JPS5942494B2 - noise reduction couch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction device, and particularly to a dbx
The present invention relates to a device for obtaining characteristics compatible with a noise reduction device known by the trade name ANR8.

Various noise reduction devices have been developed as means for circuit-wise improving the dynamic range of recording media, which is the main limiting factor for the dynamic range of audio devices.

With the exception of a few things, all of them have in common that they compress the dynamic range of the signal during recording, and expand it to the state before recording during playback.

Cassette decks, which have recently become popular, have a significantly inferior Ω/N ratio compared to disc or reel-to-reel tape decks.

Nowadays, most intermediate level players and above are equipped with Dolby noise reduction devices, which are somewhat effective.

A device called ANR8 is also used in some products, but other than a slight difference in circuit system, its effects and characteristics are the same as those of the Dolby noise reduction device.

On the other hand, recently, a method called a dbx noise reduction device has appeared.

The noise reduction effect of this device is much greater than that of Dolby noise reduction devices, etc., and it also has many features such as no need for level adjustment and high fidelity of sound quality.

Currently, it is limited to professional equipment mainly due to cost, but with the realization of lower costs, especially the use of ICs, there is a trend for it to be used in consumer cassette decks, and their S/N There are high expectations for the improvement effects.

When considering the widespread use of dbx noise reduction devices in cassette decks, the most important problem will be compatibility.

Although the dbx noise reduction device has far more advantages than the Dolby noise reduction device, the lack of compatibility is a considerable problem considering the past performance of the Dolby noise reduction device.

When recording and playing back with one device, d with higher fidelity
Even if there is no problem because a Dolby noise reduction device is used, a problem arises when playing back tapes or recorded tapes that have already been recorded on a deck equipped with a Dolby noise reduction device.

Therefore, it is desirable to incorporate at least the playback function of the Dolby noise reduction device into one cassette deck, but this becomes a major factor in increasing costs.

This invention was made in view of the above points, and uses the dbx noise reduction device built into the device, and has the functions of a Dolby noise reduction device or ANR8 device by simply adding a small number of additional circuits. To provide a noise reduction device that enables necessary functions and a rational circuit configuration with almost no increase in cost through a slight increase in the number of elements, and that solves expected compatibility problems at once. It is something.

An embodiment of the present invention will be described below in detail with reference to the drawings. Prior to that, a Dolby noise reduction device will be described with reference to FIG. 1.

FIG. 1 is a block diagram of a noise reduction device for cassette decks known as the Dolby Noise Reduction B System.

1 and 11 are variable attenuation networks, level detectors 2 and 1
The transmission characteristics are controlled by the output voltages 3 and 13 of 2.

When the level of the input signal 4 acting as a low-cut filter is sufficiently large, the variable attenuation networks 1 and 11, controlled by the level detectors 2 and 12, have a very high cut-off frequency and virtually no signal is detected. The input signal 4 is directly recorded without passing through and is directly played back.

When the input signal level falls below a certain level, the absolute values of the control voltages of the level detectors 2 and 12 decrease, and the variable attenuation network 1
A high-frequency signal passes through .

Through this process, it is possible to attenuate the noise generated on the tape 5, which is a recording medium, and to reproduce the original signal.

As described above, the present invention combines the characteristics of the Dolby noise reduction device with a level detector proportional to the logarithm of the effective value of the signal in the dbx noise reduction device and a voltage controlled variable gain circuit V having a gain proportional to the exponent of the control voltage.
The purpose of this invention is to provide a circuit device that can be obtained in common with CA, and FIG. 2 shows one embodiment thereof.

In the recording circuit shown in the figure, an input signal and high frequency components passing through a high pass filter 22 and a VCA 23 are added to an adder 21.

Therefore, in the low frequency range, the path passing through the VCA 23 can be virtually ignored, but in the high frequency range, the sum of the input signal 24 itself and the signal passing through the VCA becomes the output of the recording circuit.

The VCA 23 and the level detector 25 are connected so that the gain of the VCA 23 decreases as the input signal level to the level detector 25 increases.

Considering a state in which the input signal level in the recording circuit is sufficiently high, the gain of the VCA 23 is sufficiently small, and the amount of the signal that does not pass through the VCA 23 but passes through a direct path becomes dominant.

The same thing holds true for the reproduction circuit, and the output 26 passes through a high-pass filter 27 and a VCA 28 to a subtracter 29.
The negative feedback loop that leads to can be ignored because the gain of the VCA 28 is reduced.

Therefore, in this case, the direct signal path is dominant in both the recording and playback circuits, and each of the recording and playback circuits exhibits a flat frequency response.

