JPS6149558A - Direct current regenerating circuit - Google Patents

Abstract

PURPOSE:To make a black level invariably constant over the whole range of a contrast adjustment by detecting variation in DC voltage for contrast control and varying a reference voltage for DC regeneration according to the detection output. CONSTITUTION:A video amplifying circuit used for a TV receiver, etc., is provided with a contrast circuit and a DC regenerating circuit which control the level of a video signal with a DC voltage, and the circuit composed of resistances 16, 17, and 18 and an emitter follower transistor TRQ3 is provided at the connection point between an element 15 as a DC voltage control type resistance and the DC voltage 14 for contrast control. The TRQ3 of this circuit detects variation in DC voltage 14 and applies the detected voltage to a point (f) of the DC regenerating circuit. The detected voltage at this point (f) is superposed on a clamp reference voltage (f-point voltage) to vary the reference voltage for DC regeneration. Thus, the black level is held invariably constant over the entire range of contrast adjustment.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a DC reproduction circuit applied to a television receiver or a display monitor in which a blanking level and a black level of a video signal are different.

Conventional Structure and Problems First, an example of a conventional DC regeneration circuit is shown in FIG. 1st
The circuit shown in the figure is a contrast circuit 10 controlled by DC voltage.
0. Emitter follower circuit 101 and DC regeneration circuit 102 with constant clamp voltage. It is composed of a video amplification circuit 103.

FIG. 2 shows a detailed example of the circuit shown in FIG. The numbers shown in FIG. 2 indicate the same blocks as the numbers shown in FIG. 10
0 is a contrast adjustment circuit (hereinafter referred to as a DC contrast circuit) that controls the video signal 13 applied to point a by changing the control DC voltage 14. The element 16 is an element (for example, JET, etc.) in which when the DC voltage 14 is applied to the b terminal and changed, the resistance value between the terminals C and d changes linearly with respect to the change in the DC voltage 14.

The video signal thus applied through the C contrast circuit 100 is supplied to the emitter follower circuit 101, guided to the DC reproduction circuit 102 composed of a clamp transistor Q2, and then sent from the output terminal e to the video amplification circuit 103.
guided by.

Here, the operation of the DC regeneration circuit 102 will be described. A clamp signal synchronized with the blanking period of the video signal as shown in FIG. 3B is applied to the base of the clamp transistor Q2. At this time, the clamp transistor Q
2 is conductive only during the high level period of the clamp signal, forcibly clamping the signal level during the blanking period of the video signal to the emitter potential of the clamp transistor Q2, and charging the capacitor 7 at the same time. Thereafter, when the input clamp signal becomes low level, the clamp transistor Q2 is cut off, and the charge charged in the capacitor 7 described above operates as a bias power supply to transmit a video signal to the next connected circuit 103.

In this case, the charge is discharged by the input impedance R1 of the circuit 103, but if the capacitance of the capacitor 7 is C9, the discharge time constant is τ, and the time of the video section of the video signal is T, then τ-GRi, >>T ・River ······river··
The capacitance C of the capacitor 7 may be determined as shown in (1).

In this way, the signal level during the blanking period of the video word to be clamped is determined by the clamp transistor Q2.
It is determined by the voltage determined by the values of the resistors 11 and 12, which determine the emitter potential of the resistors 11 and 12.

A problem here arises when a video signal with a blanking level and a black level different from each other as shown in FIG. 3 is applied. Since the black level of the video signal is changed by the DC contrast circuit with reference to the blanking level, there is a drawback that the black level of the video signal changes depending on the change in the contrast as shown in FIG. For example, if the black level when the contrast is minimum is set to be the cutoff voltage of the cathode ray tube, the black level becomes gray when the contrast is maximum, making true DC reproduction impossible.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a DC reproducing circuit that can eliminate such conventional disadvantages and keep the black level constant over the entire range of contrast adjustment.

Structure of the Invention In the present invention, the reference level of the video signal is adjusted by detecting a change in the DC voltage for contrast control applied to the contrast circuit and changing the DC reference voltage for DC reproduction based on the detected output. The black level is always kept constant.

DESCRIPTION OF EMBODIMENTS FIG. 5 shows an embodiment of the present invention. Components that are the same as those in FIG. 2 are designated by the same numbers.

The feature of the circuit shown in Fig. 5 is that it has a circuit configuration with resistors 16, 17.18 and emitter follower transistor Q3, and the DC voltage is 1.
A voltage change of 4 is detected, and 0 is found at a point where it is superimposed on the clamp reference voltage (one point voltage).A detailed explanation will be given below.

In FIG. 3, the level difference between the blanking level and the black level of the video signal is V. , and if the resistance value of resistor 1 in FIG. V becomes. Here, the resistors 4, 5, 6 and the transistor Q1 are set so that the input impedance R of the next stage emitter follower circuit is R''')>R2 (3).

