ADV7188BSTZ datasheet(39/112 Pages) AD

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ADV7188

Rev. A | Page 39 of 112

CHROMA TRANSIENT IMPROVEMENT (CTI)

The signal bandwidth allocated for chroma is typically much

smaller than that of luminance. With older devices, this was a

valid way to fit a color video signal into a given overall band-

width because the human eye is less sensitive to chrominance

than to luminance.

The uneven bandwidth, however, may lead to visual artifacts

during sharp color transitions. At the border of two bars of

color, both components (luma and chroma) change at the same

time (see Figure 22).

Due to the higher bandwidth, the signal transition of the luma

component is usually much sharper than that of the chroma

component. The color edge is not sharp and can be blurred, in

the worst case, over several pixels.

LUMA

SIGNAL

DEMODULATED

CHROMA

SIGNAL

LUMA SIGNAL WITH A

TRANSITION, ACCOMPANIED

BY A CHROMA TRANSITION

ORIGINAL SLOW CHROMA

TRANSITION PRIOR TO CTI

SHARPENED CHROMA

TRANSITION AT THE

OUTPUT OF CTI

Figure 22. CTI Luma/Chroma Transition

To correct for such uneven bandwidths, the CTI block examines

the input video data. It detects transitions of chroma and can be

programmed to create steeper chroma edges in an attempt to

artificially restore lost color bandwidth. By operating only on

edges that are greater than a certain threshold, the CTI block

ensures that noise is not emphasized. Care has also been taken to

avoid edge ringing and undesirable saturation and hue distortion.

Chroma transient improvements are needed primarily for signals

that have severe chroma bandwidth limitations. For these types

of signals, it is strongly recommended to enable the CTI block

via CTI_EN.

CTI_EN, Chroma Transient Improvement Enable,

Address 0x4D [0]

0—Disables the CTI block.

1 (default)—Enables the CTI block.

CTI_AB_EN, Chroma Transient Improvement

Alpha Blend Enable, Address 0x4D [1]

This bit enables an alpha-blend function, which mixes the

transient improved chroma with the original signal. The

sharpness of the alpha blending can be configured via the

CTI_AB [1:0] bits. For the alpha blender to be active, the CTI

block must be enabled via the CTI_EN bit. The settings of the

CTI_AB_EN bit are as follows:

0—Disables the CTI alpha blender.

1 (default)—Enables the CTI alpha blender.

CTI_AB [1:0], Chroma Transient Improvement Alpha

Blend, Address 0x4D [3:2]

The CTI_AB [1:0] controls the behavior of alpha-blend

circuitry, which mixes the sharpened chroma signal with the

original one. It thereby controls the visual impact of CTI on the

output data.

For CTI_AB [1:0] to become active, the CTI block must be

enabled via the CTI_EN bit and the alpha blender must be

switched on via CTI_AB_EN.

Sharp blending maximizes the effect of CTI on the picture, but

may also increase the visual impact of small amplitude, high

frequency chroma noise.

Table 49. CTI_AB [1:0] Function

CTI_AB [1:0]

Description

Sharpest mixing

Sharp mixing

Smooth mixing

11 (default)

Smoothest mixing

CTI_C_TH [7:0], CTI Chroma Threshold,

Address 0x4E [7:0]

The CTI_C_TH [7:0] value is an unsigned, 8-bit number speci-

fying how big the amplitude step in a chroma transition must be

to be steepened by the CTI block. Programming a small value

into this register causes even, small edges to be steepened by the

CTI block. Making CTI_C_TH [7:0] a large value causes the

block to only improve large transitions.

The default value for CTI_C_TH [7:0] is 0x08, indicating the

threshold for the chroma edges prior to CTI.

DIGITAL NOISE REDUCTION (DNR) AND

LUMA PEAKING FILTER

DNR is based on the assumption that high frequency signals with

low amplitude are probably noise and that their removal therefore

improves picture quality. There are two DNR blocks in the

ADV7188: the DNR1 block before the luma peaking filter and the

DNR2 block after the luma peaking filter, as shown in Figure 23.

LUMA

OUTPUT

DNR1

LUMA PEAKING

FILTER

DNR2

LUMA

SIGNAL

Figure 23. DNR and Peaking Block Diagram

ADV7188BSTZ Datasheet(PDF) 39 Page - Analog Devices

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