BACKGROUND

The present invention relates generally to nonlinear light modulators and image projectors, and more particularly, to gamma correction circuits for use with nonlinear light modulators and image projectors that correct the grey scale linearity of images displayed thereby. Gamma correction circuits are used to correct the grey scale linearity of image projectors in which they are employed. If a gamma correction circuit is not used, the grey scale linearity of the image projector is relatively poor, causing video images to look "washed out" and have poor color rendition. In particular, the gray scale is nonlinear or unbalanced, and this translates into fewer shades of gray, which may be insufficient to provide a good quality image.

Previously implemented gamma correction circuits have been based on the use of diode circuits. The diode-based circuits are generally disadvantageous because of higher voltage swings are required to compensate for diode voltage drops. This reduces bandwidth and consumes more power. There are sharp breakpoints caused by the diode-based correction circuits that result in a somewhat discontinuous transfer functions. The diode-based correction circuits are not temperature compensated. The diode voltage drop changes with temperature causing a shift in the breakpoint. The diode-based correction circuits also have a lower bandwidth if the diodes are used in a feedback configuration. Such feedback configurations may fix the temperature drift

problem, but breakpoints are even sharper causing transfer functions to be discontinuous.

Previous implementations of gamma correction circuits have also employed digital techniques using A/D converters, a memory, and D/A converters. These circuits are relatively costly to build, especially if high speed and large bandwidth circuits are desired.

Consequently, it would be an improvement in the art to have an image light projector employing a gamma correction circuit that eliminates the above-mentioned problems associated with conventional diode-based and digital gamma correction circuits.

SUMMARY OF THE INVENTION

The present invention is an electronic circuit that implements a nonlinear system transfer function (gain) that provides for gamma correction of an image projectors, and nonlinear light modulators in particular. The nonlinearity of the gamma correction circuit is designed to compensate for the nonlinearity of the light modulator or image projector. One embodiment of the gamma correction circuit has been built for use with a liquid crystal light valve image projector. The nonlinearity of this specific gamma correction circuit compensates for the nonlinearity of a liquid crystal light valve and an image source (cathode ray tube, for example) employed in the image projector.

In general, the present gamma correction circuit provides a means for correcting the system transfer function of any system employing nonlinear control. Such systems may include sine wave shaping circuits, logarithmic amplifiers, liquid crystal based systems, analog sensor systems, such as a thermocouple-based transducer, for exam- pie, deflection circuits requiring linearity correction, and feedback circuits requiring lin¬ earity control. The present gamma correction circuit may be used with other nonlinear devices such as deformable membranes or piezoelectric light transducers, for example.

More specifically, the gamma correction circuit of the present invention com¬ prises a plurality of amplifiers that are coupled together to sum their respective output currents to provide a composite output current. Each of the plurality of amplifiers typi¬ cally comprises first and second emitter coupled pair transistors. Each of the plurality of amplifiers is adapted to implement a predetermined transfer function. A plurality of current sources are coupled to respective ones of the plurality of amplifiers to provide differing levels of current to the respective amplifiers. The plurality of current sources cooperate with the plurality of amplifiers to provide each predetermined transfer func¬ tion. Optionally, an output resistor is adapted to convert the composite output current into a corresponding output voltage of the circuit. The combination of amplifiers and

current sources is configured to compensate for nonlinearity of the image projector due to the combination of the cathode ray tube and the liquid crystal light valve.

The primary application for the present gamma correction circuit has been developed is in liquid crystal light valve-based image projectors, but it may also be adapted for other uses related to video image projection. When used in a liquid crystal light valve projector, the nonlinear transfer function of the present gamma correction circuit combines with the response functions of the liquid crystal light valve and a cathode ray tube used to input images to the liquid crystal light valve, and produces a system transfer function equivalent to that of the cathode ray tube alone. This is the desired result of using the gamma correction circuit of the present invention.

Specifically, the present invention produces a nonlinear gain function by summing the collector current derived from the plurality of interconnected emitter coupled pair amplifiers. The present invention corrects the grey scale linearity of the image projector in which it is employed. The amplifier configuration of the present gamma correction circuit does not require feedback, so its processing bandwidth remains high. The present gamma correction circuit also has "soft" breakpoints, creating a relatively smooth transfer function.

