Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1999-07-06
2001-09-25
Eisenzopf, Reinhard J. (Department: 2614)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C348S674000, C348S675000, C348S676000, C348S677000, C358S003050, C358S461000
Reexamination Certificate
active
06295098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination intensity correcting circuit for correcting the intensity of a video signal or three primary color signals.
2. Description of the Related Art
FIG. 1
is a schematic block diagram of a computer display apparatus of the related art. This computer display apparatus
100
includes a signal amplifier circuit
20
, a cathode ray tube ( hereinafter referred to as a CRT)
30
, and a deflection circuit
40
. The signal amplifier circuit
20
includes first to third amplifier circuits
21
to
23
. The CRT
30
includes an electron gun
34
. The first amplifier circuit
21
receives as an input a red primary color signal SR to amplify the same and supplies the resultant signal SR
2
to a cathode
31
of the electron gun
34
. The second amplifier circuit
22
receives as an input a green primary color signal SG to amplify the same and supplies the resultant signal SG
2
to a cathode
32
of the electron gun
34
. The third amplifier circuit
23
receives as an input a blue primary color signal SB to amplify the same and supplies the resultant signal SB
2
to a cathode
33
of the electron gun
34
.
The electron gun
34
generates electron beams BR, BG, and BB according to the output signals SR
2
, SG
2
, and SB
2
correspondingly and emits the electron beams BR, BG, and BB to a fluorescent screen
35
. This fluorescent screen
35
is made of a fluorescent material that is coated on the inside of the front glass of the CRT
30
. The deflection circuit
40
receives as inputs horizontal and vertical synchronizing signals SH and SV and generates a magnetic force by a deflecting coil (yoke) in accordance with the signals SH and SV to change magnetic field force in the CRT
30
so that horizontal and vertical scanning by the electron beams from the electron gun
34
are carried out in the CRT
30
. The three primary color signals SR, SG, and SB are analog output signals for example from digital-to-analog converters which are supplied with digital signals respectively from frame buffers used for the three primary color signals or from color look-up tables (CLUTs).
Known in the art is a “curve fitting circuit” for compressing the amplitude of an input signal by approximating the signal characteristic by a series of breakpoints connected by straight lines. For example, the curve fitting circuit is formed by a parallel-connected circuit which consists of a reference resistor (first resistor) and a serial-connected circuit that includes a diode, a second resistor, and a constant-voltage source. This constant-voltage source generates a reference voltage. If the terminal voltage of the reference resistor is smaller than the reference voltage, the diode is supplied with a reverse voltage and turned OFF, whereby the parallel-connected circuit is given a first electric resistance the same as that of the reference resistor. If the terminal voltage of the reference resistor is larger than the reference voltage, the diode is supplied with a forward voltage and turned ON, whereby the parallel-connected circuit is given a second electric resistance of the reference resistor and the second resistor connected in parallel. If the parallel-connected circuit is used as a load resistor, the electric resistance of the load resistor is changed before and after the reference voltage, so the parallel-connected circuit can form a curve fitting circuit.
However this curve fitting circuit is structured so that the diode is supplied with a reverse voltage and forms a depletion layer capacitance when the reverse voltage is supplied. Since the resistance of the parallel-connected circuit differs for a low frequency component and a high frequency component of the same input signal due to the capacitance of the depletion layer, this curve fitting circuit is not well suited for high speed operation.
Moreover the diode resistance changes according to the applied voltage of the reference resistor due to the non-linear characteristic of the diode, so this curve fitting circuit may suffer from distortion near each breakpoint in the signal characteristic.
A television receiver is sometimes provided with an illumination intensity correcting circuit for adjusting the relationship between the three primary color signals or video signal and the brightness of the image. For example, use is sometimes made of a gamma correcting circuit. As the curve of the gamma characteristic, use may be made of a convex secondary degree curve. Because the gamma correcting circuit of the related art forms a secondary degree curve by using capacitor elements, it is difficult to perform gamma-correction on the high frequency component of a video signal or three primary color signals.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an illumination intensity correcting circuit which can perform gamma-correction on the high frequency component of a video signal or three primary color signals.
Another object of the present invention is to provide an illumination intensity correcting circuit which can prevent distortion near the breakpoints in the signal characteristic.
To achieve the above objects, according to a first aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on a video signal comprising parallel-connected differential amplifier circuits for generating signals corresponding to the difference between a voltage of the video signal and the reference voltages of the differential amplifier circuits and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to the output terminals of the differential amplifier circuits, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits and the load resistor, the reference voltages of the differential amplifier circuits are set so that at least two breakpoint voltages are arranged in the range of the voltage of the video signal, and the amplification factors of the differential amplifier circuits are set so that the amplification factor of the curve fitting circuit in the range of the signal voltage between the two breakpoint voltages is smaller than the amplification factor of the curve fitting circuit outside the range between the two breakpoint voltages.
To achieve the above objects, according to a second aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on a video signal comprising differential amplifier circuits for generating signals corresponding to the difference between a voltage of the video signal and the reference voltages of the differential amplifier circuits; a synthesizing circuit including a differential amplifier circuit to synthesize output signals of the differential amplifier circuits; and a load resistor with one terminal connected to an input terminal of a source voltage and with another terminal connected to an output terminal of the synthesizing circuit, wherein a curve fitting circuit whose amplification factor is changed before and after each breakpoint voltage is formed by the differential amplifier circuits, the synthesizing circuit, and the load resistor, the reference voltages of the differential amplifier circuits are set so that a breakpoint voltage is arranged in the range of the voltage of the video signal, and the amplification factors of the differential amplifier circuits are set so that an amplification factor of the curve fitting circuit in a range of the signal voltage lower than the breakpoint voltage is larger than the amplification factor of the curve fitting circuit in the range higher than the breakpoint voltage.
To achieve the above objects, according to a third aspect of the present invention, there is provided an illumination intensity correcting circuit for performing gamma-correction on three primary color signals compr
Arimizu Akira
Mitarai Shozo
Motonakano Hitoshi
Eisenzopf Reinhard J.
Kananen Ronald P.
Natnael Paulos
Rader Fishman & Grauer
Sony Corporation
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