ADV7180WBSTZ datasheet(31/120 Pages) AD

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Data Sheet

ADV7180

Rev. G | Page 31 of 120

DCT[1:0], Digital Clamp Timing, Address 0x15[6:5]

The clamp timing register determines the time constant of the

digital fine clamp circuitry. It is important to note that the digital

fine clamp reacts quickly because it immediately corrects any

residual dc level error for the active line. The time constant from

the digital fine clamp must be much quicker than the one from

the analog blocks.

By default, the time constant of the digital fine clamp is adjusted

dynamically to suit the currently connected input signal.

Table 34. DCT Function

DCT[1:0]

Description

00 (default)

Slow (TC = 1 sec)

Medium (TC = 0.5 sec)

Fast (TC = 0.1 sec)

Determined by ADV7180, depending on the

input video parameters

DCFE, Digital Clamp Freeze Enable, Address 0x15[4]

This register bit allows the user to freeze the digital clamp loop

at any time. It is intended for users who would like to do their

own clamping. Users should disable the current sources for

analog clamping via the appropriate register bits, wait until the

digital clamp loop settles, and then freeze it via the DCFE bit.

When DCFE is 0 (default), the digital clamp is operational.

When DCFE is 1, the digital clamp loop is frozen.

LUMA FILTER

Data from the digital fine clamp block is processed by the three

sets of filters that follow. Note that the data format at this point

is CVBS for CVBS input or luma only for Y/C and YPrPb input

formats.

•

Luma antialias filter (YAA). The ADV7180 receives video

at a rate of 28.6363 MHz. (In the case of 4× oversampled

video, the ADC samples at 57.27 MHz, and the first

decimation is performed inside the DPP filters. Therefore,

the data rate into the ADV7180 is always 28.6363 MHz.)

The ITU-R BT.601 recommends a sampling frequency of

13.5 MHz. The luma antialias filter decimates the

oversampled video using a high quality linear phase, low-

pass filter that preserves the luma signal while at the same

time attenuating out-of-band components. The luma

antialias filter (YAA) has a fixed response.

•

Luma shaping filters (YSH). The shaping filter block is a

programmable low-pass filter with a wide variety of responses.

It can be used to selectively reduce the luma video signal

bandwidth (needed prior to scaling, for example). For

some video sources that contain high frequency noise,

reducing the bandwidth of the luma signal improves visual

picture quality. A follow-on video compression stage may

work more efficiently if the video is low-pass filtered.

The ADV7180 has two responses for the shaping filter: one

that is used for good quality composite, component, and SVHS

type sources, and a second for nonstandard CVBS signals.

The YSH filter responses also include a set of notches for

PAL and NTSC. However, using the comb filters for Y/C

separation is recommended.

•

Digital resampling filter. This block allows dynamic

resampling of the video signal to alter parameters such as

the time base of a line of video. Fundamentally, the resampler

is a set of low-pass filters. The actual response is chosen by the

system with no requirement for user intervention.

Figure 22 through Figure 25 show the overall response of all filters

together. Unless otherwise noted, the filters are set into a typical

wideband mode.

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. Y/C separation must aim for best possible

crosstalk reduction while still retaining as much bandwidth

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

Y/C separation can be achieved by using the internal comb

filters of the ADV7180. 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 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 the best fashion without the assistance

of the shaping filter block.

ADV7180WBSTZ Datasheet(PDF) 31 Page - Analog Devices

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