The gamma correction circuit of the present invention is useful in applications requiring a nonlinear amplifier. In particular, the present gamma correction circuit is adapted for use in a model 300 projector manufactured by the assignee of the present invention. The present gamma correction circuit may also be used in any projector or display using liquid crystal (or other nonlinear) imaging technology.

BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

Fig. 1 shows the primary display components of a liquid crystal light valve image projector in which the present invention is employed;

Fig. 2a shows a response curve of the light output versus voltage of the combination of the cathode ray tube and the liquid crystal light valve employed in the image projector of Fig. 1;

Fig. 2b shows a response curve of the light output versus voltage of the cathode ray tube of Fig. 1 ;

Fig. 2c shows a desired response curve of light output versus voltage provided by the gamma correction circuit of the present invention;

Fig. 3a shows a conventional diode resistor amplifier circuit used to create the transfer function shown in Fig. 2c;

Fig. 3b shows the response of the circuit of Fig. 3a;

Fig. 4a shows a second conventional diode resistor amplifier circuit used to create the transfer function shown in Fig. 2c;

Fig. 4b shows the response of the circuit of Fig. 4a;

Fig. 5a shows a conventional feedback loop amplifier circuit used to create the transfer function shown in Fig. 2c;

Fig. 5b shows the response of the circuit of Fig. 5a; Fig. 6a shows a conventional multi-stage amplifier circuit used to create the transfer function shown in Fig. 2c;

Fig. 6b shows the response of the circuit of Fig. 6a;

Fig. 7a shows a first gamma correction circuit in accordance with the principles of the present invention that may be employed in the projector of Fig. 1 ; Fig. 7b shows the transfer function of the gamma correction circuit of Fig. 7a;

Fig. 8a shows a second gamma correction circuit in accordance with the present invention that may be employed in the projector of Fig. 1;

Fig. 8b shows the transfer function of the gamma correction circuit of Fig. 8a;

Fig. 9a shows a third gamma correction circuit in accordance with the present invention that may be employed in the projector of Fig. 1 ;

Fig. 9b shows the transfer function of the gamma correction circuit of Fig. 9a;

Fig. 10 shows a fourth gamma correction circuit in accordance with the present invention that is adapted for use in a liquid crystal light valve image projector manufactured by the assignee of the present invention; Figs. 11a and lib show the output response of the gamma correction circuit of

Fig. 10;

Fig. 12 shows the output response of the Fig. 12 also shows the output response of the gamma correction circuit in terms of voltage;

Fig. 13 shows details of an embodiment of the gamma correction circuit of the present invention implemented in one image projector manufactured by the assignee of the present invention; and

Fig. 14 shows a generic embodiment of the present invention.

DETAILED DESCRIPTION Referring to the drawing figures, Fig. 1 shows the primary display components of a liquid crystal light valve image projector 10 in which a gamma correction circuit 20 in accordance with the principles of the present invention is employed. Although the

present invention will be described with reference to the liquid crystal light valve image projector 10, it is to be understood that it may be employed for use with any nonlinear light modulator or image projector, and is not limited to use with only liquid crystal light valve image projectors. The image projector 10 is generally comprised of a cathode ray tube (CRT) 11 that provides an image 12 that is to be displayed by the image projector 10. A relay lens 13 is adapted to focus the image 12 on an input surface of a liquid crystal light valve 14. A bias drive circuit 22 is adapted to drive the liquid crystal light valve 14 to control its output. An arc lamp 16 is focused by a lens 16a onto a polarizing beamsplitter 15 that transfers light from the arc lamp 16 onto an output surface of the liquid crystal light valve 14. This light is reflected from the output surface of the liquid crystal light valve 14 and back through the polarizing beamsplitter 15. The image 12 is "transferred" by the liquid crystal light valve 14 to the light provided by the arc lamp 16. A projection lens 17 then images the reflected light onto an image screen 18 which provides a replication of the image 12 as an image 19 that is viewable by an observer. In general, the design and operation of the liquid crystal light valve 14 and image projector 10 are well-known in the art.

