Display Enhancement

Brightness, Contrast, Saturation (Color), and Hue (Tint)

Working in the YCbCr color space simplifies the implementation of brightness, contrast, saturation, and hue controls, as shown in Figure 5.1. Also illustrated are multiplexers to allow the output of black screen, blue screen, and color bars.

Alert!

The designer should ensure that no overflow or underflow wraparound errors occur, effectively saturating results to the 0 and 255 values.

Figure 5.1. Hue, Saturation, Contrast, and Brightness Controls.

Many displays also use separate hue and saturation controls for each of the red, green, blue, cyan, yellow, and magenta colors. This enables tuning the image at production time to better match the display's characteristics.

Color Transient Improvement

YCbCr transitions should be aligned. However, the Cb and Cr transitions are usually slower and time-offset due to the narrower bandwidth of color difference information.

By monitoring coincident Y transitions, faster horizontal and vertical transitions may be synthesized for Cb and Cr. Small pre- and after-shoots may also be added to the Cb and Cr signals.

The new Cb and Cr edges are then aligned with the Y edge, as shown in Figure 5.2.

Figure 5.2. Color Transient Improvement.

Displays commonly use this technique to provide a sharper-looking picture.

Luma Transient Improvement

In this case, the Y horizontal and vertical transitions are shortened, and small pre- and after-shoots may also be added, to artificially sharpen the image.

Displays commonly use this technique to provide a sharper-looking picture.

Sharpness

The apparent sharpness of a picture may be increased by increasing the amplitude of high-frequency luminance information.

How It Works

As shown in Figure 5.3, a simple bandpass filter with selectable gain (also called a peaking filter) may be used. The frequency where maximum gain occurs is usually selectable to be either at the color subcarrier frequency or at about 2.6 MHz. A coring circuit is typically used after the filter to reduce low-level noise.

Figure 5.3. Simple Adjustable Sharpness Control. (A) NTSC. (B) PAL.

Figure 5.4 illustrates a more complex sharpness control circuit. The high-frequency luminance is increased using a variable band-pass filter, with adjustable gain. The coring function (typically ±1 LSB) removes low-level noise. The modified luminance is then added to the original luminance signal.

Figure 5.4. More Complex Sharpness Control. (A) Typical implementation. (B) Coring function.

Blue Stretch

Blue stretch increases the blue value of white and near-white colors in order to make whites appear brighter. When applying blue stretch, only colors within a specified color range should be processed.

Colors with a Y value of ∼80% or more of the maximum with a low saturation value and that fall within a white detection area in the CbCr-plane, have their blue components increased by ∼4% (the blue gain factor) and their red components decreased the same amount. For more complex designs, the white detection area and blue gain factor can be dependent on the color's Y value and saturation level.

Green Enhancement

Green enhancement creates a richer, more saturated green color when the level of green is low. Displays commonly use this technique to provide greener looking grass, plants, etc. When applying green enhancement, only colors only within a specified color range should be processed.

Dynamic Contrast

Using dynamic contrast (also called adaptive contrast enhancement), the differences between dark and light portions of the image are artificially enhanced based on the content in the image. Displays commonly use this technique to improve their contrast ratio.

Bright colors in mostly dark images are enhanced by making them brighter (white stretch). This is typically done by using histogram information to modify the upper portion of the gamma curve.

Dark colors in mostly light images are enhanced by making them darker (black stretch). This is typically done by using histogram information to modify the lower portion of the gamma curve.

For a medium-bright image, both techniques may be applied.

A minor gamma correction adjustment may also be applied to colors that are between dark and light, resulting in a more detailed and contrasting picture.

Color Correction

The RGB chromaticities are usually slightly different between the source video and what the display uses. This results in red, green and blue colors that are not completely accurate.

Color correction can be done on the source video to compensate for the display characteristics, enabling more accurate red, green and blue colors to be displayed.

An alternate type of color correction is to perform color expansion, taking advantage of the greater color reproduction capabilities of modern displays. This can result in greener greens, bluer blues, etc. One common technique of implementing color expansion is to use independent hue and saturation controls for each primary and complementary color, plus the skin color.