Television – Image signal processing circuitry specific to television – Gray scale transformation
Patent
1993-09-03
1995-10-24
Groody, James J.
Television
Image signal processing circuitry specific to television
Gray scale transformation
348744, H04N 5202
Patent
active
054614309
ABSTRACT:
Apparatus that implements a nonlinear transfer function (gain) that provides for gamma correction of nonlinear image projectors. The nonlinearity of the transfer function is designed to compensate for the nonlinearity of a nonlinear light modulator, such as a liquid crystal light valve and cathode ray tube combination, for example, that is used in the image projector. The gamma correction circuit comprises a plurality of amplifiers that include current sources coupled together to sum their output currents, and each amplifier is adapted to implement a predetermined transfer function, and provide differing levels of current. An optional output resistor may be employed to convert the composite output current into a corresponding output voltage. Each of the plurality of amplifiers typically comprises first and second emitter coupled pair transistors plus their associated current sources. The present invention corrects the grey scale linearity of the image projector in which it is employed. The amplifier configuration of the circuit does not use feedback, so its processing bandwidth remains high. The present invention also produces "soft" breakpoints, creating a relatively smooth transfer function. The gamma correction circuit of the present invention is useful in any application using a nonlinear amplifier. The present gamma correction circuit may also be used in any image projector or display using liquid crystal or other nonlinear imaging technology. If gamma correction is used in conjunction with dynamic threshold correction then the gamma correction is made dynamic by adding the threshold correction signal to the base of one of the transistors in the gamma correction circuitry.
REFERENCES:
patent: 3708693 (1973-01-01), Ferrier et al.
patent: 4847524 (1989-07-01), Van Rooy et al.
patent: 4868668 (1989-09-01), Tavernetti
Grant Chris
Groody James J.
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