The gamma correction circuit 20 of the present invention is adapted to receive an input drive signal that comprises an input video signal derived from a video tape recorder, for example. The gamma correction circuit 20 is coupled by way of an amplifier 21 to an input of the cathode ray tube 11. The amplifier 21 is adapted to pro¬ vide a gain stage for increasing the signal strength of the output of the gamma correc¬ tion circuit 20 prior to its application to the cathode ray tube 11. The gamma correction circuit 20 is adapted to generate a nonlinear transfer function using several emitter coupled pair amplifiers whose collectors are tied together. This will be described in detail below with reference to Fig. 10. By tying the collectors of the emitter coupled pair amplifiers together, the output of the gamma correction circuit 20 is the sum of currents from each amplifier stage, which forms a composite response. To understand the present invention more fully, background material is presented below. The response of the liquid crystal light valve 14 is not linear. It is a light amplification device that works by "programming" its input side with an image that modulates a variable mirror on the output side of the device. This mirror reflects varying amounts of projection light provided by the arc lamp 16 thus creating a light amplifier. The image on the input side of the liquid crystal light valve 14 is applied using the cathode ray tube 11. The transfer function of the cathode ray tube 11 and

liquid crystal light valve 14 combination between the input and output is shown in Fig. 2a. As is seen in Fig. 2a, the transfer function is very nonlinear.

An image projector 10 built using a cathode ray tube 11 and liquid crystal light valve 14 combination needs to have its transfer function corrected. Video signals for broadcast, or from any conventional source, are normally pre-corrected for the response of the cathode ray tube 11. The transfer function for cathode ray tube 11 is shown in Fig. 2b. The transfer function is approximately equal to VIN to the power of 2.5 (VIN**2.5). For proper grey scale imagery, the image projector 10 should have the transfer function shown Fig. 2b. To make an image projector 10 having proper display characteristics, and when employing the cathode ray tube 11 and liquid crystal light valve 14 combination, gamma correction is used. Referring again to Fig. 1, a video input signal is corrected using a nonlinear amplifier implemented in the present gamma correction (compensation) circuit 20, such that when the video input signal is applied to the cathode ray tube 11 and liquid crystal light valve 14 combination, the transfer func- tion of the entire system is correct, and has the shape shown in Fig. 2b. Fig. 2c shows the correction function that is provided by the present gamma correction circuit 20, and that is necessary to produce the system transfer function of Fig. 2b.

There are several conventional ways to create a gamma correction circuit that produces the system transfer function shown in Fig. 2c, but the system transfer functions are produced with varying degrees of success. Fig. 3a is a simple diode resistor example, which has the nonlinear response shown in Fig. 3b. The circuit shown in Fig. 4a is another example, and its response curve is shown in Fig. 4b. The problem with these two circuits is that the breakpoint shifts with temperature because of the diodes used therein. Also, if breakpoints need to occur at a low VJN, then a high maximum VJ is needed. The lowest available diode drop is about 0.3 volts using a germanium or Schottky diode. If the first breakpoint is at 10% VIN, then the maximum signal VIN is about 3 volts. This is difficult to achieve for fast amplifiers. To keep the signal path fast, low impedance devices must be used and this also requires a good deal of power at a 3 volt operating voltage. Another conventional approach is to put the diodes in a feedback loop of an operational amplifier. Fig. 5a shows one conventional circuit that achieves this, and this circuit has a transfer function shown in Fig. 5b. The output of this circuit is constant until a threshold is reached after which it has linear gain. The breakpoint is extremely sharp which is not good for image light amplifier projector applications. This type of circuit has a low bandwidth because it uses negative feedback.

The desired transfer function may be achieved using several amplifier stages. Fig. 6a shows a three operational amplifier circuit that accomplishes this. The first two

operational amplifiers use diodes in their feedback to create transfer functions similar to that shown in Fig. 5 b. These transfer functions are summed together along with the input voltage in different proportions to get the composite transfer function shown in Fig. 6b. The breakpoints and gain of each stage are programmable to obtain the desired shape. Using feedback eliminates the need for large voltage swings and removes the temperature dependence of the diode voltage characteristic. However, the main drawbacks with this circuit are its sharp breakpoints and low bandwidth.

To overcome the problems associated with the above-described circuits, the gamma correction circuit 20 of the present invention is comprised of a plurality of inter- connected emitter coupled pair amplifiers 30. The simplest form of one such amplifier 30 is shown in Fig. 7a, which shows an emitter coupled pair amplifier 30 that acts as a switch. The emitter coupled pair amplifier 30 is comprised of first and second transis¬ tors 31, 32 that have their emitters coupled together. Current (IEE) either flows in one collector 34 or the other collector 35 of the respective transistors 31, 32 of the emitter coupled pair amplifier 30. However, there is a small region of linear operation where both collectors 34, 35 share the current If VΓN is substantially equal to VREF, then the collectors 34, 35 share the current equally (assuming matched transistors 31, 32). An output signal may be taken from either collector 34, 35, or from both collectors 34, 35 to provide signals having opposite polarity. The key is that the collectors 34, 35 are not tied together. A collector resistor 33 (Re) generates a voltage across it proportional to the collector current. The transfer function of this circuit 20 is shown in Fig. 7b. The linear range is approximately from VREF - 0.1 volts to VREF + 0.1 volts. The end- points are somewhat smoothed and are not very sharp. This is a beneficial characteris¬ tic of the present gamma correction circuit 20. Once VJN is outside the linear range, the output current, or voltage across the collector resistor 33 (Re), is constant.

Fig. 8a shows a modified gamma correction circuit 20a, similar to the gamma correction circuit 20 of Fig. 7a, that provides greater linear range. By using resistors 36, 37 coupled to the emitters 34, 35, the range is extended. The transfer function of the gamma correction circuit 20a of Fig. 9a is shown in Fig. 8b. The linear range is from VREF minus (IEE)(RE) minus 0.1 volt to VREF plus (IEEXRE) plus 0.1 volt. Outside of the linear range, the output voltage is constant.

Another modification of the gamma correction circuit 20 is shown in Fig. 9a. This gamma correction circuit 20b uses a resistor 38 for the current source IEE- If a negative voltage supply is infinite in voltage, then the current source provided by the resistor 38 is constant. By using a low supply voltage, this current does not stay con¬ stant once it is outside of the linear range, but this occurs only in the collector 34, 35 on the selected input side of the circuit 20b. Fig. 9b shows that outside the linear range IQI

is constant, but I02 s not. When VI is low, I ₀ 2 is constant, but when VIN is high, I ₀ 2 equals IEE, and IEE changes. The right side of the circuit 20b essentially becomes an emitter follower amplifier where the emitter resistance is RE plus REE- This may be used to smooth out the transition region of the transfer function even further. Fig. 10 shows an embodiment of a gamma correction circuit 20c that is used in a Model 300 image projector manufactured by the assignee of the present invention. This gamma correction circuit 20c uses three amplifiers 30a, 30b, 30c to generate outputs illustrated by the transfer function shown in Fig. 10. Since the three amplifiers 30a, 30b, 30c provide output currents with voltage compliance, they are connected together to sum the output currents. This sum is a composite output current which is converted to a voltage across the collector resistor 33 (Re). Each amplifier 30a, 30b, 30c may be programmed to have its own gain, its own linear range, and its own reference voltage produced by resistors 38a, 38b, 38c. This gamma correction circuit 20c may be made to generate the desired transfer function as shown in Fig. 2c. It may also be modified to generate many different nonlinear transfer functions by selectively summing currents taken across either the left collector of a selected amplifier 30a, 30b, 30c. The transfer function of the left side of a selected amplifier 30a, 30b, 30c has a polarity opposite to that of the right side of the selected amplifier 30a, 30b, 30c, which may be used to "cancel" out part of the overall gain. For example, if both collectors of a selected amplifier 30a, 30b, 30c were tied together, the output thereof would be constant current and the contributions of the two sides of the selected amplifier 30a, 30b, 30c would cancel out. The left hand side of one the amplifier 30a, 30b, 30c may be used to constructively cancel out part of the gain produced by the transfer function of the right hand side of a different amplifier 30a, 30b, 30c. Thus, virtually any transfer function may be produced using many amplifiers 30 in accordance with the principles of the present invention, and the present invention is not limited to the three amplifiers 30a, 30b, 30c described above. This is also the heart of the present invention.

