ADV7181BST datasheet(26/96 Pages) AD

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ADV7181B

Rev. 0 | Page 26 of 96

Y Shaping Filter

For input signals in CVBS format, the luma shaping filters play

an essential role in removing the chroma component from a

composite signal. YC separation must aim for best possible

crosstalk reduction while still retaining as much bandwidth

(especially on the luma component) as possible. High quality

YC separation can be achieved by using the internal comb filters

of the ADV7181B. Comb filtering, however, relies on the

frequency relationship of the luma component (multiples of the

video line rate) and the color subcarrier (Fsc). For good quality

CVBS signals, this relationship is known; the comb filter

algorithms can be used to separate out luma and chroma with

high accuracy.

In the case of nonstandard video signals, the frequency

relationship may be disturbed and the comb filters may not be

able to remove all crosstalk artifacts in an optimum fashion

without the assistance of the shaping filter block.

An automatic mode is provided. Here, the ADV7181B evaluates

the quality of the incoming video signal and selects the filter

responses in accordance with the signal quality and video

standard. YFSM, WYSFMOVR, and WYSFM allow the user to

manually override the automatic decisions in part or in full.

The luma shaping filter has three control registers:

•

YSFM[4:0] allows the user to manually select a shaping

filter mode (applied to all video signals) or to enable an

automatic selection (dependent on video quality and video

standard).

•

WYSFMOVR allows the user to manually override the

WYSFM decision.

•

WYSFM[4:0] allows the user to select a different shaping

filter mode for good quality CVBS, component (YPrPb),

and S-VHS (YC) input signals.

In automatic mode, the system preserves the maximum possible

bandwidth for good CVBS sources (since they can successfully

be combed) as well as for luma components of YPrPb and YC

sources, since they need not be combed. For poor quality

signals, the system selects from a set of proprietary shaping

filter responses that complements comb filter operation in order

to reduce visual artifacts.

The decisions of the control logic are shown in Figure 10.

YSFM[4:0] Y Shaping Filter Mode, Address 0x17 [4:0]

The Y shaping filter mode bits allow the user to select from a

wide range of low-pass and notch filters. When switched in

automatic mode, the filter is selected based on other register

selections, for example, detected video standard, as well as

properties extracted from the incoming video itself, for

example, quality, time base stability. The automatic selection

always picks the widest possible bandwidth for the video input

encountered.

•

If the YSFM settings specify a filter (i.e., YSFM is set to

values other than 00000 or 00001), the chosen filter is

applied to all video, regardless of its quality.

•

In automatic selection mode, the notch filters are only used

for bad quality video signals. For all other video signals,

wideband filters are used.

WYSFMOVR Wideband Y Shaping Filter Override,

Address 0x18,[7]

Setting the WYSFMOVR bit enables the use of the WYSFM[4:0]

settings for good quality video signals. For more information,

refer to the general discussion of the luma shaping filters in the

Y Shaping Filter section and the flowchart shown in Figure 10.

When WYSFMOVR is 0, the shaping filter for good quality

video signals is selected automatically.

Setting WYSFMOVR to 1 (default) enables manual override via

WYSFM[4:0].

ADV7181BST Datasheet(PDF) 26 Page - Analog Devices

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