JPS639385A - Magnetic video reproducing device - Google Patents

Abstract

PURPOSE:To improve S/N by detecting the level of video signals by a level detecting circuit and controlling the frequency passing band of reproduced video signals by a frequency characteristic variable circuit based on the detected signals. CONSTITUTION:In a case where reproduced video signals from a clamping circuit 8 are white, APL of the reproduced video signals is higher than discrimination level that appears in a discrimination end 93, and accordingly, an off signal is outputted from a comparator 94. A low pass filter is inputted to a video signal processing circuit 6 in an inactive state, and after subjected to specified process, appears in an output end 7. In a case where reproduced video signals are black, APL is lower than the discrimination level, and accordingly, an ON signal is outputted from the comparator 94, and the low pass filter is activated. High band component of > about 23 MHz is removed and inputted to the video signal processing circuit 6, and after specified processing, appears in the output end 7.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a magnetic video reproducing device such as a magnetic recording/reproducing device, and particularly to a magnetic video reproducing device that can control the amount of noise according to a reproduced video signal. It is.

[Conventional technology]

Figure 2 shows a conventional magnetic video reproducing device.
In 2), +11 is a video head that reproduces a video signal recorded on a tape (not shown), (21d is a reproduced video amplifier that amplifies the reproduced minute video signal from this video head, and (3) is from this reproduced video amplifier. a limiter that suppresses amplitude fluctuations of an FM carrier signal in a reproduced video signal;
(4) is a demodulator that demodulates the reproduced video signal from this limiter, and (5) is a de-emphasis circuit to which the reproduced video signal from this demodulator is input, which has the opposite characteristics to the pre-emphasis circuit provided on the recording side. It is
It has a characteristic of attenuating signals at frequencies higher than around λOMHz. (6) is a video signal processing circuit that performs predetermined signal processing on the reproduced video signal from the de-emphasis circuit and outputs it to the output terminal (7).

In the magnetic video reproducing apparatus configured in this way, the video head [11] reproduces the reproduced minute video signal from the tape and the reproduced video amplifier (2), limiter +31.
Predetermined signal processing is performed via a demodulator (4), a de-emphasis circuit (51), and a video signal processing circuit (6) before appearing at the output terminal (7). These signal processings are well known, so , specific detailed explanation will be omitted here.

Next, in the magnetic video reproducing device configured in this way,
The influence of noise when noise exists in the minute reproduced video signal reproduced by the video head fi+ will be explained. To explain the case where the reproduced video signal is black and the case where it is white, in the case of black, as shown in FIG. 3(a), the carrier signal of the FM carrier signal in the minute reproduced video signal is 3-7 MHz, The noise spectrum that coexists with the FM carrier signal is from 1.0 MHz to approximately 5 MHz (3rd
(indicated by N in Figure (a)). The FM carrier signal in which the noise spectrum coexists in this way is demodulated by the demodulator (4), and in this demodulator (4), it is demodulated as a λ7 MHz f 0 point, which is the carrier signal. The noise from (4) is output as a signal with a frequency band of 2.7 MHz!!, as shown in Figure 4 (al). This output noise is transmitted to the de-emphasis circuit (5) and the video The signal passes through the signal processing circuit (6) and appears at the output end (7) without being attenuated.

On the other hand, in the case of white, the carrier signal ij of the FM carrier signal in the slightly reproduced video signal is 4.4 MHz, as shown in Figure 3 (bl), and the noise spectrum coexisting with the py carrier signal is the same as in the case of black. The same, 1.
0MHz to about 5MHz. The FM carrier signal with the noise spectrum coexisting in this way is demodulated by the demodulator (4), and since it is demodulated by the demodulator (4) as a 4,4 MHz f 0 point, which is the carrier signal,
The noise from the demodulator (4) is output as having a frequency band up to 14 MHz, as shown in FIG. 4(b). This output noise is passed through the de-emphasis circuit (5) in 3. Signals with frequencies higher than around OMHz are attenuated and passed through the video signal processing circuit (6) to reduce the amount of noise in the high range.
It will appear in

[Problem that the invention seeks to solve]

Therefore, in the IB magnetic video playback device configured as described above, when the playback video signal is white, the noise coexisting in the FM carrier signal is transferred to the de-emphasis circuit (5) in the high frequency range.
Although the S/N ratio is good because the signal is attenuated by The S/N ratio was poor. That is, a black screen seems to have more noise than a white screen, making it difficult to see, especially when the entire screen displays a dark image.

