JPS60223307A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To attain reproduction of a signal close to an original waveform having no noise by reproducing a signal while a demodulated output is replaced into the signal changing in the changing direction of a demodulated output voltage before noise is generated when the noise is generated in the demodulated output of a demodulator. CONSTITUTION:If a dropout takes place and a level of a value to be modulated at an input terminal 1 is small, a detector 2 is activated so as to turn of switches 5 and 13. A capacitor 17 acts like holding an output voltage of an amplifier 4 before the noise is generated at this point of time and a capacitor 14 acts like holding an output voltage of an amplifier 12. Since an output voltage of the amplifier 12 includes a differentiated output of the amplifier 6, a voltage difference across the capacitors 7 and 14 when the switches 5, 13 are turned off represents the changing direction of the demodulated output voltage before the dropout takes place and this voltage difference is lost gradually because a resistor 9 is connected. The voltage across the capacitor 7 changes in the direction in which the demodulated output voltage is changed, contrary to the state that the switches 5, 13 are turned off.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a noise reduction circuit that can be effectively used in equipment that demodulates and reproduces modulated audio signals such as video tape recorders and video discs.

Conventional configuration and its problems FIG. 1 shows an example of the configuration of a conventional noise reduction circuit.

Modulated wave A reproduced from a recording medium such as a videotape
is applied to input terminal 1, passes through detector 2, and is demodulated by demodulator 3. The demodulated signal is amplified by an amplifier 4, passes through a switch 5, and is applied to a hold capacitor q and an amplifier B. If there is a dropout in the modulated wave, noise will occur in the demodulated output BK of the demodulator 3. However, if there is a dropout, the detector 2 will act and turn on the switch 6. In this case, the noise is not transmitted to the amplifier 6, but only the voltage held by the capacitor 7 is amplified, and the output C of the amplifier with reduced noise is obtained from the output J terminal 8. FIG. 2 shows the waveforms of each part in FIG. 1 at this time. Although the hold circuit can reduce noise in this way, it has the disadvantage that it cannot obtain the original waveform.

OBJECTS OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to eliminate noise in the demodulated output that occurs when there is discontinuity in the modulated wave caused by the dropout of the nuclear modulated wave or the switching of the rotating head of the videotape recorder. The aim is to reduce

Configuration of the Invention In addition to the conventional configuration that holds the demodulated output when noise occurs in the modulated wave, a second configuration that differentiates the output of the hold circuit and holds this output at the same timing as the aforementioned hold circuit is used. By providing a hold circuit and obtaining the output of the sum of both hold circuits, the noise reduction effect is increased.

Description of examples An embodiment of the present invention will be explained based on FIG.

In FIG. 3, the same members as those in FIG. 1 are given the same numbers and their explanations will be omitted.

9 is a resistor whose one end is connected to the input terminal of the amplifier 6, 10 is a capacitor whose one end is connected to the output terminal of the amplifier 6, and 11
is a resistor connected in parallel to the capacitor 10, and 12 is an amplifier connected to the other end of the capacitor 10. The amplifier 12 and the capacitor 1o constitute a differentiator. 13 is detector 2
A switch 14 transmits the output of the amplifier 12 to the other end of the resistor 9 in response to the output of the resistor 9. A switch 14 is a capacitor whose one end is connected to the output end of the switch 13 and the input end of the resistor 9, and whose other end is grounded. A frequency-modulated signal reproduced from a recording medium such as a video tape by a reproducing circuit (not shown) by the switch 5 and the capacitor 7 is applied to the input terminal 1. This signal passes through a detector 2 that detects the magnitude of the signal, is demodulated by a demodulator 3, and is converted into an audio signal, a music signal, or the like. The demodulated signal is amplified into signal E by amplifier 4,
The signal passes through the switch 5 which is in the on state, is further amplified by the amplifier 6, and is transmitted to the output terminal 8. A signal F is outputted to the output terminal 8, which can drive a power amplifier and make a speaker sound. A hold capacitor 7 and a resistor 9 are connected to the input terminal of the amplifier 6, but since the output impedance of the amplifier 4 is set low, this does not affect the frequency characteristics and does not affect the audible band. Normally, signals recorded on a recording medium such as a videotape can be reproduced and listened to in this way. However, in a helical scan video tape recorder using a rotating head, the recorded tracks are not connected, so the modulated wave becomes discontinuous during playback. Furthermore, if the tape is scratched, a dropout will occur in the reproduced modulated wave. In such a case, the demodulator 3 generates noise at discontinuous portions or dropout portions of the modulated wave, and the operation when this dropout occurs will be described in detail below.

