JPS6017173B2 - noise reduction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction device suitable for use in magnetic recording/reproducing devices, etc., and in particular to reduce noise fluctuation phenomena during operation of a noise reduction device using a compressor and an expander. This invention relates to a noise reduction device that can reduce noise.

In recent years, many noise reduction devices have been proposed for the purpose of expanding the dynamic range of magnetic recording and reproducing devices, and some have appeared with an S/N ratio improvement of 30 to 4 MB. However, this type of noise reduction device has a drawback in that the greater the degree of improvement in the S/N ratio, the more noticeable the phenomenon of noise fluctuations during signal compression/expansion becomes. This noise fluctuation phenomenon is particularly noticeable when a transmission system with a poor S/N ratio is used. Here, a description will be given based on a conventional example of a typical noise reduction device shown in FIG. In FIG. 1, reference numeral 1 denotes a variable gain amplifier, for example, a divider is used when constructing a compressor.

Reference numeral 2 denotes a control signal generating section, which generates a control signal proportional to the amplitude of the signal applied to the input terminal of the control signal generating section 2 to control the variable gain amplifier 14.

3 is a transmission system, which corresponds to an electromagnetic conversion system in the case of a magnetic recording/reproducing device.

4 is a variable gain amplifier like 1, but since it constitutes an expander here, a multiplier is used.

In the noise reduction device shown in Figure 1, the input signal is logarithmically compressed to 1/2 and recorded, and during playback, the recorded signal is logarithmically expanded to 2 times, resulting in the overall characteristics of recording and playback. It works to maintain constant input and output characteristics. In the process of expanding the above operation, the noise in the transmission system is reduced by the amount of expansion. The above-mentioned input-output characteristic diagram is shown in FIG. In FIG. 2, A is the compression characteristic during recording, B is the expansion characteristic during playback, and C is the overall characteristic during recording and playback. In the noise reduction device having the characteristics shown in Fig. 2, the S/N ratio improvement degree for the MB input signal is B, and the noise in the transmission system is output as is without being reduced. Since the signal level during reproduction is lowered to the 18th generation for the input signal of , an SN ratio improvement of 4B can be obtained after all. Therefore, in such a system, the noise level always fluctuates in response to increases and decreases in the signal level. Generally, when the volume is high, noise becomes difficult to hear due to the masking effect, but when the input level becomes low, the masking effect of the main signal becomes less effective, and fluctuations in noise are particularly noticeable when the signal is intermittent, such as a piano solo. It had become.

This noise fluctuation phenomenon is very unnatural and has been a major hindrance in high-end audio equipment where sound quality is important. One method for solving the above-mentioned problems is disclosed in Tokokukan Sho 51-124304.

This method detects the level of the input signal to be compressed or expanded a certain time in advance, generates a noise signal corresponding to that level, and adds this to the main signal to generate noise fluctuations caused by changes in the level of the input signal. It makes it less noticeable. In order to realize the above, many auxiliary circuits such as a main signal delay circuit, a noise generator, a noise application level adjuster, etc. are required, and it is undeniable that the device becomes large-scale and complicated. .

Furthermore, in the above system, there is a risk that noise may be added even when compressing and expanding a signal in which the noise fluctuation phenomenon is not noticeable. In other words, the noise fluctuation phenomenon is more noticeable when
Experiments have revealed that most cases occur only when companding intermittent signals. The present invention has been made with attention to the above points, and by gently controlling the gain change of the companding device only when companding intermittent signals, the noise fluctuation phenomenon is substantially made inconspicuous. The present invention provides a novel noise reduction device capable of reducing noise. The present invention will be described in detail below with reference to the drawings. Fig. 3 is a block diagram of an expander used during reproduction of the noise reduction device according to the present invention.

In FIG. 3, 7 is a control signal generator, and 4 is a variable gain amplifier, which may be any variable gain amplifier used from the slave.

Here, the difference from the conventional expander is the control signal generator 7, which has two types of control signal generators: an amplitude information control signal generator 5 and a time information control signal generator 6. be. The amplitude information control signal generating section 5 is exactly the same as the conventional control signal generating section 2 shown in FIG. 1, and is a section that generates a control signal in response to a change in the level of an input signal.

Therefore, during the period in which no control signal is generated from the time information control signal generator 6, the noise reduction apparatus is configured to operate exactly the same as the conventional noise reduction apparatus. As mentioned earlier, when an intermittent input signal with noticeable noise fluctuations arrives, the time information control signal generator 6 uses an intermittent characteristic in order to gently control the gain change of the variable gain amplifier 4. Simultaneously with the discrimination of the input signal having the signal, a control signal is generated to make the gain change of the variable gain amplifier 4 gradual when the intermittent signal arrives.

In other words, when controlling the variable gain amplifier 4 using only amplitude information as in a conventional expander, the gain of the variable gain amplifier 4 increases rapidly when a large input signal is applied, and the gain suddenly increases at the moment the signal is interrupted. Since the gain decreases, the noise also fluctuates drastically in response to the rapid increase or decrease in gain. Therefore,
As mentioned above, when the signal is interrupted and the amplitude information control signal begins to decrease rapidly, the time information control signal generator 6 generates a control signal that gradually increases, and the gain of the variable gain amplifier 4 is increased. This method gently controls the decrease.
If the gain changes are gradual, the noise fluctuations are also gradual, and therefore the noise fluctuations are less noticeable.
A specific circuit example of the time information control signal generator 6 is shown in FIG.