This invention has been made in view of the above-mentioned points, and an object thereof is to obtain a magnetic video reproducing device that can improve the S/N ratio and display an easy-to-read screen with less noise even when the reproduced video signal is black. That is.

[Means for solving problems]

A magnetic video playback device according to the present invention includes a level detection circuit that detects the level of a playback video signal and outputs a detection signal;
The frequency characteristic variable circuit is provided with a frequency characteristic variable circuit that controls the frequency pass band of the reproduced video signal in accordance with the intelligence signal from the level detection circuit.

[Effect]

In this invention, there are nine frequency characteristic variable circuits.

The frequency passband of the reproduced video signal is controlled by the detection signal from the level detection circuit, and even when the reproduced video signal is white or black, the amount of noise coexisting in the F'M carrier signal is made to be the same at the output. It is something.

〔Example〕

An embodiment of the present invention will be explained below based on FIG. 1. In FIG. A clamp circuit (9) that fixes the electric potential receives the reproduced video signal from this clamp circuit and adjusts the level of the reproduced video signal, that is, pedicle level @o%, white (D level is 100ch, excluding blanking period). average level of the video signal (hereinafter referred to as APL)
The level detection circuit detects the APL and outputs a detection signal according to the APL, and includes a resistor (91) whose one end is connected to the output terminal of the clamp circuit (8), and the other end of this resistor (91). and a capacitor (92) connected between it and ground.

An inverting input terminal is connected to the connection point between the resistor (91) and the capacitor (92), and a non-identifying terminal (93) is connected to the discriminating terminal (93) to which the discriminating level, which is approximately the middle level between the white level and the black level, is input. Comparator (9) to which the inverting input terminal is connected
4), and consists of a resistor (91) and a capacitor (
APL of the reproduced video signal appears at the connection point with 92), and when the reproduced video signal is white, an off signal of O potential appears at the output terminal of the comparator (94), and when it is black, @H"
A potential ON signal will appear at the output of the comparator (94).

αrJVi A frequency characteristic variable circuit that receives a detection signal from the level detection circuit (9) and a reproduced video signal from the clamp circuit (8) and controls the frequency pass band of the reproduced video signal according to the detection signal; Clamp circuit (8
) and the input terminal of the video signal processing circuit (6) and a resistor (101) connected between the output terminal of the video signal processing circuit (
A capacitor (102) with one end connected to the input end of 6)
When activated, a collector-emitter path is connected between the low-pass filter that removes high-frequency components above around 2.3 MHz and the other end of the capacitor (102) and the ground, and the level detection circuit is connected. It consists of a transistor (103) which serves as a switching means to which the detection signal from the circuit (9) is input to the base, and when the reproduced video signal is white, the off signal from the comparator (94) turns off the transistor (103). 103) is off, the low-pass filter is inactive, and the clamp circuit (8
) is input to the video signal processing circuit (6) without being attenuated via the resistor (101), and in the case of black, the on signal from the comparator (94) turns on the transistor (103). ) is turned on, the low-pass filter is activated, and the reproduced video signal from the clamp circuit (8) is processed into the low-pass filter, and the low-pass filter is activated.
High-frequency components around Hz or higher are removed and input to the video signal processing circuit (6).

In the magnetic video reproducing apparatus configured in this way, the reproduced minute video signal reproduced from the tape by the video head (1) is transmitted to the reproduced video amplifier [2+, IJ mitter (31,
Predetermined signal processing is performed in a demodulator (41), a de-emphasis circuit (5), and a clamp circuit (8).The clamp circuit (8) performs sufficient processing on the APLt of the reproduced video signal for white and black. The end level of the synchronizing signal is kept at a constant potential so that it can be detected correctly.When the reproduced video signal from the clamp circuit (8) is white, the resistor (91) and capacitor (92) are connected to each other. Since the APL in the reproduced video signal appearing at the connection point of is higher than the identification level appearing at the identification end (93), the comparator (93)
4) outputs an off signal and the transistor (105
) is off, so the resistor (101) and capacitor (1
The low-pass filter consisting of 02) remains inactive, and the signal is input to the video signal processing circuit (6) as it is, undergoes predetermined processing, and appears at the output terminal (7).

