JPH0347011B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a noise reduction circuit, which is used, for example, in a VTR that modulates an audio signal and records and plays it using a rotating head. It is an object of the present invention to provide a circuit that can reduce low-frequency noise components that are relatively audible problems due to the influence of a circuit provided for the purpose of reducing low-frequency noise components without changing sound quality only when the low-frequency noise components are easily detected. purpose.

Prior Art This technique is used in VTRs that use FM modulation to record and play back audio signals using a rotating head.
FM modulated audio signals generally contain noise components (so-called triangular noise) as they reach higher frequencies.
increases. Therefore, it is necessary to reduce this noise component, and there are complementary reduction circuits that place particular importance on noise reduction effects in high frequencies.

Problems to be Solved by the Invention The noise reduction circuit as described above changes its noise reduction operation depending on the signal level or frequency. For signals with energy components, etc.), the noise low-frequency effect in the low frequency range is relatively reduced. Such a phenomenon is generally not detected due to the masking effect of signal components present in the low frequency range in most cases, but when there is no signal in the low frequency range, the noise component is audible and is undesirable from the auditory sense.

There is also a noise reduction circuit that holds the signal level just before the head is switched for the duration of the noise to prevent the switching noise from being transmitted to the subsequent stage in order to eliminate the noise that occurs when switching the rotating head. When the frequency of is high, the difference between the true signal level and the hold potential becomes large, which causes new noise in the frequency range below the signal frequency.

It is possible to improve the low-frequency noise as described above to some extent by appropriately selecting the low-frequency characteristics, but it is not effective beyond a certain limit because the level of the low-frequency signal itself changes and compatibility deteriorates. There was a problem that the noise could not be reduced sufficiently.

Means for Solving the Problems and Effects Thereof The present invention provides a wave reduction means for extracting low frequency components of an FM modulated audio signal, which is an input signal;
A high-frequency means for extracting high-frequency components of an input signal is compared with the output of the low-frequency means and the output of the high-frequency means to determine whether the low-frequency components extracted from the low-frequency means are below a predetermined level. a first state in which high-frequency components exist at a predetermined ratio or more compared to low-frequency components;
detects a second state other than the state of
control signal generating means for outputting a control signal according to the state of the input signal; The above problem is solved by a configuration consisting of a variable wave means that outputs the low frequency component of the input signal as it is, and the following is as follows.
One embodiment will be described with reference to the drawings.

Embodiment FIGS. 1 and 2 respectively show a block system diagram and a specific circuit diagram of an embodiment of the present invention, and the same components in each figure are given the same numbers. In the figure, the reproduced audio signal input to terminal 1 is filtered through low-pass filter circuit 2 (the cut-off frequency is selected to be approximately the same as the frequency of the low-frequency noise component, and has a slope of -12 dB/octave above the cut-off frequency). , whose frequency characteristics are shown in curve I in FIG. 3, where the low frequency components are separated. That is, it is detected here whether or not the reproduced audio signal includes a signal component that masks the band noise component.

Here, the output of the amplifier OP ₁ is inverted and amplified by the amplifier OP ₂ , and the output of the variable filter circuit ₄ is inverted and amplified _.
The transistor _X 1 is connected to the line from terminal 1 through the connection point of low frequency R ₆ and R ₂ , and is controlled to turn on and off by the output signal of the control signal generation circuit 3, which will be described later. Whether or not the output of the amplifier _OP2 is added to the signal from the terminal 1 is controlled by turning it on or off. The transistor _X1 can be replaced with a variable impedance circuit that can smoothly vary the impedance by changing the voltage, but most easily it can be replaced with an on/off controlled transistor as shown in the second diagram.

Since the low-frequency signal that is the output of amplifier OP ₂ is sufficiently amplified, resistor R ₂ can be set larger than resistor R ₁ , and the difference in output level between when adding and when not adding can be made small. . The value of the resistor R ₂ is the transistor X ₁
is selected so that the low frequency signal level from amplifier OP ₂ cancels out the low frequency input signal level when is off.

The output of amplifier OP ₂ is supplied to amplifier OP ₃ to further reduce the high-frequency gain, and from this, even if there is a high-level signal at a high level, this effect is extracted on the output of low-pass filter circuit 2. There isn't. Amplifier
The output of OP ₃ is filtered to remove components below a predetermined band by a filter composed of a resistor R ₁₈ and a capacitor C ₁₂ , and is then supplied to the control signal generation circuit 3 .

On the other hand, the reproduced audio signal is supplied to a high-frequency filter circuit 5 (the frequency characteristics of which are shown by the curve in FIG. 3), where a high-frequency signal that tends to generate low-frequency noise is extracted. This makes it possible to indirectly detect low-frequency noise that is difficult to directly measure. The output of the high-pass filter circuit 5 is supplied to the control signal generation circuit 3.

The signal supplied to the control signal generation circuit 3 is passed through a detection circuit with diodes D ₁ and D ₂ and a detection circuit with diodes D ₃ ,
They are each rectified by a detection circuit using D ₄ and
The signals shown in the curve in the figure (input voltage waveform diagram of the comparator COMP measured using an actual music signal) are supplied to the + and - terminals of the comparator COMP, respectively, and compared with the level. Note that the detection circuit using diodes D ₁ and D ₂ includes a resistor.
A DC offset voltage is applied to R ₂₂ , R ₂₃ , capacitor C ₁₈ , and power supply Vcc.

In Fig. 4, during the period when the output of the high-pass filter circuit 5 (curve in Fig. 4) is larger than the output of the low-pass filter circuit 2 (curve in the same figure), low-frequency noise is likely to occur. There is no signal to mask low-frequency noise, and there is no significant change in sound quality even if the gain of the low-frequency signal is lowered. Conversely, during a period in which the output of the low-pass filter circuit 2 is larger than the output of the high-pass filter circuit 5, a signal capable of masking low-pass noise is included in the reproduced audio signal.