In FIG. 4, 21 is an input terminal of the time information control signal generator, and 22 is an output terminal. The transistor 8 is used to detect the presence or absence of an alternating current signal, and is a type of rectifier circuit. The transistor 11 is a DC amplifier circuit and operates as a switch. Transistor 18 operates as a NOR gate with two input terminals formed by resistors 16 and 17 connected to its base. 15 is a monostable multipipulator, whose input terminal 23 is connected to the collector of the transistor 11 by a capacitor 13 and a resistor 14.
The output terminal 24 is connected to one end of the NOR gate of the transistor 18.

FIG. 5 is a signal waveform diagram in which the signal waveforms of each part of the control signal generator are illustrated as a model for the convenience of explaining the operation of the noise reduction device of the present invention.

Suppose now that an intermittent input signal as shown in FIG. 5D is applied to the input terminal of the expander.

The input signal ei is directly applied to the input terminals of the amplitude information control signal generation section 5 and the time information control signal generation section 6.

The amplitude information control signal generator 5 generates a control signal substantially proportional to the input signal ei as shown in FIG. 5E, which acts on the variable gain amplifier 4 to control the gain of the variable gain amplifier 4. Become. On the other hand, the time information control signal generator 6
The input signal ei applied to is applied to input terminal 21 in FIG. The transistor 8 is turned ON during the signal presence period and turned OFF during the signal disappearance period. Since the transistor 11 directly connected to the collector of the transistor 8 operates in the opposite direction to that of the transistor 8, the input signal existence period becomes N of the OFF extinction period. Therefore, the collector waveform of transistor 1 is as shown in FIG. The signal F obtained from the collector of the transistor 11 is connected to the capacitor 13 and the resistor 1.
It is differentiated by 4.

Assume now that the monostable multivibrator 15 is triggered by a pulse in the negative direction and has a metastable time of time t. Therefore, the signal shown in FIG. 5G is obtained at the output terminal 24 of the monostable multipipulator 15. Figure 5 F,
The G signal is applied to the NOR gate of transistor 18 through resistor 16 and resistor 17, respectively. Transistor 18 is turned off only when signals F and G are both at ground potential. However, the capacitor 20 between its collector and ground
Since it has a collector potential of resistor 1 when it is OFF,
9 and the time constant determined by the capacitor 20. This state is shown in Figure 5. In this embodiment, the control signal E obtained from the amplitude information control signal generator 5
and the control signal date obtained from the time information control signal generator 6, the final control signal is as shown in FIG. It is maintained at a reasonable level.

This allows the gain change of the variable gain amplifier 4 to be controlled gently. Naturally, the more continuous the input signal is, the smaller the time information control signal becomes, and in the case of a continuous signal, it becomes completely zero.

When the signal extinction period lasts longer than the metastable time of the monostable multipipulator 15, the time information control signal is automatically set to zero and the gain of the variable gain amplifier is lowered to lower the noise level. Note that the degree of change in the control signal during the signal extinction period can be changed by adjusting the time constant determined by the resistor 19 and capacitor 20, and the voltage divider and level may be adjusted as necessary so that the magnitude of the control signal becomes an optimal value. You can adjust it using a limiter. In the above embodiment, the decompressor was explained, but the present invention can also be used as it is in the control circuit on the compressor side.
On the compressor side, background noise is not unnecessarily amplified when the signal disappears, so it can be used in combination with a decompressor to achieve even greater effects.

As described above, according to the present invention, when compressing and expanding an intermittent signal in which noise fluctuations are easily detected, it is possible to gently control the gain change of the variable gain amplifier, thereby slowing down the noise fluctuation phenomenon. It is something that can be done. In addition, when a highly continuous signal arrives, it is configured to function exactly like a conventional compression/expansion device, so it does not unnecessarily deteriorate the S/N ratio. Although the circuit configuration is relatively simple, it has great effects. It has the excellent feature of

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional noise reduction device, FIG. 2 is its input/output characteristic diagram, FIG. 3 is a block diagram of a noise reduction device according to the present invention, and FIG. 4 is a time information control signal generator according to the present invention. FIG. 5 is a signal waveform diagram of each part of the control signal generator according to the present invention. 4...Variable gain amplifier, 7...Control signal generator, 5
......Amplitude information control signal generation section, 6...
Time information control signal generator. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. A control signal generator that is a noise reduction device that uses compression/expansion and controls the gain of a variable gain amplifier is composed of an amplitude information control signal generation section and a time information control signal generation section, and is configured to handle the intermittent nature of the input signal. Or, in the case of an intermittent signal, the continuous nature is discriminated by the time information control signal generator, and in the case of an intermittent signal,
A noise reduction device characterized in that the control signal generated by the time information control signal generator and the amplitude information control signal are added together when the signal disappears to control the gain of the variable gain amplifier so that it does not change suddenly. .