Liquid Crystal Display (LCD) devices and other display devices use a variety of techniques to generate voltages that correspond in some fashion to a gamma curve, which is a non-linear curve that maps pixel luminance values, such as pixel grey-level values, to drive voltage values.
A gamma curve voltage generator circuit comprises a first linear resistor string and a second linear resistor string. The first linear resistor string comprises resistors of a first resistor value and corresponds to a first portion of a gamma curve. A first end of the first linear resistor string is ohmically coupled to a first end of the second linear resistor string. The second linear resistor string comprises resistors of a second resistor value and corresponds to a second portion of the gamma curve. The first resistor value is different from the second resistor value.
The drawings referred to in this Brief Description of Drawings should not be understood as being drawn to scale unless specifically noted. The accompanying drawings, which are incorporated in and form a part of the Description of Embodiments, illustrate various embodiments of the present invention and, together with the Description of Embodiments, serve to explain principles discussed below, where like designations denote like elements, and:
The following Description of Embodiments is merely provided by way of example and not of limitation. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
Herein, various embodiments are described that display devices, display device drivers, and methods that facilitate improved, usability. Discussion begins with description of some example gamma curves for a variety different display panels (e.g., Liquid Crystal Display panels). An example display device, which includes a display such as an LCD panel is then described. The display device includes a gamma curve voltage generator circuit, which is then described in greater detail. The gamma curve voltage generator circuit is configured to generate gamma curve voltages for a variety of display panels. A particular gamma curve based on the adjustment values for the particular display panel. Operation of a gamma curve voltage generator circuit is described in further detail in conjunction with description of a method of gamma curve voltage generation.
In the embodiment illustrated in
In one embodiment, the gamma curve for which gamma curve voltages are generated may be selected from a set of gamma curves, for example gamma curve 110 may be selected from a plurality of gamma curves for a single display panel 210 (e.g., a red gamma curve for display panel 210, a blue gamma curve for display panel 210, a green gamma curve for display panel 210, etc.) and/or from a plurality of gamma curves for different displays (e.g., a red gamma curve for a display made by manufacturer A, a red gamma curve for a display made by manufacturer B, and a red gamma curve for a display made by manufacturer C, etc.). The selection may be based on the desired sub-pixel display color and/or the manufacturer. In other embodiments, a single gamma curve may be used. In further embodiments, the gamma curve may be hardwired within the circuitry and/or firmware of display device 200A.
Gamma curve voltage selector 290 is configured to select first gamma curve voltage from a set of gamma curve voltages 280 that comprise the first plurality of gamma curve voltages, the second plurality of gamma curve voltages, and additional pluralities of gamma curve voltages when more than two resistive modules 275 are utilized. Gamma curve voltage selector 290 is further configured to couple the first gamma curve voltage with a respective pixel of pixel array 220 in display panel 210. The first and second pluralities of gamma curve voltages correspond to first and second subsets of a set of grey-level values. In one embodiment, the set of grey-level values may comprise 256 values. In other embodiments, different amounts of values may be used. In various embodiments, the grey-level values may be based on a grey-level code. For example, the 256 grey-level values may be based on an 8-bit grey-level code values. In other embodiments, other numbers of code values may be used.
In one embodiment, gamma curve voltage generator circuit 270, and corresponding resistive modules 275 generate a different set of reference gamma curve voltages for each different gamma curve. In one embodiment, each sub-pixel color may have a corresponding gamma curve; for example, in one embodiment, a red gamma curve corresponding to red sub-pixels, a green gamma curve corresponding to green sub-pixels, and a blue gamma curve corresponding to blue sub-pixels. In another embodiment, a red gamma curve corresponding to red sub-pixels, a green gamma curve corresponding to green sub-pixels, a blue gamma curve corresponding to blue sub-pixels and a white gamma curve corresponding to white sub-pixels. In other embodiments, different display device manufacturers may have corresponding gamma curves. In yet further embodiments, each display device manufacture may have a gamma curve corresponding to each sub-pixel color. The gamma curves may be stored within a storage device, and may be selected based on the display device manufacturer and/or sub-pixel color to be displayed. In one embodiment, gamma curve voltage generator circuit 270 selects the gamma curve. In other embodiments, the gamma curve is selected externally from gamma curve voltage generator circuit 270 and communicated to gamma curve voltage generator circuit 270. External selection can take place at various times and locations. For example, in one embodiment external selection of a gamma curve occurs as a part of manufacture of a display device 200. In another embodiment, gamma curve selection can occur just prior to generating gamma curve voltages during operation of display device 200A.
