JP2668974B2 - Peripheral light correction circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a more uniform electrical correction in a projector such as a projection color television in which the periphery of the screen is essentially dark. The present invention relates to a peripheral light amount correction circuit for obtaining a screen of brightness.

2. Description of the Related Art Conventionally, in a projection type color television, a luminance correction circuit has been introduced in order to compensate for insufficient luminance in a peripheral portion.

As an example, as shown in FIG. 3, a multiplication circuit is basically used. As the input multiplied signal,
The pedestal clamp circuit 1 adds a normal video signal pedestal clamped to the base of the transistor Q1 and the reference voltage of the pedestal clamp to the base of the transistor Q2, and the transistors Q3 and Q6 forming a differential amplifier are used as multiplication signals. A horizontal and vertical period parabolic voltage is applied between the common base and the common base of the transistors Q4 and Q5,
The necessary correction waveform was obtained as an output signal from one of the common collectors.

FIG. 4 shows the relationship between the input signal and the modulation signal (multiplication signal) output signal when such a circuit configuration is employed. Here, the vertical cycle is shown as an example. Transistor here
The correction is zero for the equal portion (pedestal level) of both base potentials of Q1 and Q2, and transistors Q1 and Q2
The correction proportional to the potential difference between them will be added.

Basically, by adding the signal thus corrected to the cathode ray tube, it is possible to correct the decrease in the peripheral light amount in a projection type car television or the like, and it is possible to realize a screen with more uniform brightness.

However, when a signal with a low APL (average brightness level) is input, the central part of the screen becomes almost dark and only the peripheral part slightly shines, which causes the side effect. The screen looks unnatural.

Ideally, on a screen with a low APL, the correction should be zero, and a signal with APL of 40 to 50% or more should be sufficiently corrected to give the most natural feeling.

An object of the present invention is to solve the problem of the conventional circuit and to realize a peripheral light amount correction circuit in which image projection is more natural.

Means for Solving the Problems The peripheral light amount correction circuit of the present invention is designed to perform correction only on a signal of an arbitrary APL or more, and is a difference between two sets of transistors in which a common current source transistor is connected to a common emitter. A dynamic amplifier is provided, and a normal video signal is input to one of the bases of the constant current source transistors of each differential amplifier as a signal to be multiplied, and the video signal is arbitrarily input to the base of the other constant current source transistor. The signal clipped at the level of is added, the multiplication signal is input between the two common bases of the two sets of differential amplifiers, and the third differential amplifier is provided, and both bases of the third differential amplifier are provided. , A normal video signal which is a signal to be multiplied and a video signal clipped at an arbitrary level are added, and the collector of the third differential amplifier is connected to the force collectors of the two differential amplifiers. It is characterized by continuing.

Effect According to the present invention, since the peripheral light amount correction is applied to a signal of a predetermined APL or more, a screen with uniform brightness can be obtained, and the peripheral light amount correction is not applied to a video signal of less than that. Therefore, a natural screen can be obtained.

FIG. 1 shows an embodiment of the present invention. In the figure, 1 is a pedestal clamp circuit, 2 is a reference voltage generation circuit, 3 is this multiplier, and it is composed of two sets of differential amplifiers Q3, Q4, Q5, Q6, constant current source transistors Q1, Q2 and load resistance RL. It 4
Is a clipping circuit, and 5 is a level correction circuit, which comprises differential amplifiers Q7 and Q8 and a constant current source transistor Q9. Reference numeral 6 denotes a volume for setting a clip level.

As a signal to be multiplied to the multiplier 3, a normal video signal pedestal clamped by the pedestal clamp circuit 1 and a signal clipped at an arbitrary signal level via the clip circuit 4 are respectively added to the bases of the transistors Q1 and Q2. To enter. From the basic principle of multiplier 3, transistor Q
If the same input signal is input to the bases of 1, Q2, even if a modulation signal is applied between the bases of transistors Q3 and Q6,
On the output side, no influence of the modulation signal appears. Therefore, when the normal video signal and the video signal from the clipping circuit 4 are input to the bases of the transistors Q1 and Q2, the correction is applied only to the signal clipped by the clipping circuit 4 and having the IRE level or higher. It will be a matter. The IRE level to which the correction is applied can be arbitrarily controlled by the level setting volume 6. However, if the normal video signal and the signal obtained by clipping the normal video signal are added to the bases of the transistors Q1 and Q2, respectively, the output signal is compressed to 1/2 or more at the clipped level or higher. It will be shaped like.

For ease of understanding, the description will be made with reference to FIG. 2 assuming that the modulation signal input is zero. In FIG. 2, a is a collector current of the transistor Q4 with respect to the base input signal of the transistor Q1, FIG. 2b is a collector current of the transistor Q6 with respect to the base input signal of the transistor Q2, and FIG.
Is a signal current waveform flowing through the load resistor RL. If a modulation signal (parabolic voltage) is applied in this state, correction will be applied to signals above the level shown by the dotted line in FIG. 2c, but the output waveform when the correction is zero is different from the input waveform. turn into.

In order to solve this problem, in FIG. 1, the level correction circuit 5 to which the level correction circuit 5 is added is composed of a pair of differential amplifiers Q7 and Q8, and one of the differential input bases has a normal The video signal and the other clipped video signal are input.

By doing so, as the collector currents of the differential amplifiers Q7 and Q8, only the current of the clipped signal portion flows. FIG. 2d shows a collector current waveform of the transistor Q8. By connecting the collector of the transistor Q8 and the collector of the transistor Q6, it is possible to obtain a waveform equal to the input waveform as shown by e in FIG. 2 as the final current flowing through the load resistor RL. By adding a modulation signal, it becomes possible to apply correction only to the IRE portion above the dotted line level shown in FIG. 2e.

EFFECTS OF THE INVENTION As described above, by using the circuit of the present invention, it is possible to freely apply the modulation for the peripheral luminance correction only in the signal portion of IRE or more by changing the clip level setting volume. This makes it possible to freely perform natural peripheral brightness correction according to the characteristics of the lens and screen of the projection type color television.

[Brief description of the drawings]

1 is a circuit diagram of a peripheral light amount correction circuit in an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, FIG. 3 is a circuit diagram of a conventional peripheral light amount correction circuit, and FIG. FIG. 4 is a waveform diagram for explaining the operation. 3 ... Multiplier, 4 ... Clip circuit, 5 ... Level correction circuit, 6 ... Clip level setting volume, Q1, Q2, Q9
…… Constant current transistor, Q3, Q4, Q5, Q7, Q8 …… Differential amplifier.

Claims (1)

(57) [Claims] 1. A pair of differential amplifiers, each having a common emitter connected to a transistor for a constant current source, are provided, and as a signal to be multiplied, one of the bases of the transistors for a constant current source of each differential amplifier is a normal video. A signal obtained by clipping the video signal at an arbitrary level is added to the base of the other transistor for a constant current source, and a multiplication signal is applied to the base of the other transistor.
A third differential amplifier is provided while inputting between the common bases of the pair of differential amplifiers, and a normal video signal, which is the multiplied signal, is supplied to both bases of the third differential amplifier. A peripheral light amount correction circuit characterized in that a video signal clipped at an arbitrary level is added and the collector of the third differential amplifier is connected to the output collectors of the two sets of differential amplifiers.