JP3059140U - Low-pass filter for analog audio signals with excellent noise blocking characteristics - Google Patents

Abstract

(57) [Summary] [Purpose] To improve the quality of reproduced sound of an analog audio signal by improving the noise cutoff characteristics of a low-pass filter. [Constitution] A low-pass filter is combined with an isolation amplifier using a linear type photocoupler.

Description

[Detailed description of the invention] [Industrial applications]

The present invention is to improve the quality of reproduced sound of digital audio equipment.

[Prior art]

[0002] Digital recording and playback devices have replaced conventional analog recording and playback devices due to their superior physical properties. However, the reproduced sound is not superior to the analog system in all aspects. The cause is that high frequency noise peculiar to a digital circuit generated at the time of D / A conversion or A / D conversion is mixed into an analog audio signal from various signal paths and changes its spectrum. As a countermeasure, the separation of analog and digital circuits and the high precision of D / A and A / D conversion have been attempted. In recent years, the sampling frequency has been increased and the format has been improved by increasing the number of bits. It is scheduled. In the case of reproduction, the improvement of the performance of the D / A converter has been especially focused. However, it is impossible to completely eliminate the generation of various types of noise that can be called the fate of digital circuits. Normal low-pass filters cannot remove high-frequency noise mixed into the signal. Using a digital filter to reduce the role of a low-pass filter also generates another type of noise. In order to make the audio output of a digital device containing such complicated noise close to the original signal without noise, an additional output transformer may be added to the output of the D / A converter, but complete isolation is not possible. In addition, the results are not entirely satisfactory due to coil resonance and narrow frequency band. Even if the low-pass filter is improved, noise in the high frequency range cannot be cut off. In conclusion, for the above reasons, it is true that the reproduced sound of the digital recording / reproducing device remains at this level less than the reproduced sound of the analog recording / reproducing device. In other words, even if that part is only a small part, there is something that is sufficient for the purpose of communication such as conversation, but insufficient for reproducing music as art.

[Problems to be solved by the invention]

[0003] Immediately after D / A conversion of a digital signal, a low-pass filter is required because noise outside the audible band is included even if oversampling is performed by a digital filter. Since the sound quality is degraded as the level is increased, there is a contradiction in which the minimum necessary must be used, and the performance is not sufficient for high frequency noise.

[0004] Digital noise is mixed into the analog audio signal again at a later stage through the power supply line and the ground, thereby deteriorating sound quality. This cannot be completely removed even with a capacitor or an L component element.

[0005] Even if an output transformer having excellent isolation characteristics is provided in addition to the low-pass filter, an audible narrow-band feeling, which is a drawback of the transformer, is generated, and this is not a complete solution.

[Means for Solving the Problems]

[0006] The present invention has an object to solve such a conventional disadvantage. The characteristic configuration of the low-pass filter of the present invention, which has been implemented to achieve the above-mentioned object, includes a low-pass filter, a photocoupler, and an amplifier.

[0007] The low-pass filter (1) attenuates high frequencies outside the audible band. Its frequency, attenuation characteristics, and method are determined from the sound quality balance of the entire device. Whether it is installed before or after the input of the photocoupler is determined by the sound quality balance of the entire device. The photocoupler (2) uses a linear output photocoupler for analog circuits with excellent noise characteristics, response speed, and output linearity. It is also desirable to use a device with a built-in negative feedback element to improve the total drift characteristics. The amplifier (3) is provided with a power supply that is separate from or independent of the circuit before the photocoupler input. The power supply section (4) uses a high-quality sound designed for the audio circuit. The above basic circuit is shown in FIGS. FIG. 3 shows a simple measure in which there is one power supply unit but the supply unit is separated.

[Action]

The output of the D / A converter input to the low-pass filter (1) of FIG. 1 is connected to the input of the photocoupler (2) after the high-frequency band outside the audible band is attenuated. The higher frequency band is further attenuated due to the limitation of the band related to the photocoupler response speed. Common mode noise transmitted through the power line and ground and flying through the capacitor is cut off for isolation or independent power supply from the isolation characteristics configured by the photocoupler. The photocoupler output is reconstructed by an amplifier (3) driven by a separate or independent power supply. This output signal has very little noise that impairs sound quality as compared to the input signal due to the characteristics of this device. Reconstruction of the reproduced sound by the amplifier (3) when converting light into an electric signal is the most important in the present apparatus, in the same way that the sound quality of the record reproduced sound is determined by the performance difference of the cartridge for reproducing the analog record. Is the point.

FIG. 2 is similar to that of FIG. 1 except that the low-pass filter (1) is placed after the amplifier (3). FIG. 3 is the same as FIG. 1 in terms of the operation of the apparatus except that there is only one power supply unit.

The present device works not only as a low-pass filter of a D / A converter, but also as a sound quality improvement device for an independent analog audio signal. That is, the sound quality can be improved by adding this device to the output of a D / A converter completed as a device with a built-in low-pass filter.

Furthermore, since the sound quality of any analog audio signal can be improved without being limited to the reproduction signal of a digital audio device, the sound quality of an electronic musical instrument output, a PA device, a studio device, and the like can be improved.

【Example】

[0012] This device was incorporated into a commercially available D / A converter. A remarkable improvement was confirmed in comparison with the original sound quality, and a natural reproduction sound was obtained as well as the record reproduction sound of a high-end analog record player.

[Effect of the invention]

[0013] About 10 people, including piano tuners, conductors, singers, performers, audio designers, audio shop owners, and audio enthusiasts, were asked to monitor one after another, and almost all were satisfied with the performance and the sound quality. Stated. In addition, when this device was removed and switched to the attached low-pass filter on the spot, all of them recognized significant deterioration in sound quality.

Since the present invention dramatically improves the sound quality of digital audio equipment, past recordings, including the recordings of excellent performers who can be said to be the historical heritage of mankind, were left by SP recording on 78-turn shellac in the past. Good playback of great recordings is also possible with digital audio equipment. This is not only a pleasure for ordinary music appreciators, but also makes it possible for the performers to reaffirm the greatness of past musicians, and will greatly contribute to the improvement of the world's music culture. The biggest advantage of the present invention is that the cost of this device is extremely low compared to the main part of the current digital audio equipment, which suggests the rapid spread of the present invention. The implementation of the present invention will be a great help for music arts and music education.

[Brief description of the drawings]

Fig. 1 Basic block diagram

[Explanation of symbols]

 1-Low-pass filter 2-Photo coupler 3-Amplifier 4-Power supply

FIG. 2 is a block diagram in which a low-pass filter is provided at a subsequent stage.

FIG. 3 is a simplified block diagram having a common power supply unit.

Claims (2)

[Utility model registration claims] 1. A low-pass filter for a D / A converter having improved noise cut-off characteristics by combining an active or passive low-pass filter with an isolation amplifier using a linear-type photocoupler. 2. An analog audio signal low-pass filter having improved noise cutoff characteristics by combining an active or passive low-pass filter with an isolation amplifier using a linear type photocoupler.