In various embodiments, gamma curve voltage selector 290 is configured to select a gamma curve voltage 280 corresponding to the sub-pixel color to be displayed by display device 200A. In one example embodiment, where there are 256 grey-level values, a gamma curve voltage selector 290 connects exactly one of these voltages to an associated pixel, according to the 8-bit value for that pixel's red, green or blue sub-pixel. Note that a given gamma curve voltage 280 output from gamma curve voltage generator circuit 270 may be connected to none of the pixels or to any number of the pixels. This depends on the sub-pixel data
In
In one embodiment, for a given pixel, the associated 256:1 gamma curve voltage selector 290 (290-1, 290-2, 290-3, . . . 290-n) connects exactly one of these voltages to an associated buffer amplifier 291, according to the 8-bit value for that pixel's red, green or blue sub-pixel. Note that a given reference gamma curve voltage, of gamma curve voltages 280, that is output from gamma curve voltage generator circuit 270 may be connected to none of the buffer amplifiers 291 or to any number of the buffer amplifiers 291. This depends on the sub-pixel data. In one embodiment, each gamma curve voltage selector 290 couples the selected, voltage from gamma curve voltages 280 to a buffer amplifier 291 (291-1, 291-2, 291-3, . . . 291-n) and the buffer amplifier 291 drives a buffered replica of this selected gamma curve voltage onto the corresponding pixel. In embodiments where there are three sub-pixel colors, the gamma curve voltage is connected through a 1:3 selector 292 (292-1, 292-2, 292-3, . . . 292-n) to the appropriate red, green or blue sub-pixel column via sub-pixel column lines. In other embodiments, the size of a selector 292 corresponds to the available colors of the sub-pixels. In embodiments where there are more sub-pixel colors, a selector 292 may be larger and in embodiments where there are less sub-pixel colors, a selector 292 may be smaller. The sub-pixel (in the row currently-selected by row select logic 225) and the parasitic capacitance of the sub-pixel column are charged to this voltage. In various embodiments, this process occurs for each color of the sub-pixels. In one embodiment, gamma curve voltage selector 290 comprises a voltage selector corresponding to each column line of display device 200A. In other embodiments, each gamma curve voltage selector 290 corresponds to more than one column of the display device.
In the embodiment depicted in
As is illustrated by the gamma curves of
With reference again to
Tap adjustments 320 may be utilized to program a selected tap point at the input of a resistive module, at the output of a resistive module, or at some combination of both the input and output. Programmable tap points located on the output of, for example, a resistive module map a specific voltage generated from the resistive module to a corresponding grey level code value (i.e., position on the gamma curve).
In some embodiments, the tap point associated with one or more voltages supplied by voltage sources V0, V1, V16, V250, and V255 may also be programmable. In gamma curve voltage generator circuit 270, the driven tap points of one or more resistive modules may be varied both in voltage and position in order to generate a desired gamma curve. Further, due to the combination of programmable voltages and programmable tap points, matching to multiple gamma curves is possible with reduced amount of ripple in the error between the obtained gamma curve and the desired gamma curve (as compared to an approach with adjustable voltages and only fixed tap points). In a display panel 210 the reduction in ripple reduces contouring in smooth image regions of images displayed on the display panel.
With reference to
The resistors of a first value in resistive module 275-1 and the resistors of a second value in resistive module 275-2 have differing resistance values from one another. For example, in sonic embodiments the resistors of a second value each have a resistance value of 1R (where R is a fixed positive value in ohms) and the resistors of a first value each have a greater resistance than 1R. For example, in some embodiments, the resistors of a first value may each have a resistance value that is a multiple of 1R, such as 2R, 3R, 4R (as illustrated), or the like. The resistors of a second value and the resistors of a third value (resistive module 275-3) may also have differing resistance values from one another. For example, in some embodiments when the resistors of a second value each have a resistance value of 1R, the resistors of a third value may each have a resistance value that is a multiple of 1R, such as 2R (as illustrated), 3R, 4R, or the like. Whole or fractional number multiples are possible. In various embodiments, depending upon the nature of the gamma curve being replicated, the resistors of a third value may be of the same or different resistance value than the resistors of a first value. In general, larger resistance values are used where (1) the gamma curve being produced is steep such that there is a large voltage change across each resistor and (2) where the total resistance from the output nodes to the nearest two driven taps (i.e., the Thévenin equivalent) is desired to remain at an acceptably low value. By using larger resistors where the voltage per resistor is high, power dissipation is minimized, as will be describe further herein.
