Tuesday, November 23, 2010

Gamma Correction: what to know, and why it’s as important as ever

Mike Ogier, Intersil Corporation

10/31/2010 5:33 PM EDT

Rapid growth in the active matrix, thin-film transistor liquid-crystal display (TFT-LCD) market has redefined how many people live, work, and recreate. From High Definition (HD) televisions, to desktop and laptop computers, to smart phones, and automotive infotainment, the TFT-LCD in some form or another has become rather ubiquitous.

Although "gamma" itself is not generally used by TV makers as a selling feature, the term has been around since the days of cathode ray tube (CRT) TVs and is still an important behind-the-scenes characteristic of current TFT-LCDs.

When talking about modern LCD TVs, gamma has taken a slightly new definition. Every LCD TV maker must at some point in the development process pay attention to gamma. If not, then they could have the absolute best panel technology in the world, but no one would buy their panel because it would recreate images incorrectly. Where, “incorrectly” means that the color representation and luminance intensity would be far from what it should be to accurately reproduce/recreate the intended source image.

So what is this mysterious so called “gamma” in TVs? It’s not same thing as high energy “gamma radiation” many may associate with radioactive decay. First, we'll have a brief discussion about what gamma means in a CRT TV system. Then we'll have a discussion about how that translates to TFT-LCD TV systems, and why the average consumer should or would care about it.

Gamma and CRT TVs

CRT TVs operate by using an electron beam to excite phosphors on the screen. An image can be “painted” on the screen by scanning the electron beam accordingly, and exciting these phosphors. The relationship between the control voltage applied to the electron gun and the resulting light intensity on the screen is inherently non-linear. It can be approximated in first order by a power law equation (in the form of Y(x)=x a), and this is known as the gamma response of the CRT.

The human eye naturally has a non-linear sensitivity to luminance intensity (perceived brightness), which is referred to as “lightness”. The eye is most sensitive to changes at lower gray scale (darker) levels. It just so happens that the natural response of the eye is very close to being the inverse of the inherent response of the CRT. This is an unintended, but extremely useful, side effect, which allows for a single correction to be performed on source data to compensate for system nonlinearity and to create the perception of uniform luminance changes to the eye.

Source data must be encoded in a way that accounts for the CRT response and the known lightness response of the eye. Gamma correction is performed by the camera on the red, green, and blue color components (RGB) of a video signal. Simply, video needs to be encoded in such a way that the camera system reacts to luminance changes similarly to how the human eye does. Since the CRT has an inverse response to this, then the resulting luminance intensity will be perceptually linear. Gamma correction also has benefits such as reducing video signal noise and increasing effective resolution at low levels (both of which relate to creating uniformity).

In the power law equation, for a given system, the light intensity is (simply) equal to the applied electron gun voltage raised to some power. This power is the gamma coefficient (γ), and the formula approximately defines the overall transfer function of the CRT. In general, a typical CRT gamma would be about 2.2 to 2.5. Higher-number gamma coefficients tend to yield more image contrast but increase the amount of darkened area (due to an increase in resolution at low levels). Lower-number gamma coefficients tend to wash out or flatten the image (due to a decrease in resolution at low levels).

A system with a gamma response of 1.0 is considered to be linear and is not good for various reasons. The most critical is that it doesn’t reproduce gamma-corrected images with perceptually correct color and contrast.

In summary, gamma correction is needed compensate for the system gamma response and ensure that (at least approximately) the intensity of what goes into the camera is the same as what comes out of the CRT TV, thus yielding correct image representation to the eye, Figure 1.

Figure 1: Basic gamma correction and gamma response curves

Gamma and TFT-LCD TVs

The TFT-LCD has very little in common with a CRT, except that fact that they both can display video images. Most notably, an LCD does not have an electron gun and phosphors to generate intensity. Instead it uses voltage-controlled pixels to control the transmittance of light through the pixel. The light in sourced from a backlight, which is generally a cold cathode fluorescent lamp (CCFL) or light emitting diode (LED) array. In short, there is a voltage-transmission (V/T) curve that defines the panel’s response to an input and thus its luminance intensity.