At the neck where padrosopout occurs, a demodulated output is obtained at the output terminal 8 as described above. The demodulated output is also applied to the amplifier 12 through the capacitor 10 and the resistor 11, and the combination of the capacitor 10 and the amplifier 12 functions as a differentiator, and the output of the amplifier 12 is the sum of the demodulated output and the differential value of the demodulated output. is obtained. The output of the amplifier 12 is passed through the switch 13 in the on state to the capacitor 14.
is applied to the input terminal of the amplifier 6 via a resistor 9. When dropout occurs and the level of the modulated wave becomes small, detector 2 is activated and switches 5 and 13 are activated.
turn off. Therefore, the noise generated in the demodulator 3 is not transmitted to the output terminal 8. switches 5 and 13
At the time when the amplifier 4 is turned off, the capacitor 7 works to hold the output voltage of the amplifier 4 before noise is generated, and the capacitor 14 works to hold the output voltage of the amplifier 12. By the way, the output voltage of the amplifier 12 includes the differential value of the output of the amplifier 6, so when the output voltage of the amplifier 6 is increasing, it becomes higher than the input voltage of the amplifier 6, and the output voltage of the amplifier 6 decreases. When the voltage is on, the input voltage of the amplifier 6 becomes lower. In other words, the voltage difference across capacitors 7 and 14 when dropout occurs and switches 5 and 13 are turned off indicates the direction of change in the demodulated output voltage before dropout occurs. This voltage difference gradually disappears because the resistor 9 is connected. At this time, the voltage across capacitor 7 is
The demodulated output voltage changes in the direction in which it was changing compared to when 13 and 13 were in the off state. This operation is shown in FIG. Symbols A, B, and C in FIG. 4 represent signals A, 13, . Indicates C. Further, FIG. 5 shows operating waveforms when the dropout time is long. As can be seen from FIGS. 4 and 6, when the dropout time is short, a signal close to the original can be reproduced, and the noise reduction effect is greater than that of the conventional method. In addition, if the dropout time is long, it is difficult to distinguish the original signal waveform, and the capacitor 7
and 14 are maintained at a constant voltage that is equal to each other to reduce noise. In this case, a current may be supplied to the capacitor 7 from the input terminal of the amplifier 6 so that the demodulated output approaches the voltage across the capacitor 8 when there is no signal.

Although we have explained the case where dropout occurs, it is also possible to similarly reduce demodulated output noise caused by discontinuity in the modulated wave that occurs when discontinuous tracks recorded on a tape are sequentially played back. can. In this case, when switching the track to be reproduced, the switches 5 and 13 may be turned off in response to the track switching signal for a period of time during which the modulated wave becomes discontinuous, for example, about 10 microseconds.

Detailed Description of the Invention As described in detail, in the present invention, when noise occurs in the demodulated output of a demodulator, the demodulated output is replaced with a signal that changes in the direction of change in the demodulated output voltage before the noise occurs. Therefore, a noise-free signal close to the original waveform can be reproduced. When the dropout time of the modulated wave to be demodulated becomes longer, it works to output a constant voltage, so that noise generated in the demodulator can be removed.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional noise reduction circuit, FIG. 2 is an operation waveform diagram of each part of FIG. 1, FIG. 3 is a block diagram of a noise reduction circuit according to an embodiment of the present invention, and FIG. FIG. 5 is an operational waveform diagram of each part in FIG. 3. 2...Detector, 6,13...Switch, 6...Amplifier, 7,10.14...Capacitor, 9...Resistor, 12...Amplifier. Figure 1 Figure 2 Figure 4 @ Figure 5

Claims (1)

[Claims] a first hold circuit that holds an input signal voltage; an amplifier that amplifies the output of the first hold circuit; a differentiator that differentiates the output of the amplifier; and a second hold circuit that holds the output voltage of the differentiator. a hold circuit; and a detector for detecting the occurrence of noise in the input signal; the first and second hold circuits are connected by a resistor; A noise reduction circuit characterized in that the first and second hold circuits are configured to perform a hold operation.