JPH0623109Y2 - Video control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a video control circuit used in a television receiver and the like.

(B) Conventional technology In recent television receivers, IF / AG is used to reduce snow noise and color noise in a weak electric field state.
By using the C voltage, the gain of the high frequency part of the video signal and the gain of the color signal are reduced when the electric field is weak. For example, Japanese Patent Application Laid-Open No. 55-82876 discloses a method for switching the frequency characteristics of a video signal to lower the high frequency part, and Japanese Patent Application Laid-Open No. 55-55688 discloses a method for reducing the gain of a color signal. It is described in the official gazette.

(C) Problems to be solved by the invention However, in each of the above-mentioned conventional circuits, a transistor is used to perform a switching operation for gain switching, or the collector-emitter impedance of the transistor is used. The frequency characteristics were changed. Therefore, in the former case, the gain change becomes discontinuous, so that the unnaturalness cannot be wiped out, and in the latter case, the active range of the transistor is relatively narrow, so the above-mentioned frequency characteristic changes gently with respect to the input electric field strength. I couldn't do it. In addition, since the transistor is not relatively stable against temperature changes, there is a drawback that the electric field strength for starting the above-mentioned control operation changes.

Therefore, an object of the present invention is to provide a video control circuit which eliminates the drawbacks of the conventional circuit.

(D) Means for Solving the Problems In the video control circuit of the present invention, the first and second FETs to which the AGC voltage is applied to each gate are provided, and the input electric field strength obtained from the first FET is adjusted. The DC voltage is supplied as the control voltage of the color gain control circuit, and one end of the second FET is connected to the video signal processing circuit to switch the frequency characteristic of the video signal.

(E) Action According to the above configuration, since the gate-source rising voltage is large in the FET, a wide active region can be taken, and thereby the output voltage range and impedance change range of the FET can be widened.

(F) Embodiment FIG. 1 is a circuit diagram of an essential part of an embodiment of the present invention. (1) is a positive IF / AGC voltage output from a VIF / IC (not shown) in a television receiver. Each of the gates (G) of the first and second N-channel junction type FETs (3) and (4) is connected to the terminal (1) via the resistor (2), and each source (S) A constant DC voltage is applied to () by the voltage dividing resistors (5) and (6), the variable resistor (7), the zener diode (8), the diode (9), and the smoothing capacitor (10).

The drain (D) of the first FET (3) has resistors (11) (12) (13)
The base of a PNP transistor (15) biased by is connected to the collector of the PNP transistor (15) via a resistor (16) and a reverse current blocking diode (17).
Chroma / deflection 1-chip IC: μPC1401CA (18)
Is connected to a pin for applying a color gain control voltage. Further, a DC voltage obtained from the variable resistor (19) for adjusting the saturation through the resistor (20) is also applied to this pin.

On the other hand, the drain (D) of the second FET (4) is connected to the VIF.
The video signal output from the IC is connected to the pin of the IC (18) input via the delay line (21) via the frequency characteristic changing capacitor (22). Therefore, the capacitor (22) and the drain of the second FET (4)
The series circuit of the impedance between the sources acts as a high cut filter that attenuates the high frequency part of the video signal.

The transistor (23) commonly connected to the gates (G) of the first and second FETs (3) and (4) is supplied with a high level voltage to the base when an external video signal such as a VTR reproduction signal is input. It is turned on, which causes the IF / A
This is for grounding the GC voltage to stop the video control operation.

In such an embodiment, the IF / AGC voltage is lower than the DC voltage applied to the source (S) of the first and second FETs (3) and (4) in the strong electric field state, and therefore, the respective FETs thereof are (3)
All of (4) are off, and the transistor (15) is off. Therefore, the color gain control operation of the IC (18) is not affected by the AGC voltage in any way, and the frequency characteristic changing capacitor (22) is in a free state. The frequency characteristic of the video signal input to the pin is also constant regardless of the AGC voltage.

However, as the input electric field strength decreases, the AGC
Since the voltage is increasing, this AGC
When the voltage reaches a weak electric field state in which the DC voltage of the source (S) is exceeded, the first and second FETs (3) and (4) both start to turn on. After that, the conductivity of the first and second FETs (3) and (4) increases as the input electric field strength decreases. Then, the potential of the drain (D) of the first FET (3) decreases accordingly, so that the transistor (15) is turned on and its collector potential increases. Therefore, the control voltage at the pin of the IC (18) eventually increases as the input electric field strength decreases, and the degree of decrease in the gain of the color signal increases.
In addition, increasing the conductivity of the second FET (4)
The drain-source impedance is reduced. Therefore, this impedance and capacitor (22)
That is, the high cut filter effect consisting of increases as the input electric field strength decreases, and the degree of attenuation in the high frequency part of the video signal increases.

Here, the first and second FETs (3) and (4) are completely turned on (saturated) only when the gate potential exceeds the source potential and then rises by 1.6 V, so that the IF applied to the gate (G) A
The range in which the first and second FETs (3) and (4) operate in the active state with respect to the change in the GC voltage is sufficiently wider than when a normal transistor is used. Therefore, the control voltage obtained at the collector of the transistor (15) and the second FET (4) and the capacitor
This means that the attenuation of the high cut filter composed of (22) can be gently changed depending on the input electric field strength.

(G) Effect of the invention According to the video control circuit of the present invention, the gain control of the color signal and the switching of the frequency characteristic of the video signal according to the input electric field strength are performed by using the FETs. Can be continuously and gently changed, and the operation is stable against temperature drift.

[Brief description of drawings]

The drawing is a circuit diagram showing a main part of an embodiment of the present invention. (1): IF / AGC voltage application terminal, (3) (4): 1st and 2nd F
ET, (18): Video, Chroma, Deflection IC

Claims (1)

[Scope of utility model registration request] 1. A first and second FET in which an AGC voltage according to an input electric field strength is applied to each gate is provided, and each FET is provided.
A DC voltage for setting the reference electric field strength is applied to the source of the first FET, the drain output voltage of the first FET is supplied as the control voltage of the color gain control circuit, and one end of the terminal of the second FET is supplied to the video signal processing circuit. A video control circuit that is connected to switch the frequency characteristics of the video signal.