The change in DC voltage 14 is expressed as ΔE14. Terminal C- of element 16
If the change in resistance value R2 between elements 15 and 6 is ΔR2, then element 15
Since the resistance R2 between the terminals C and 6 changes linearly due to the DC voltage 14, the relationship between them is ΔR2=, ・ΔE14 −・ ・・−(@(K,
: constant of proportionality). In addition, the relationship between ΔR2 and the amount of change ΔV in the level difference from the blanking level of the video signal to the black level due to contrast change is given by equation (2), where Δv=2・ΔR2 ( 9) However, there is the following relationship. Therefore, from equations (8) and (9), Δv=
, -2-ΔE+a """"' (11), and ΔL4 and JV are in a proportional relationship.

Here, if the base voltage of the emitter follower transistor Q3 is vB3e, and the resistance value of the resistor 16 is R16, and the resistance value of the resistor 17 is R17, then the base voltage vB3 due to the change in the DC voltage E, 4 is as follows.
The change amount ΔvB3 is. From equation (12). Also, Q
3 is an emitter follower transistor, so the base voltage change ΔvB3 and the emitter voltage V. 3 change Δv
83 are equal, so

Further, if the voltage at point f is vf, the resistance values of resistors 11.12 are R11 and R12, and the voltage value of power supply v1 is vl, then the change ΔV/ in the voltage at point f with respect to the change ΔvE5 in vE3 is (16 ) From the formula, JVf=;, 1-)-R12°, JV,,-°゛=°(
16) ', and in the end, the change Δvf in the voltage at point f with respect to the change in DC voltage E14 is (14), (16)
From the formula......(17). Resistance value R11 of resistors 11.12 and 16.17,
Select R12, R16, and R17 so as to satisfy the following formula, that is, from formula (10), from formulas (11) and (17), Δvf−Δv
81109311169.1. (20).

In this way, as shown in Figure 6a, if the amount of change in the level difference from the blanking level to the black level of the video signal due to the contrast change is Δma at the 0 point, then in the conventional example, the clamp voltage is constant. As shown in FIG. 6, the blanking level becomes constant and the black level shifts by JV. Therefore, as shown in Figure 6C, a voltage (ΔV,) equal to the amount of change Δma in the level difference from the blanking of the video signal to the black level due to the contrast change is used as the clamp voltage (voltage at one point) for DC reproduction. If the blanking level of the 0-point video signal is changed by JV, the black level can be made to match the black level before the contrast change.

As a result, the DC voltage value of the black level can be kept constant over the entire range of contrast adjustment.

As described in detail, according to the present invention, a change in DC voltage for contrast control during contrast adjustment is detected;
By changing the reference voltage for DC reproduction according to the change in contrast, the DC voltage value of the black level can be kept constant over the entire range of contrast and trust adjustment in the DC reproduction circuit. The effects are incredible.

[Brief explanation of drawings]

Figure 1 is a block diagram of a conventional contrast/DC regeneration circuit, Figure 2 is a circuit diagram showing the detailed circuit configuration of the circuit in Figure 1, and Figure 3 is a block diagram of a contrast/DC regeneration circuit in a conventional example. FIG. 4 is a waveform diagram of a video output signal showing black level deviation in a conventional example. FIG. 5 is a circuit diagram of a DC reproduction circuit according to an embodiment of the present invention. FIG. 6 is a conventional example. FIG. 3 is a waveform diagram showing a black level shift in the image data and a video output signal according to the present invention. 1- Contrast attenuation setting resistor, 2... Bihas capacitor, 3- Coupling capacitor, 4゜6...
・Bias setting resistance, 6...Emitter resistance, 7.
8...Coupling capacitor, 9...Resistor, 1o
-...Bypass capacitor, 11.12...
Clamp voltage setting resistor, 13...Video signal source, 14...
・-・DC voltage for contrast control, 15・Element as DC voltage controlled resistor, Q2 ・Clamp transistor, 16.17 ・・Contrast control voltage 14
0 division resistance, 18...emitter resistance, Vl, V2.
··power supply. Name of agent: Patent attorney Toshio Nakao and one other person - Ward
Ward 0]

! tv&

Figure 5 (a CA,

Claims (1)

[Claims] A video amplification circuit having a contrast circuit and a DC reproduction circuit for controlling the magnitude of the video signal using a DC voltage is provided, and a black level, which is a reference level of the output video signal of the video amplification circuit, is determined by detecting a change in the DC voltage. A direct current reproducing circuit comprising means for changing the reference voltage for direct current reproducing according to a change in contrast so as to maintain the reference voltage constant.