Though not the same as a CRT, the TFT-LCD does have “gamma,” however, the term must be slightly redefined. In order for an LCD to illuminate a pixel, the liquid crystal (LC) pixel cell needs to have a voltage applied across it, thus allowing it to transmit light which is then perceived as brightness on the screen (note that the “light source” is the LCD panel’s backlight, which is generally CCFL or LED).

The analog voltage applied to a pixel is determined by a digital/analog converter, where the digital code is generated based on the incoming source image/video data (when talking about TV video content, historically it is gamma encoded from the days of CRT TV). The relationship between applied voltage and transmittance (the V/T transfer function) is the gamma response of the TFT-LCD and is inherently non-linear due to the nature of the transmittance capability of LC cell. Although, this response is not necessarily the desired non-linear response like we saw in the CRT TV.

Thus, LCD TV gamma is often desired to closely simulate the gamma response of a CRT. This is for a few main reasons: first is historical, since all legacy video has been gamma corrected for CRTs, and second is to take advantage of the natural response of the human eye (lightness). Of course, not all LCDs have the same inherent gamma response, which can vary by panel technology and manufacturer. A panel’s gamma response may be altered by the manufacturer, whom may choose to force a particular gamma response based on desired visual performance of the end system, Figure 2.

Figure 2: Various system gamma response curves

Digital video data (often in the form of LVDS) must be converted using a digital-to-analog converter (DAC) to generate an analog voltage for the pixel. Gamma is roughly corrected (intentionally made non-linear) by the use of piecewise non-linear DACs in the panel’s source/column drivers. The source driver DACs determine how many different voltage steps can be applied to the pixels (e.g. an 8-bit DAC yields 28 or 256 steps of possible grayscale). The perception of changes in gray scale intensity caused by each voltage step is relative to the panel’s gamma response (V/T curve) and the response of the eye, Figure 3.

If luminance changes were simply allowed to be linear with code changes then a system would require many more bits of resolution (e.g. 12-bits or 14-bits instead of 8-bits) to achieve the sensitivity at low levels needed for the eye to not perceive changes from code to code differently (to keep changes uniform) across the entire range. This is, in fact, another advantage of using gamma correction. It allows a lower number of bits to be used to encode the video data in a form that “compresses” low level luminance data (most noticeable to the eye) while not affecting high level luminance data where small changes are not as distinguishable. This can also be viewed as a way to reduce noise in the video signal, such that potential small errors in the digital code are not as recognizable.

Figure 3: Changes in relative intensity compared to gamma response

To change the panel’s gamma response to a desired V/T transfer function, the panel’s source (column) driver DACs may use various reference voltages applied at multiple tap points. These voltages force the DAC(s) to have a certain desired non-linear operation. The reference voltages are often supplied by “gamma buffer” integrated circuits (ICs), which are generally buffer amplifiers that drive the analog voltages to the DAC taps. The gamma buffer ICs can be static or programmable.

Some such IC devices are Intersil’s EL5411/20T operational amplifiers, EL5421T buffer, and EL5x26 I2C programmable gamma buffers family. These devices are designed to drive accurate and steady DC reference voltages to a TFT-LCD source driver. For a simplified system block diagram of a TFT-LCD panel, Figure 4.

Figure 4: Simplified TFT-LCD block diagram

With the capability to control the DAC taps, LCD TV makers can fine-tune the voltages to further adjust/calibrate the panel’s non-linear gamma response, this is called gamma calibration. For example, this allows a TV maker to ensure that all their TV’s panels of a certain model exhibit the same gamma response from panel to panel. This means that any potential visual performance variations caused by things such as variances in LCD fabrication and manufacturing can be minimized, and thus the TV maker has a more consistent product. The consumer can then be assured that the TV they purchase and take home will visually perform like the one they saw on the sales floor at the local dealer.