With respect to the illustrated resistive modules 275-1, 275-2, and 275-3, the respective 4R, 1R and 2R per step sections, create a piecewise-linear gamma curve from which voltages may be selected when circuit 270 is active. For example, with reference to
Most LCD gamma curves have a large voltage difference between V0 and V1. This is illustrated in the vicinity of staring point 101 in example gamma curves 110, 120, and 130 of
For similar reasons, resistive modules 275-1, 275-2, and/or 275-3 are used. As described above, within a given resistive module, the individual resistor elements (such as individual resistors in a resistor string) all have the same value, but this value is may be different for each of the resistive modules. In one embodiment, resistive module 275-1, between V1 and V16, have large resistance values because desired gamma curves are steep in this region. Thus, the voltage across each resistor is relatively large. By using larger resistor sizes (4× those in the center of the gamma curve in this example), the power dissipation is reduced. In the middle of the gamma curve, located between V16 and V250, is resistive module 275-2. The voltage across each resistor in resistive module 275-2 is smaller than that of resistive module 275-1, thus resistors of small resistance value may be used without drastically increasing the power dissipation. It is desirable to use smaller resistors in this portion of the gamma curve because the driven taps are, in general, further apart. By using smaller resistor values, the Thévenin resistance of each output voltage node is reduced. In one embodiment, the resistance values of the individual resistors in resistive module 275-3, between V250 and V255, are somewhat larger than those of resistive module 275-2 because the gamma curve is somewhat steep, but not as steep as between V1 and V16.
Because both the tap voltages and tap points of the driven taps within each resistor string are adjustable by the improved method of this invention, it is possible to get very good matching between the resulting piecewise-linear curve and a desired smooth gamma curve.
Utilizing the techniques described herein with respect to gamma curve voltage generator circuit 270, good matching between a desired and generated gamma curve can be obtained even if the programmable tap points are optionally limited to just odd numbered taps or even-numbered taps (as depicted in
The following is a calculation of the power that would be dissipated in a resistor string for an actual gamma curve in a display driver using the new techniques described herein and when not using these techniques (i.e., in a conventional manner). The power savings calculations are based upon a gamma curve produces by driving programmable voltage sources at tap points 0, 1, 6, 8, 16, 38, 108, 180, 226, 250, 254 and 255. As in
Using the new techniques described herein, with R=220 ohms, the resistance per step between taps 1 and 16 is 880 Ohms (4R), between taps 16 and 250 the resistance per step is 220 ohms (1R), and between taps 250 and 255 the resistance per step is 440 ohms (2R). Tap 0 is driven directly and is not connected to the resistive modules. This results in a calculated power of 437 microwatts.
When not using the new techniques described herein, and resistance values are equal to 220 ohms, the power increases to 1184 microwatts.
To adequately drive the required load, the maximum Thévenin output impedance must be kept small. In both cases, the maximum Thévenin output impedance of any tap occurring in the middle of the “1R” resistive module 275-2 is 3960 ohms (which is satisfactory). However, utilizing the new techniques described herein reduces the power dissipation by a factor of 2.7 (1184/437=2.7).
In order to reduce the power without the new techniques described herein, the value of R must be increased, but this also increases the maximum Thévenin output impedance by a factor of 2.7 to a value of (1184/437)×3960=10,729 ohms (which is not desirable or satisfactory). Thus, the new techniques described herein allow the power to be greatly reduced without increasing the maximum Thévenin output impedance of a gamma curve voltage generator circuit.
At 410 of flow diagram 400, in one embodiment, a first subset of a plurality of voltages is driven onto a first linear resistor string. In one embodiment, the first linear resistor string comprises resistors of a first resistor value and corresponds to a first portion of a selected gamma curve. With reference to
At 420 of flow diagram 400, in one embodiment, a second subset of a plurality of voltages is driven onto a second linear resistor string. The first linear resistor string is ohmically coupled to a first end of the second linear resistor string. The second linear resistor string comprises resistors of a second resistor value which correspond to a second portion of a selected gamma curve, and the first resistor value is different from the second resistor value. With reference to
At 430 of flow diagram 400, in one embodiment, gamma curve voltages are output from both of the first linear resistor string and the second linear resistor string. The gamma curve voltages correspond to grey level code value mappings of the selected gamma curve which has been generated. For example, gamma curve voltages 280 that are output from resistive modules 275-1 and 275-2 correspond to grey level code value mappings of the bottom and middle portions of the selected gamma curve which has been generated.
At 440 of flow diagram 400, in one embodiment, a third subset of the plurality of voltages is driven onto a third linear resistor string. The third linear resistor string is ohmically coupled to a second end of the second linear resistor string, and the third linear resistor string comprises resistors of a third resistor value that correspond to a third portion of the selected gamma curve. The third resistor value is different from the second resistor value. With reference to
At 450 of flow diagram 400, in one embodiment, additional gamma curve voltages are output from the third linear resistor string. The additional gamma curve voltages correspond to grey levels mappings to the selected gamma curve. For example, gamma curve voltages 280 that are output from resistive module 275-3 correspond to grey level code value mappings of the upper portion of the selected gamma curve which has been generated.
The embodiments and examples set forth herein were presented in order to best explain the present invention and its particular application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed.