JPS623970Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a circuit for limiting brightness, etc. in a teletext receiver that can receive teletext such as teletext, synthesize it with a video signal, and reproduce it. The object of the present invention is to prevent so-called blooming when an image is received by mixing or compositing the characters, and to enable character signals to be read clearly. When a character signal transmitted by a teletext system, etc. and a general TV video signal are mixed or combined and reproduced and displayed on the screen of a receiver, the video signal as the background of the characters is high brightness and high contrast. If so, so-called blooming occurs in that part, making it impossible to distinguish and read characters, and it becomes necessary to limit the brightness and contrast of the video signal. 1 and 2 represent a conventional example for achieving this purpose. Below, FIG. 1 and FIG. 2 will be explained for a while. The drawing number T ₁ indicates a first transistor of the NPN type which is basically connected to operate as a grounded emitter amplifier circuit, the emitter resistor R ₁ being an AC bypass circuit (resistor) forming the contrast adjustment circuit 2 .
R ₂ , capacitor C, and contrast adjustment volume Rv ₁ ), and the contrast is limited by opening the first switch S ₁ that is linked to the operation of the text/video mixed display mode switch or the control circuit 1 . This is done by a resistor R ₃ inserted in series. T ₂ designates a second transistor of the PNP type connected to basically operate as an emitter follower type amplifier circuit, the base of which is connected to the first transistor.
It is directly connected to the collector of transistor _T1 .
A voltage dividing circuit 3 consisting of a series circuit of resistors R ₄ , R ₅ and a brightness adjustment volume Rv ₂ and a resistor R ₆ connected between the voltage dividing point A and the base of the second transistor T ₂ are as follows: A brightness adjustment circuit 4 is formed, and the brightness is limited in the mixed display mode or the control circuit 1.
This is achieved by the resistor R ₇ inserted in series with the voltage divider circuit 3 by opening the switch S ₂ in conjunction with the operation of the voltage divider circuit 3 . However, with this configuration, the effect of inserting resistor _R7 will occur regardless of the setting position of brightness adjustment volume _Rv2 , and the brightness will be lowered, making it difficult to watch TV at low brightness. In text and video display mode, the brightness is too low and the video disappears, which is a drawback. In order to prevent this, as shown in Figure 2, a resistor _R8 is added in parallel with the brightness adjustment volume _Rv2 via a switch _S3 , so that the brightness remains constant regardless of the set position of the volume _Rv2 . Although it is possible to adopt a configuration that maintains
Three switches and three resistors are essential, which inevitably increases cost and reliability. In view of this, the present invention has been developed to improve brightness in a new teletext receiver, etc., which can achieve reliable brightness and contrast limitations at low cost by basically adding one switch and a pair of resistors. It provides a limiting circuit. The details of the present invention will be described below with reference to the basic circuit shown in FIG. 3, the example circuit shown in FIG. 4, and the operational waveform diagram shown in FIG. The basic circuit of the present invention shown in FIG. 3 differs from the conventional example shown in FIG. 1 in that it lacks the resistors R ₃ and R ₇ and the corresponding parallel switches S ₁ and S ₂ , and that the first transistor T ₁ collector resistance R ₉ and resistor
Mixed character and video display using the voltage dividing point B of the voltage dividing circuit 5 formed by the resistors R ₁₀ and R ₁₁ whose values are lower than those of R ₆ , R ₄ and R ₅ as the base of the second transistor T ₂ Elements that are essentially the same as those of the conventional example in that they are configured to be connected via the switch _S4 which is closed in the mode are given the same figure numbers and explanations thereof will be omitted. In such a configuration, when the switch _S4 is turned off in a mode other than the text/video mixed display mode, it functions as a simple ordinary video amplification circuit. Next, in the text/video mixed display mode, when the switch _S4 that enables the electronic switch is turned on by the output of the control circuit 1 , the resistors _R9 ,
Resistors R ₁₀ and R ₁₁ are connected in parallel to R ₆ ,
The AC load resistance of the first transistor _T1 is equivalently reduced. Generally, the voltage amplification degree Av of an emitter grounded type transistor amplification circuit is Av=-aRL/re+rb (1-a) where a: Current amplification rate of the transistor re: Equivalent emitter resistance Rb: Equivalent base Since the resistance RL is expressed as AC load resistance, it can be seen that when the AC load resistance RL becomes equivalently smaller, the voltage amplification degree of the amplification circuit decreases. Therefore, if the values of the voltage dividing resistors R ₁₀ and R ₁₁ are selected to be smaller than those of the resistors R ₉ , R ₆ , R ₄ and R ₅ , the base voltage of the second transistor T ₂ will be Voltage at point B, V _B = E ₀ · R ₁₁ /R ₁₀ +R