[Often, TVs "on display" in a store will have a special “store” mode activated. This mode is intended to make the display have maximum contrast and extremely saturated colors to attract customers to the vividness of colors and brightness; however the consumer in a home environment would not generally use this setting.]

TV makers ultimately decide how they want to calibrate the gamma response (e.g. γ = 2.2, γ = 2.0, γ = 1.8, or even a combination of gammas such that the gamma changes based on the intended brightness), so that their display has a certain look. It’s noteworthy that gamma characteristics tend to shift with wide viewing angles and different ambient lighting conditions. So, when shopping for a new TV it is always a good idea to compare different TVs under similar conditions for a more apples-to-apples comparison.

Figure 5 compares the same image but with different overall gamma. The differences can be easily discerned by the viewer. The middle image is the nominal (original) gamma (e.g. γ = 2.2) while the top image is less than nominal and the lower is more than nominal. The top looses contrast as the darks disappear, and becomes more “washed out.” The lower image gets more contrast but overall has more darkened area.

Figure 5: Image comparison: shift in overall gamma: top is low gamma; middle is nominal gamma; bottom is high gamma (click on individual images to enlarge each).

Gamma and TV Settings

Historically, CRT TVs (and as much as these names don’t make sense) have a “brightness” control which changes the black level, and a “contrast” or “picture” control that changes the maximum white level. The gamma of the TV defines the response inside these end points. Ideally, these TV settings would be totally independent of gamma, however, once the maximum white level is fixed any changes to the black level effectively moves the gamma curve in the opposite directions (its shape changes).

This means the gamma coefficient has changed and now the system’s gamma response has been affected by a simple change to the black level. In the case of LCDs the end point levels are generally fixed by the source driver but digital adjustments can be made to the signal. Also, LCDs have the ability to have the intensity of the backlight changed, which adds another dimension to the performance of the system.

Some TFT-LCD and digital TVs offer other settings, which can be adjusted via a user control menu. These settings can be things like a “gamma” adjustment (generally not quantified, rather a +/- gamma adjustment), dynamic enhancements for contrast or blacks, different picture/video modes, and other manufacturer named features. These settings can alter some or all of the TV settings including the gamma curve. For example, a gamma that may have been defined by a single coefficient (e.g. γ = 2.2) in normal operation changes into a curve that has properties of multiple gamma coefficients, thus the TV response changes over the range of brightness (codes or voltages of the video signal).

It is also worth noting that ambient lighting from the environment or room can affect the perceived response of a video system. Unfortunately you will never know the exact conditions that were used when video editing was done to your favorite DVD movie or TV show and what the setting on their equipment was, thus it’s hard to set up viewing in your home to yield accurate reproduction. Likely your living room conditions will be very different from a studio and will change constantly.

For example, a dark room can reduce the perceived black level since you now reference it to the pitch-black ambient light (the blacks appear not as dark). Conversely, if you turn the lights on brightly then the ambient light can overpower the luminance of the panel, thus the perception is that the image looses contrast due to the loss of detail at the lower end (dark levels).

Overall, gamma may not be something simple to define, but as a consumer of hi-tech video products, it’s best to try and understand the concept with a system-level or top-down approach. It’s definitely a good thing that when you buy a new LCD TV, you are not required to calibrate the gamma. This would be quite frustrating and impossible for most. The TV manufacturer does the initial tough work by setting the transfer function of the source driver DACs such that the panel has their desired non-linear gamma response.

However, most TVs have a range of user-accessed adjustments, which control--to some extent--the response of the system, although many argue that more gamma-specific user adjustments controls would be valuable. Ultimately, the consumer doesn’t have much direct control of a TV’s gamma, but this tends to be perfectly fine for the average TV viewer.

About the author

Mike Ogier is a senior applications engineer for the Analog and Mixed-Signal Group at Intersil Corporation. He currently supports devices for LCD and video applications and has experience in test also. He earned a BSEE from San Jose State University.

Origianl Article Linke: http://www.eetimes.com/design/analog-design/4210265/Gamma-Correction--what-to-know--and-why-it-s-as-important-as-ever

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