JPH02137168A - Video tape recorder - Google Patents

Abstract

PURPOSE:To prevent the distortion of a sink chip level by lowering the demodulation efficiency of an FM demodulating circuit in response to the detecting output of a dropout detecting (DOC) circuit. CONSTITUTION:A demodulation curve is made smooth only during a noise period and the amplitude of a noise is relatively lowered. Thus, a noise generation period is detected by a DOC circuit 8. Namely, when a luminance signal to include the noise period to be a noise bar is impressed, the DOC circuit 8 generates the detecting output in response to the impression and the values of a variable current source 9 and a variable resistor are controlled in a direction, in which the values go to be small, in response the detecting output. In such a case, the characteristic of the demodulation curve is simultaneously adjusted by both axes. Thus, the noise to be generated at the time of VTR reproducing is reduced and the sink chip distortion of a reproducing luminance signal can be prevented.

Description

Detailed Description of the Invention (A) Industrial Application Field The present invention is directed to a home VTR (video tape recorder) equipped with an FM demodulation circuit that demodulates an FM-modulated luminance signal.
In particular, it relates to a VTR with reduced noise amplitude level generated during special playback.

(b) Conventional technology Home VTRs are equipped with an FM demodulation circuit for reproducing FM-modulated luminance signals recorded on tape.
The FM demodulation characteristics are as shown in FIG. 2 in the case of the VH3 system. In the VH8 system, the key frequency at the video signal sync tip (the tip of the synchronization signal) is 3 to 4 MHz, and the white peak is 4 to 4 MHz, and the frequency deviation between them is IMHz. It is standardized to be.

For example, the FM demodulation circuit of a home VTR is
It is described in Publication No. 81273.

(c) Problems to be Solved by the Invention By the way, it is known that noise bars occur in a search mode such as double-speed playback of a VTR. In the period of the noise bar, the above-mentioned 3 and 4 MHz frequency is present in the FM signal before demodulation.
4, and many signals with frequencies other than 4 MHz are included. Therefore, when the signal is demodulated, large-amplitude noise is generated, which causes the problem that the noise is clamped by a clamp circuit located after the demodulator.

(d) Means for Solving the Problems The present invention has been made in view of the above points, and includes a circuit for FM demodulating an FM-modulated luminance signal, and a dropout detection circuit for detecting a dropout of the luminance signal. The video tape recorder is characterized in that the demodulation efficiency of the FM demodulating circuit is reduced in accordance with the detection output of the dropout detection circuit.

(*) Effect According to the present invention, when the dropout detection circuit detects that a video signal contains a lot of noise in search mode, etc., the demodulation efficiency of the FM demodulation circuit is reduced accordingly. Therefore, the level of the output signal during that period can be lowered, and the sync tip level can be prevented from rising.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (1) is a limiter circuit that controls the level of the FM-modulated luminance signal from the input terminal (2), (3) is an output transistor to which the output signal of the J-mitter circuit (1) is applied to the base, <4> is an FM demodulator that performs FM demodulation of the luminance signal from the output transistor (3), and (5> is the FM demodulator) A current mirror circuit that inverts the output current of the device (4), (6
> is a variable resistor that converts the output current of the current mirror circuit (5) into voltage and performs level control, (7) is a smoothing circuit, and (8) detects a lack of brightness signal from the input terminal (2). The limiter circuit (
1> is a DOC (dropout detection circuit) circuit that changes the values of the variable current source (9) and the variable resistor (6).

Next, the operation will be explained. The internal circuit of the FM demodulator (4) is detailed in Japanese Utility Model Application Laid-open No. 61-81273, so the explanation will be omitted and its operation will be briefly explained. When the FM signal of the amplitude is applied from point A to the FM demodulator (4), the FM demodulator (4)
) operates as a mono multi-by-break using the rise and fall of the FM signal as a trigger. Therefore, as shown in Figure 3 (B), point B is exposed for a certain period of time (') compared to Figure 3 (A).
ro) A delayed delayed pulse is generated. This time TD is T
D=CV/I.・・・・・・
... 1〉. And Figure 3 (b)
By smoothing the pulses in the smoothing circuit (7), a demodulated luminance signal as shown in FIG. 3(c) can be obtained.