₁₁ , and changes in brightness can be ignored depending on the value of the brightness adjustment volume, and the brightness at that time is approximately equal to the resistance.
It is determined by the division ratio of R ₁₀ and R ₁₁ . An example of the value of the above-mentioned resistor is as described later, but if the value is too small, the amplification degree of the first transistor _T1 will be small and there will be a disadvantage that contrast will not be obtained. The range of change must be carefully considered when selecting. The difference between the more practical embodiment of FIG. 4 and the basic circuit of FIG. 3 is that the first and second transistors T ₁ and T ₂
A parallel circuit of peaking coil L ₁ and resistor R ₁₂ is inserted in series in the direct connection path of , and resistor R ₄
is the parallel circuit of resistor R ₄₁ and thermistor T _H and the resistor
The point formed by the series connection of R ₄₂ , the point that the voltage divider circuit 3 is connected to the resistor R ₆ via the volume,
and essentially, a switch _S5 which is opened (off) depending on the output of the character/video mixed display mode control circuit 1 to release the ground fault at point B, and between the point B and the base of the second transistor _T2. The diode _D1 for backflow blocking is connected as shown in the figure. In this Fig. 4 as well, parts and elements common to Figs. 1 and 3 are given the same figure numbers.
I will omit that explanation. In cases other than the text/video mixed display mode, the switch _S5 is closed, the diode _D1 is turned off, and the video amplification circuit operates normally. Figure 5A shows the video staircase wave output in such a case.
H indicates a horizontal synchronizing signal, and a indicates a black level. In the text and video mixed display mode, the control circuit
When the electronic switch S5 is opened by the output of _the second transistor T2, the diode _D1 is turned on only when the base voltage of the second transistor _T2 becomes lower than the potential at the point B. The potential at point B is set to the black level, which is the cut-off level of the cathode ray tube, so when the diode _D1 is on, the first
As the equivalent load resistance of the transistor _T1 becomes smaller, the voltage amplification degree decreases, and the contrast is limited, and even if the brightness is increased by the brightness adjustment volume _Rv2 , the change will not be affected over the entire video signal level. is suppressed without distortion as shown in Figure 5B. The potential at the point B is lower than the black level, for example, b
Once set to level, the second transistor
The base input waveform of T ₂ is as shown in Figure 5 (c),
Since the gradation of the video signal will be distorted, a part of the resistor _R11 forming the voltage dividing circuit 5 is
Configure Rv ₆ and set the potential at point B accurately to the black level. As an attempt, the values of the main resistances in one embodiment are shown. Resistor R ₉ = 2.7KΩ Resistor R ₅ = 680Ω Resistor R ₆ = 1.0KΩ Resistor R ₁₀ = 680Ω Resistor R ₄₁ = 2.2KΩ Resistor R ₁₁ = 330Ω Resistor R ₄₂ = 1.5KΩ Resistor Rv ₃ = 500Ω The present invention is as described above. Because of its simple circuit configuration, it can be used in highly reliable teletext receivers, etc., to prevent the brightness from dropping too much at low brightness and the image disappearing, and to prevent the blooming phenomenon at high brightness. This makes it possible to realize a brightness limiting circuit.

[Brief explanation of the drawing]

1 and 2 relate to a conventional example, with FIG. 1 being an overall circuit diagram and FIG. 2 being a main part circuit diagram. 3 to 5 relate to the present invention; FIG. 3 is a basic circuit diagram, FIG. 4 is an implementation circuit diagram, and FIG. 5 is an operating waveform diagram. _T1 ...First transistor, _T2 ...Second transistor, 2 ...Contrast adjustment circuit, 4 ...Brightness adjustment circuit, _S5 ...Switch, _D1 ...Diode, 1 ...Control circuit.

Claims (1)

[Scope of utility model registration request] The connection point between the first amplification circuit equipped with a contrast adjustment circuit and the second amplification circuit equipped with a brightness adjustment circuit equipped with a brightness adjustment volume is used to display the potential at the time of mixed reception of both text and video signals.The cut-off level of the cathode ray tube. is connected via a switching means to the voltage dividing point of a voltage dividing circuit set to By keeping the brightness (or brightness and contrast) of the video signal constant,
A brightness limiting circuit in a teletext receiver, etc. configured to prevent blooming.