As shown in FIG. 4, in the period from the beginning to about 6 or 7 seconds, at several points between about 12 seconds to 14 seconds, and at about 16 seconds to 18 seconds, the output of the high-pass filter circuit 5 is switched to the low-pass filter. This is larger than the output of circuit 2, so that the output of comparator COMP is set to L level.
The output of the low-pass filter circuit 2 is the output of the high-pass filter circuit 5.
is greater than the output of the comparator COMP
The output of is set to H level. comparator
The output of COMP is diode D ₅ , resistor R ₂₅ , R ₂₄ ,
It is integrated by an integrating circuit made up of capacitor _C19 . Comparator by diode D ₅
When the COMP output goes to L level, a faster response can be obtained than when it goes to H level, and the attack time can be set smaller than the recovery time.

The output of the above integration circuit is supplied to the transistor X ₁ of the variable filter circuit 4 via the diode D ₆ , and when the output of the integration circuit is at H level (normally), the transistor X ₁ is turned on and the amplifier OP ₂ is turned on. The output is grounded, and the variable filter circuit 4 has no frequency characteristics. On the other hand, when the output of the integrating circuit is at L level (when low-frequency noise components occur), _transistor
is turned off, and the output of the amplifier OP ₂ is added to the signal supplied via the resistor R ₁ and taken out from the output terminal 6. As a result, the frequency characteristics of the variable filter circuit 4 can be varied and the low frequency can be lowered during a period in which the reproduced audio signal has a spectrum where low frequency noise is likely to occur and there is no signal component to mask this.

As shown in the curve of FIG. 5, according to this embodiment, compared to the conventional example (curve V), the harmonics of around 30 Hz (the rotational frequency of the rotating head of the VTR) account for a large proportion of the low-frequency noise component. (shown) more than 30dB, 200
Up to Hz, each can be improved by about 10dB.

Note that during no-signal regeneration, only the DC offset voltage applied to the diodes D ₁ and D ₂ is supplied to the comparator COMP, and as a result, the control signal is set to H level, and the variable filter circuit 4 has no frequency characteristics. .

Furthermore, the diode _D6 is provided to improve the transient characteristics of the transistor _X1 during switching, and is not necessarily necessary.

Effects As described above, the noise low frequency circuit according to the present invention includes a wave reduction means for extracting the low frequency components of the FM modulated audio signal, which is the input signal, and a high frequency means for extracting the high frequency components of the input signal. , by comparing the magnitude relationship between the output of the low frequency means and the output of the high frequency means, it is determined that the low frequency component extracted from the low frequency means is below a predetermined level and the high frequency component is at a predetermined level compared to the low frequency component. control signal generating means for detecting a first state existing at a ratio greater than or equal to the ratio and a second state other than the first state and outputting a control signal according to the first and second states; The variable wave means lowers the low-frequency component of the input signal in the first state and outputs the low-frequency component of the input signal as it is in the second state, depending on the control signal. When the output of the wave means becomes larger than the output of the low frequency means, the low frequency component level is varied to a small value, and in this case, there is a low frequency signal at a level that sufficiently masks low frequency noise in the low frequency means. Since it is possible to detect whether low-frequency noise actually occurs to the ear or not, it is possible to lower the low-frequency noise without changing the sound quality only when the low-frequency noise actually causes an audible problem to the ears. Since the occurrence of low-frequency noise is detected in the input signal, for example,
The circuit can be configured more easily than the one that directly detects low-frequency noise generation by separating only 30Hz and its harmonics with a comb filter, etc., and the variable wave means can be taken out from the low-frequency means. Since the low-frequency components of the input signal are reduced by adding the low-frequency components in opposite phase to the input signal, a part of the low-frequency wave means can be shared with the variable wave means, and the circuit can be easily configured. It has characteristics.

[Brief explanation of drawings]

Figures 1 and 2 are a block system diagram and a specific circuit diagram of an embodiment of the circuit of the present invention, respectively, Figure 3 is a frequency characteristic diagram of a low-pass filter circuit and a high-pass filter circuit, and Figure 4 is an actual circuit diagram. FIG. 5 is a diagram of the input voltage waveform of the comparator for a music signal, and FIG. 5 is a frequency characteristic diagram obtained by the conventional circuit and the circuit of the present invention. 1...Reproduction audio signal input terminal, 2...Low frequency filter circuit, 3...Control signal generation circuit, 4...Variable filter circuit, 5...High frequency filter circuit, 6...Output terminal, OP ₁ to OP ₄ ...Amplifier, COMP... Comparator, X ₁ ...Transistor.

Claims (1)

[Claims] 1. A low frequency means for extracting a low frequency component of an FM modulated audio signal which is an input signal, a high frequency means for extracting a high frequency component of the input signal, and a low frequency means for extracting a high frequency component of the input signal. Comparing the magnitude relationship between the output and the output of the high frequency means, the low frequency component extracted from the low frequency means is below a predetermined level and the high frequency component is at least a predetermined ratio compared to the low frequency component. A first state that exists, and a second state other than the first state.
control signal generating means for detecting the state of the input signal and outputting a control signal according to the first and second conditions; A noise reduction circuit comprising: variable wave means for reducing a low frequency component of the input signal and outputting the low frequency component of the input signal as it is when in the second state. 2. The variable wave means is a means for reducing the low frequency components of the input signal by adding the low frequency components extracted from the low frequency means in opposite phase to the input signal. The noise reduction circuit according to item 1.