Now, in the FM demodulator (4), when the frequency of the input signal is zero, that is, when a DC voltage is applied, the value of the output current becomes the maximum value and is constant as t current I of the current source (11). is output as it is, is reversed by the current mirror circuit (5), and flows to the variable resistor (6). Therefore, the voltage at point B at this time is the voltage at the variable resistor (6).
Letting the resistance value of R6 be Io.Ro. Furthermore, when the input signal is applied, the dowel changes accordingly, and becomes zero when the frequency becomes 1/(2TD) or more.

To summarize the above, the demodulation characteristics are as shown by the dotted line in FIG. Therefore, in the present invention, the demodulation curve shown by the dotted line in FIG. 4 is made gentler by the noise period to relatively reduce the amplitude of the noise. For that purpose, first, D.
The OC circuit (8) detects the period of noise occurrence. D.O.
When the C circuit (8>) is applied with a luminance signal including a noise period (a) that becomes a noise bar as shown in FIG. 5(a), it generates the detection output shown in FIG. 5(b) in response. ,
The values of the variable current source (9) and variable resistor (6) are controlled according to the detection output. In this case, the values of the variable current source (9) and variable resistor (6) are controlled to decrease. Then, the demodulation curve in that case becomes as shown by the solid line in FIG. In this case, since the characteristics of the demodulation curve are adjusted simultaneously on both axes, the change in demodulation efficiency near the center frequency is small and the sense of discomfort on the television screen can be reduced.

By the way, as a method to reduce noise, a DOC circuit (
One possible method is to clamp the level of the demodulated luminance signal to a certain level (black, white, gray, etc.) using the detection output in 8), but this would make it look unnatural on the screen. In other words, in the noise bar period, a large amount of noise is certainly included, but a luminance signal is also present and has sufficient luminance information. Therefore, even during the noise bar period, it is still better to perform demodulation in terms of image quality.Therefore, in the present invention, demodulation is performed after lowering the demodulation efficiency.

In the explanation of Fig. 1, the value of the variable current source (9) is changed in order to reduce the demodulation efficiency, but the value of the load resistance (12>) of the limiter circuit (1) may also be changed. death,
Further, the capacitance of the capacitor (1o) of the FM demodulator (4) may be changed.

(g) Effects of the Invention As described above, according to the present invention, it is possible to reduce the noise generated during reproduction of a VTR, and therefore it is possible to prevent sync tip disturbance of the reproduced luminance signal. especially,
If operated during special playback such as search mode, the demodulation operation will continue during that period, so deterioration of image quality can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention.
A characteristic diagram for explaining the conventional FM demodulator, Fig. 3 (
A) to (C) are waveform diagrams for explaining FIG. 1, FIG. 4 is a characteristic diagram for explaining FIG. 1, and FIG.
b) and (b) are waveform diagrams for explaining Fig. 1. (1>... limiter circuit, (4)... FM demodulator,
(6>...variable resistance, (7)...smoothing circuit, (
8)...DOC circuit. Figure 1

Claims (2)

[Claims] (1) In a video tape recorder comprising a circuit for FM demodulating an FM-modulated luminance signal and a dropout detection circuit for detecting a dropout of the luminance signal, the FM demodulation is performed according to a detection output of the dropout detection circuit. 1. A video tape recorder characterized in that the demodulation efficiency of the circuit is reduced. (2) a limiter circuit that limits the amplitude of an FM-modulated luminance signal; a first means for changing the level of the output signal of the limiter circuit; and an FM demodulator that demodulates the output signal of the first means. a dropout detection circuit for detecting a dropout of the luminance signal; a second means for changing the level of the demodulated output of the FM demodulator; and a smoothing circuit for smoothing the output signal of the second means. A video tape recorder comprising the following: wherein the first and second means are adjusted in accordance with the detection output of the dropout detection circuit.