Clamp and gamma correction circuit, and image display apparatus and electronic machine employing the same

• US 5,940,058 A
• Filed: 11/06/1997
• Issued: 08/17/1999
• Est. Priority Date: 11/08/1996
• Status: Expired due to Term

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Koyama, Fumio
• Primary Examiner(s): Kostak, Victor R.
• Application Number:
  US08/965,683
• Time in Patent Office: 649 Days
• Field of Search: 348/674, 348/254, 348/675-679, 348/691, 348/692, 348/751, 348/761, 348/766, 348/790, 348/89-99, 345/207-214
• US Class Current: 345/89
• CPC Class Codes: G09G 2310/0297 Special arrangements with multiplexing or demultiplexing of display data in the drivers for data electrodes, in a pre-processing circuitry delivering display data to said drivers or in the matrix panel, e.g. multiplexing plural data signals to one D/A converter or demultiplexing the D/A converter output to multiple columns
• CPC Class Codes: G09G 2320/0276 for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general
• CPC Class Codes: G09G 3/3648 using an active matrix G09G3/367 - G09G3/3696 take precedence
• CPC Class Codes: G09G 3/3688 suitable for active matrices only
• CPC Class Codes: H04N 5/18 by means of "clamp" circuit operated by switching circuit
• CPC Class Codes: H04N 5/202 Gamma control circuits for controlling camera response irrespective of the scene brightness H04N23/82
• CPC Class Codes: H04N 5/66 Transforming electric information into light information scanning details H04N3/00

Abstract

A clamp and gamma correction circuit which processes an image signal through a clamp circuit and then a gamma correction circuit in that order to perform a proper gamma correction with a clamp voltage adjusted to its appropriate value. An image signal varying between a black level and a white level is applied to the clamp circuit. The clamp circuit outputs an image signal, the black level of which is clamped to the clamp voltage, while the amplitude between the black level and the white level is maintained at constant level. The gamma correction circuit amplifies the image signal output by the clamp circuit at different amplification factors based on a reference voltage. A voltage setting circuit varies the clamp voltage and the reference voltage but with the voltage difference therebetween kept constant, and applies the clamp voltage and the reference voltage to the clamp circuit and the gamma correction circuit, respectively.

Claims

1. A clamp and gamma correction circuit comprising:

a clamp circuit which receives an image signal varying between a first voltage and a second voltage and clamps one of the first and the second voltages to a predetermined clamp voltage while keeping constant an amplitude between the first and the second voltage;

a gamma correction circuit which receives the output of the clamp circuit and amplifies the output of the clamp circuit at different amplification factors depending on whether the output of the clamp circuit exceeds at least one predetermined reference voltage; and

a voltage setting circuit that sets the clamp voltage and the at least one predetermined reference voltage to the clamp circuit and the gamma correction circuit, respectively, while varying both the clamp voltage and the at least one predetermined reference voltage, a voltage difference between the clamp voltage and the at least one predetermined reference voltage is kept constant.

2. The clamp and gamma correction circuit according to claim 1, wherein said voltage setting circuit comprises:

a variable voltage circuit;

a constant voltage generator circuit which receives a varying input voltage and generates a constant voltage regardless of the value of the varying input voltage; and

a voltage setting circuit that sets the clamp voltage and the at least one predetermined reference voltage to said clamp circuit and said gamma correction circuit, respectively, based on the output of said variable voltage circuit and the output of said constant voltage generator circuit.

3. The clamp and gamma correction circuit according to claim 2, wherein the variable voltage circuit comprises a transistor, the base of which is supplied with the input voltage, and the voltage between the base and the emitter of the transistor is the constant voltage.

4. The clamp and gamma correction circuit according to one of claim 2, wherein the voltage setting circuit comprises a voltage divider circuit that divides said constant voltage.

5. An image display apparatus comprising the clamp and gamma correction circuit according to claim 1 and a display device that provides an image based on an image signal that is processed through the clamp and gamma correction circuit.

6. The image display apparatus according to claim 5, wherein said image display device is a liquid-crystal panel that comprises a liquid crystal interposed between a pair of substrates.

7. An electronic machine comprising the image display apparatus according to claim 5.

8. An electronic machine comprising the clamp and gamma correction circuit according to claim 4.

9. An image display apparatus comprising the clamp and gamma correction circuit according to claim 2 and a display device that provides an image based on an image signal that is processed through the clamp and gamma correction circuit.

10. An image display apparatus comprising the clamp and gamma correction circuit according to claim 3 and a display device that provides an image based on an image signal that is processed through the clamp and gamma correction circuit.

11. An image display apparatus comprising the clamp and gamma correction circuit according to claim 4 and a display device that provides an image based on an image signal that is processed through the clamp and gamma correction circuit.