The gamma correction circuit 20c of Fig. 10 has an output response shown in Figs. 11a and lib. The response of the individual amplifier 30a, 30b, 30c is shown in Fig. 11a and their composite sum in Fig. 1 lb. The ouφut voltage across the collector resistor 33 (Re) then has the exact shape and polarity as shown in Fig. 2c. Fig. 12 also shows the output response of the gamma correction circuit 20 in terms of voltage, which corresponds to the desired function shown in Fig. 2c. The formulas used to calculate the current values are given by IEE = [(gain)(2)(V _H - V _R )] / R _C) and R _E = (V _H - V _R - 0.1) / I _EE .

Once the desired output shape of the transfer function is determined, a number of line segments may be created to linearly approximate it. This determines the number

of breakpoints and the number of amplifiers 30. The gain for each line section, and the range for each amplifier 30 is then determined. The gain is the parameter for the slope of the desired system transfer function., corresponding to VQAVI . Once the straight line approximation is determined, the gain is set, as are VH and VR. Then the equations above are applied to obtain the desired values. The above equations use VH for the high end of the linear region and VR for VREF-

It is to be understood that the amplifiers described above are generally well known in the art and may be readily constructed to implement the various gamma correction circuits 20 in accordance with the teachings of the present invention. Fig. 12 shows the ouφut response of the graph of Fig. 10 in terms of voltage.

The benefits of the gamma correction circuit 20 are high bandwidth capabilities, smooth breakpoints, temperature compensation, low voltage breakpoint capability, and its programmability for many different transfer function shapes. The high bandwidth results from the fact the each differential amplifier 30 has a fixed gain and does not need feedback to set it, and hence comprises an open loop amplifier. Using two transistors 31, 32 in each amplifier is also faster than using operational amplifiers. The present invention is also very well-suited for integrated circuit fabrication.

The gamma correction circuit 20c of Fig. 11a was simulated using a well- known SPICE simulation program. The transfer function derived from the simulation was nearly perfect with very smooth breakpoints. A gamma correction circuit 20d developed using the SPICE simulation program was constructed and test data was measured. This gamma correction circuit 20d is shown in Fig. 13, which also shows resistor and capacitor values for the components along with the model of the transistors used therein. The test results using the constructed gamma correction circuit 20d were virtually identical to the ouφut of the SPICE simulation. The gamma correction circuit 20d shown in Fig. 13 is adapted for use in a Model 300 image projector manufactured by the assignee of the present invention.

Fig. 14 shows a generic embodiment of the gamma correction circuit 20 of the present invention. The gamma correction circuit 20 shown in Fig. 14 includes a plural- ity of amplifier stages 30a-30n, and ouφuts from selected left and right side transistors thereof are coupled to provide an appropriate summed current ouφut across an optional ouφut resistor 33. The ouφuts of the unused sides of the respective amplifier stages 30a-30n are shown with an arrow. In general, the generic gamma correction circuit 20 comprises two transistors, two resistors and a current source, as has been described above with respect to other drawing figures.

Thus there has been described new and improved gamma correction circuits for use with nonlinear light modulators and image projectors that correct the grey scale

linearity of images displayed thereby. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.

For example, the present gamma correction circuit may be adapted to correct the system transfer function of any system employing nonlinear control. Such systems include sine wave shaping circuits, logarithmic amplifiers, liquid crystal based systems, analog sensor systems, such as a thermocouple-based transducer, deflection circuits requiring linearity correction, feedback circuits requiring linearity control, deformable membranes, or piezoelectric light transducers, for example.