However, as in the conventional example, the amount of noise shown in FIG. 4 (bl) is reduced by the de-emphasis circuit (5) removing high-frequency components above 1QMHz.

On the other hand, when the reproduced video signal from the clamp circuit (8) is black, the APL in the reproduced video signal appearing at the connection point between the resistor (91) and the capacitor (92) is at the identification end (9).
Because it is lower than the discrimination level that appears in 3).

An on signal is output from the comparator (94), turning on the transistor (103), and the resistor (101) and capacitor (
102) is activated.

The noise coexisting in the F'M carrier signal in the reproduced video signal shown in FIG. 4(a) is input to the video signal processing circuit (6) after removing high-frequency components around 2.3 MHz or higher. A predetermined process is performed and the signal appears at the output terminal (7).

Therefore, in the magnetic video reproducing apparatus configured in this manner, even when the reproduced video signal is black, the low-pass filter of the one frequency characteristic variable circuit α is activated, and the FM
The high-frequency components of noise coexisting in the carrier signal are attenuated, and S
/N ratio is improved. Moreover, when the passband of the low-pass filter is set to a predetermined value, the amount of removal of the high-frequency component of the noise coexisting in the FM carrier signal can be the same for both white and black, and the S/N ratio can also be improved. They can be made the same. Therefore, it feels the same whether it is a black screen or a white screen, and even when a whole screen or a dark image is displayed, the influence of noise is suppressed and it becomes easier to see.

In the above embodiment, the clamp circuit (8), level detection circuit (9) and frequency characteristic variable circuit (I[1t
- De-emphasis circuit (5) and video signal processing circuit (
6), but the present invention is not limited to this; for example, it may be provided between the demodulator (4) and the de-emphasis circuit (5), or the clamp circuit (8).
and a level detection circuit (9) are provided between the demodulator (4) and the de-emphasis circuit (5).
Even if it is provided between the lli de-emphasis circuit (5) and the video signal processing circuit (6), the same effect can be achieved.

〔Effect of the invention〕

As described above, in this invention, the level of the video signal is detected by the level detection circuit, and based on this detection signal, the frequency pass band of the reproduced video signal is controlled by the frequency characteristic variable circuit. This has the effect that the S/N ratio can be improved even in a friend video where the S/N ratio was poor in the past.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention; FIG. 2 is a block diagram showing a conventional magnetic video reproducing device; FIG. 3 is a diagram showing the FM carrier level and noise spectrum in a reproduced video signal; The figure is a diagram showing the demodulation level of a demodulated reproduced video signal. In the figure, (9) is level detection circuit 1 (94)
is a comparison circuit, (10 is a variable frequency characteristic circuit, (1
01') (102) is a resistor and capacitor that constitute a low-pass filter. (103) is a transistor which is a switching child. Note that the same reference numerals in each figure indicate rows or corresponding parts.

Claims (3)

[Claims] (1) A level detection circuit that receives a reproduced video signal, detects the level of this reproduced video signal, and outputs a detection signal according to the level; the detection signal of this level detection circuit and the reproduced video signal are input; A magnetic video reproducing device includes a frequency characteristic variable circuit that controls a frequency passband of an input reproduced video signal according to a detected signal. (2) The level detection circuit has a first input terminal into which the average video level of the reproduced video signal is input, and a second input terminal into which the identification level is input, and when the average video level is lower than the identification level, The frequency characteristic variable circuit includes a comparison circuit that outputs a detection signal that is an ON signal, and a frequency characteristic variable circuit that includes a low-pass filter to which a reproduced video signal is input, and a low-pass filter that activates the low-pass filter upon receiving an ON signal from the comparison circuit. and removes high-frequency components of the reproduced video signal input to the low-pass filter, and if no on signal is output from the comparison circuit, the low-pass filter is left inactivated and the reproduced video signal input to the low-pass filter is unchanged. 2. The magnetic image reproducing apparatus according to claim 1, further comprising switching means for passing the magnetic image. (3) Both the reproduced video signal input to the level detection circuit and the reproduced video signal input to the variable frequency characteristic circuit are signals in which the video signal from the de-emphasis circuit has the tip level of the synchronization signal fixed to a constant potential. A magnetic image reproducing device according to claim 1 or 2, characterized in that: