JPS60103559A - Video signal reproducing device - Google Patents

Abstract

PURPOSE:To suppress white noise on a reproduced pattern and also to prevent a normal signal from being spoiled by controlling the amplitude limit level of a video signal amplitude limit circuit in response to an output level of an envelope detection circuit to prevent buzz sound. CONSTITUTION:When missing of signal for a comparatively long period as shown in Fig. (a) takes place in a reproduced FM signal, each output of a missing signal compensating circuit 5, a limiter 6 and an FM demodulation circuit 7 is as shown in Figs. b-d respectively. On the other hand, the output of the envelope detection circuit 46 is as shown in Fig. (f), and an output of a white level amplitude limit circuit 47 is as shown in Fig. (g), where the amplitude limit level of the white level amplitude limit circuit 47 when the output of the envelope detection circuit is at high level and low level is VH and VL respectively. In reproducing the signal like this, since gray noise is obtained, the noise becomes unremarkable comparatively. In reproducing a picture through the connection of a television receiver to an RF output terminal 11, an RF converter 10 does not cause overmodulation.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a video signal reproducing device, and in particular to a missing signal compensation device used in a magnetic recording and reproducing device, or a device for remodulating a reproduced video signal into a television broadcast signal or the like. The feature is that it prevents overmodulation when

Conventional configurations and their problems A reproduction signal is obtained from a recording medium on which a video signal modulated by a modulation method that does not involve a change in amplitude, such as frequency modulation, phase modulation, or pulse coding modulation, is recorded. BACKGROUND ART In a video signal reproducing device, for example, a video tape recorder (hereinafter referred to as a VTR) using a frequency modulation recording and reproducing method, a relatively long-term dropout portion may occur in a reproduced frequency modulated signal (hereinafter referred to as an FM signal). When such an FM signal is demodulated, noise with a larger amplitude than the standard amplitude of the video signal is generated.

To explain this problem, for example, the configuration of the video signal reproducing circuit of a VTR is as shown in Figure 1.The FM signal recorded on a magnetic tape 1 is reproduced by a magnetic head 2 installed on a rotating cylinder or the like. After being amplified by the amplifier 3 and supplied to the missing signal detection circuit 4 and the missing signal compensation circuit 5, the amplitude fluctuation component is removed by the limiter 6, and the FM
The signal is demodulated by the demodulation IAj circuit 7°, amplified to a predetermined level by the video amplifier 8, and supplied to the video output terminal 9, and also converted to a television broadcasting signal by the RF converter 1o and supplied to the RF output terminal 11. It has come to be used for image playback, etc.

In such a VTR, when a relatively long period of signal loss as shown in Fig. 2 & occurs due to scratches on the magnetic tape in the raw FM signal, or due to I/lasoging deviation of special playback, etc. Since the missing signal-M compensation circuit 9 is of a general type that delays the signal from one horizontal scanning period (1H) before mixing or switching, the input and output of the limiter 6 are shown in FIG. become that way. Therefore, large amplitude noise as shown in FIG. 2d appears in the output of the FM demodulation circuit 7. When such a signal is reproduced as an image, it appears as white noise, which is very unsightly. Also,
When a television receiver is connected to the RF output terminal 11 and images are reproduced, the RF converter 10 causes overmodulation due to large amplitude noise as shown in Fig. 2(d), and the signal reaches the audio signal region, resulting in abnormal noise. There is a problem in that it generates a buzz sound (hereinafter referred to as a buzz sound).

In order to prevent buzzing noise, which is a particular problem in practice, conventional methods have been used to limit the amplitude of the white side of the video signal.

An example is shown in FIG.

