteriorate due to complicated changes in the high-frequency characteristics of the capacitor.

Means for Solving the Problems The D/A conversion output DC removal circuit according to the present invention is a D/A conversion output DC removal circuit.
A low-pass filter that extracts the DC component included in the analog signal output from the A conversion circuit, an A/D converter that converts the analog output of the low-pass filter into a digital signal, and the A/D converter. It includes a subtracter that subtracts the output of the converter from the digital input supplied to the D/A converter circuit, and an overflow limiter installed between the subtracter and the D/A converter.

Operation In the DC removal circuit of the present invention, even if high-frequency distortion components occur due to parasitic inductance in the capacitor in the analog low-pass filter that constitutes the negative feedback loop,
This is smoothed out by rounding associated with quantization during A/D conversion. Therefore, high-frequency distortion components are no longer included in the digital DC signal and fed back. In one embodiment of the present invention, in order to emphasize the smoothing by rounding,
A/D conversion is performed with a smaller number of quantization steps than in the original digital audio signal.

Further, according to an embodiment of the present invention, the original digital
The above-mentioned smoothing effect is further emphasized by the combination of A/D conversion at a sampling frequency lower than that of the audio signal and multiplication of the sampling frequency by subsequent scaling/sampling.

Hereinafter, further details of the present invention will be explained together with examples.

Embodiment FIG. 1 is a block diagram showing the configuration of a D/A conversion output DC removal circuit 20 according to an embodiment of the present invention, together with the configuration of a D/A conversion circuit IO.

The D/A conversion circuit 10 includes a D/A converter 11, a deglitch circuit 12, a low-pass filter 13, and an output amplifier 14.

Further, the DC removal circuit 20 includes an analog low-pass filter 2
1. Level adjuster 22, sample/hold circuit 23,
It consists of an A/D converter 24, a summation circuit 25, a subtracter 26, and an overflow limiter 27.

The digital audio signal supplied to the input terminal IN passes through a subtracter 26 and an overflow limiter 27 to D.
The signal is converted into an analog audio signal by the /A converter 11, and is supplied to the output terminal OUT via the deglitch circuit 12, the low-pass filter 13, and the output amplifier 14.

On the other hand, the DC component contained in the analog output on the output terminal OUT is extracted by the analog low-pass filter 21. This analog DC component is sent to the level adjuster 22 and the sample
The signal is supplied to the A/D converter 24 via the hold circuit 23, where it is converted into a digital DC component. This digital DC component is negatively fed back to the subtraction input terminal of the subtracter 26 via the sampling circuit 25. The subtracter 26 outputs a digital audio signal from which the DC component has been removed by subtracting the negative feedback digital DC component, which is supplied to the D/A converter 11 and converted into a corresponding analog audio signal. Ru.

Sampling by the sample/hold circuit 23 and A
The conversion into digital DC components by the /D converter 24 is as follows:
The period Ts/n (n=1.
2.3...). 2. The sampling circuit 25 has a sampling period Ts by repeatedly outputting the digital DC component supplied from the A/D converter 24 at a sampling period of Ts/n n times at a period n times that of the digital DC component. It is supplied to the subtraction input terminal of the subtracter 26 while converting it into a digital DC component.

The number of quantization steps in the A/D converter 24 is set to a value smaller than the number of digitization steps of the digital audio signal supplied to the input terminal IN, and quantization is performed at a coarse level. Smoothing by rounding is performed on the high frequency distortion components generated by the capacitors in the low pass filter supply 21.

The overflow limiter 27 is provided with a subtracter 26 for the purpose of preventing polarity reversal due to an overflow that may occur due to a phase shift or offset during negative feedback of a digital DC component, thereby preventing the generation of noise. and the D/A converter 11.

Effects of the Invention As explained in detail above, the D/A conversion output DC removal circuit according to the present invention has a configuration in which the negative feedback loop of the DC component is realized by a digital circuit. Even if high-frequency distortion components occur with the capacitor,
This is smoothed by rounding associated with quantization during A/D conversion and is no longer fed back. Therefore, deterioration of the high-frequency characteristics of the original audio signal is effectively prevented.

In one embodiment of the present invention, in order to emphasize the smoothing due to rounding, A/D conversion is performed using fewer quantization steps and a lower sampling frequency than in the original digital audio signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a D/A conversion output DC removal circuit according to an embodiment of the present invention together with the configuration of the D/A conversion circuit, and FIG. 2 is a block diagram showing the configuration of a conventional DC removal circuit. A
FIG. 2 is a block diagram shown together with the configuration of a conversion circuit. 10...D/A conversion circuit, 20...DC removal circuit for D/A conversion output, 21...analog low-pass filter,
22... Level adjuster, 23... Sample/hold circuit, 24... A/D converter, 25... Sampling circuit, 26... Subtractor, 27... Overflow limiter . Patent applicant: NEC Home Electronics Co., Ltd.

Claims (1)

[Claims] A low-pass filter that extracts a DC component included in an analog signal output from a D/A conversion circuit, and an A/D that converts the analog output of the low-pass filter into a digital signal.
a converter; a subtracter that subtracts the output of the A/D converter from the digital input supplied to the D/A conversion circuit; and an overflow installed between the subtractor and the D/A conversion circuit.・D/ which is characterized by being equipped with a limiter.
DC removal circuit for A conversion output.