On the other hand, when the input signal level decreases to a certain extent, the signal passing through the VCA 23 and high-pass filter 22 cannot be ignored, and the recording circuit creates a frequency response that is the sum of the direct signal path and the signal passing through the VCA 23, increasing the high frequency range. Conversely, the reproducing circuit exhibits a frequency response in which the high range is further reduced due to the effect of the negative feedback loop passing through the VCA 2B.

The effect of suppressing noise generated in the tape 30 through this process is similar to that of the Dolby noise reduction device.

However, when the level becomes extremely small, the gain of the VCA increases dramatically.

In reality, the amount of compression and expansion of high frequencies is constant (1 in the case of Dolby).
odB), so the level detection circuit 25
and 31 must be added with a function that exhibits saturation characteristics when a low level input is applied, as shown in FIG. 3, 25.

The level detector used in the dbx noise reduction device has linearity over a very wide input range (usually at least 50 dB or more), as shown at 36.

Such characteristics can be easily realized using a nonlinear circuit as shown in FIG. 4, for example.

However, according to the method shown in FIG. 5, it can be realized more easily and practically.

That is, in FIG. 5, 51 is a part of a level detector, and an input signal is applied to an input terminal 52 through a resistor R1 of 53 to a terminal 54.

Further, a steady DC current is passed through the level detector through the 55 resistor R2.

In that case, the output of the level detector is proportional to the logarithm of the effective value of the sum of the current proportional to the input signal and the steady DC current.

Therefore, in a region where the input signal level is sufficiently high, the input signal is dominant, and in a region where the level is sufficiently low, the DC current is dominant, so that the saturation characteristic in the low level region as shown in FIG. 3 is exhibited.

This method has the advantage of being easy to implement and not having temperature characteristics.

As described above, according to the present invention, it is possible to provide the function of a Dolby noise reduction device by simply connecting a small number of additional circuits to the level detector and VCA included in the dbx noise reduction device.

Therefore, since it is not necessary to provide both functions independently in one cassette deck, no extra burden is placed on the user.

In the above embodiment, the input of the level detector is taken from the input of the addition circuit 25 on the recording side, and taken from the output of the subtraction circuit 27 on the playback side, but it is taken from the output of the addition circuit on the recording side. , and on the reproduction side, it may be taken from the input of the subtraction circuit; however, in that case, it is necessary to connect a high-pass filter independent of 22 and 30 to the input of the level detector.

As another modification example, as shown in Figure 2, the adder is replaced with a subtracter, the subtracter is replaced with an adder, and the connection between the VCA and the level detector is changed so that an increase in the input of the level detector increases the gain of the VCA. The level detector may be connected so that the characteristic of the level detector has a saturation characteristic at a high level, contrary to the saturation characteristic at a low level shown in FIG. 35.

This can be easily achieved with diode limiters or other non-linear circuits.

This circuit is flat when the level is low, and when the level is high, the recording circuit lowers the high frequency and the playback circuit exhibits a high frequency increasing characteristic, so to compensate for this, the recording circuit raises the high frequency and the playback circuit lowers the high frequency. Must include circuitry.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a conventionally used Dolby noise reduction device, and FIG. 2 is a block diagram showing an embodiment of the noise reduction device according to the present invention.
Figure 3 is a saturation characteristic diagram that a level detector should have;
FIG. 5 and FIG. 5 are diagrams showing specific configuration examples of the level detector. 21... Adder, 22.27... High pass filter, 23, 28... VCA, 29
...Subtractor, 30 ...Tape, 25,
31...Level detector.

Claims (1)

[Claims] 1 The recording circuit includes a high-pass filter that passes the high-frequency components of the recording signal, a voltage-controlled variable gain circuit that amplifies the high-frequency signal components that have passed through this filter with a gain proportional to the exponent of the control voltage, and Generating a signal proportional to the logarithm of the output signal level of the control variable gain circuit and exhibiting saturation characteristics at a low level in order to determine the upper limit of the gain of the voltage control variable gain circuit, and supplying the signal as the control voltage to the voltage control variable gain circuit. a level detection circuit for adding the output of the voltage-controlled variable gain circuit and a recording signal to record the signal; a voltage-controlled variable gain circuit that amplifies the passed high-frequency component with a gain proportional to the exponent of the control voltage; and a voltage-controlled variable gain circuit that is proportional to the logarithm of the output signal level of the voltage-controlled variable gain circuit and that sets an upper limit of the voltage-controlled variable gain circuit. a level detection circuit that generates a signal exhibiting saturation characteristics when the level is as low as possible and supplies it to the voltage controlled variable gain circuit as the control voltage; and a circuit that subtracts the output of the voltage controlled variable gain circuit from the reproduced signal. A noise reduction device characterized in that it exhibits a flat frequency response when the signal level is high, and when the signal level is low, a recording circuit exhibits a high-frequency increasing characteristic and a reproduction circuit exhibits a high-frequency decreasing characteristic.