In FIG. 3, the video signal input to the terminal 31 is clamped at the potential on the synchronous signal 8 side by a clamp circuit composed of a capacitor 32, an NPN l-transistor 33, and resistors 34 to 37, and then passed through a PNP transistor 39. Level shift diode 40. The signal is supplied to an output terminal 42 via an emitter follower made up of a resistor 41. Terminal 43 is a power supply terminal. The emitter of a PNP transistor 38 is connected to the output terminal 42, and its base is connected to the connection point between the resistors 36 and 36. Therefore, the video signal appearing at the output terminal 42 is amplitude limited to a level equal to the potential difference V36 between the ends of the resistor 36 from the potential at the tip of the synchronizing signal, resulting in a signal 1L) as shown in FIG. 2e. That is, an amplitude limiting circuit as shown in FIG. 3 is connected to the output side of the FM demodulation circuit 7 and the video amplifier 8 in FIG.
0 input side, and by appropriately setting the value of the resistor 36, it is possible to prevent buzzing noise.

However, if the amplitude limit level is set to be below the white level of the video signal, the amplitude will be limited to the normal (-1υ゛), so in reality it should be set to a level slightly higher than the white level. Therefore, the disadvantage is that white noise appears in the reproduced image and is very noticeable.

Purpose of the Invention The present invention eliminates the drawbacks of the above-mentioned conventional example, and prevents buzzing sound and suppresses white noise on the 41f live screen even when signal loss occurs for a relatively long period of time. In addition, the present invention provides an image reproducing apparatus that does not reduce the number of normal images.

QU)l(n pieces+412) The present invention detects the amplitude of the reproduced modulation signal and controls the limiting level of the amplitude limiting circuit or the gain of the gain control circuit of the demodulated video signal according to the level of the amplitude.
This is to achieve the above purpose.

DESCRIPTION OF EMBODIMENTS FIG. 4 is an example of a VTR playback circuit block diagram when a method for controlling the amplitude limit level is used in the video signal playback apparatus according to the present invention, and the same parts as in FIG. 1 are the same. Indicated by number.

In FIG. 4, the FM signal recorded on the magnetic tape 1 is
is reproduced by a magnetic head 2 provided on a rotating cylinder or the like, amplified by an amplifier 3, and then supplied to a missing signal detection circuit 4, a missing signal compensation circuit 6, and an envelope detection circuit 46. An amplitude fluctuation component is removed from the output of the missing signal compensation circuit 5 by a limiter 6, demodulated by an FM demodulation circuit 7, and a white side amplitude limiting circuit 47 whose amplitude limiting level is controlled by the output of the envelope detecting circuit 46 After being amplified to a predetermined level by a video amplifier 8 and supplied to a video output terminal 9, both are converted to television broadcasting υ by an RF converter 10 and supplied to an RF output terminal 11. Used for image playback, etc.

In such a VTR, when a relatively long-term signal loss as shown in FIG. 5a occurs in the reproduced FM signal, the loss signal compensation circuit 5. The outputs of the limiter 6 and the FM demodulation circuit 7 are shown in FIG. 5, respectively.

C9 and d. The output of the envelope detection circuit 46 is as shown in FIG. ,
, vL, the output of the white side amplitude limiting circuit 47 is as shown in FIG. 6g.

When such a signal is reproduced as an image, the noise becomes gray and becomes relatively inconspicuous. Also, even when connecting a television receiver to the RF output terminal 11 and playing back images, the RF
Converter 1o does not cause overmodulation.

Next, a specific example of the white side amplitude limiting circuit 47 is shown in FIG.

The same parts as in FIG. 3 are indicated by the same numbers.

In FIG. 6, the demodulated image (g
The number is capacitor 32. The potential on the synchronizing signal side is clamped by a clamp circuit composed of a transistor 33 (N, PN) and a resistor 37.66, and a PNP transistor 39. The signal is supplied to an output terminal 42 through an emitter follower composed of a level shift diode and a 40° resistor 41. terminal 43
is the power supply terminal. The emitter of a PNP transistor 38 is connected to the output terminal 42, and its base is connected to the connection point between the resistor 36 and the resistor 6γ.
5 to the transistor rotor. The collector of the transistor 4 (PNP) is grounded, the base thereof is grounded via a resistor 63, and connected to the input terminal 61 via a resistor 62. The output of the envelope detection circuit 46 is applied to the input terminal 61.

Here, resistor 62゜63 f: Select Id so that transistor 4 turns off when input terminal 61 is at high level and turns on when input terminal 61 is at low level.
, P N P When the input terminal 61 is at a high level, the base potential of the transistor 38 is the potential vH' obtained by dividing the power supply voltage by the resistor 36 and the resistor 67.
When the input terminal 61 is at a low level, the potential vL' is obtained by dividing the power supply voltage by the parallel connection of the resistor 66 and the resistor 67 and the resistor 35, and when the input terminal 61 is at an intermediate level, the potential vH' and vL
′ becomes the intermediate potential. That is, the white side amplitude limit level of the video signal supplied to the output terminal 42 can be controlled by the potential applied to the input terminal 61.

Therefore, the input terminal 31 is connected to the output of the FM demodulation circuit 7 shown in FIG. 4, and the input terminal 61 is connected to the envelope detection circuit shown in FIG.
The output terminal 42 is connected to the output of the circuit 46 in FIG.
; By connecting the input points of r-=7, it is possible to realize the white side amplitude limiting circuit 47 as described above.

This example story 7th. Figure, white side shown, 1. . l'i+
(Although the limit level of the limit circuit 47 is continuously controlled by the output level of the envelope/rope detection circuit 46, the limit level is discretely set to two or more values. Envelope detection circuit 46
A similar effect can be obtained by switching the restriction level depending on the output. Moreover, in this embodiment, as shown in FIG. 4! Although the configuration is such that the missing signal detection circuit 4 and the envelope detection circuit 46 are configured to detect each independently, the same effect can be obtained by using a configuration in which both are used for the sake of circuit simplification.

Further, the same effect can be obtained when a gain control circuit is provided in place of the white side amplitude limiting circuit 47 in FIG. 4, and the gain of the gain control circuit is controlled by the output of the envelope detection circuit 46. It is easy to infer that this is the case.

Further, in this case, an even greater effect can be obtained by connecting a clamp circuit to the output side of the gain control circuit.

As described in detail, according to the present invention, with a very simple configuration, it is possible to prevent buzzing sound even when a reproduction modulation signal is missing for a relatively long period of time, and it is possible to This makes it possible to make noise that occurs less noticeable. In particular, according to the present invention, since the amplitude suppression level during normal operation and the amplitude suppression level during loss can be freely set, the above-mentioned effects can be obtained without affecting the reliability of the 1L video. This is extremely advantageous in practice.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an example of a conventional video signal reproducing device, Fig. 2 is a waveform diagram of output signals from each part of the device, and Fig. 3 is a block diagram showing an example of a conventional video signal reproducing device.
The figure is a circuit diagram showing a specific example of a conventional white-side amplitude limiting circuit, FIG. 4 is a block diagram of a video signal reproducing device that is an embodiment of the present invention, and FIG. 5 is a waveform of input and output signals of each part of the device. Figures 1 and 5 are circuit diagrams showing specific examples of the white side amplitude limiting circuit used in the present invention. 1... Magnetic tape, 2... Magnetic head, 3... Amplifying circuit, 4... Missing signal detection circuit, 6... Missing Signal compensation 11η path, 6...
...Limiter circuit, 7...FM demodulation circuit,
8... Video signal 7-amplifier circuit, 9... Video output terminal, 10... RF converter, 11...
...RF output terminal, 31 ... Demodulated video signal input terminal, 42 ... Output terminal, 43 ...
...Power supply terminal, 61...Control signal input terminal. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] (1) A video signal is a reproduced modulated signal from a recording medium that has been modulated by a variable J1!1 method that does not involve a change in amplitude, such as frequency modulation, phase modulation, or pulse coding modulation. It is equipped with an envelope detection circuit that detects the amplitude and a video amplitude limiting circuit that limits the amplitude of the video signal demodulated from the reproduced modulation value code, and the video value is determined according to the output level of the envelope detection circuit. - A video value ゛υ' reproducing device characterized in that KL controls the withdrawal limit level of the amplitude limit circuit. (2) The video signal amplitude limiting circuit is configured with one gain limiting circuit, and the gain of the gain side circuit j is controlled according to the output of the envelope detection circuit. A video signal reproducing device falling within the scope of item 1 above.