ADV7182ABCPZ datasheet(29/102 Pages) AD

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Data Sheet

ADV7182A

Rev. 0 | Page 29 of 102

Clamp Operation Controls

The following sections describe the I2C signals that can be used

to influence the behavior of the clamping block.

CCLEN, Current Clamp Enable—Address 0x14, Bit 4

The current clamp enable bit allows the user to switch off all the

current sources in the AFE simultaneously which is useful if the

incoming analog video signal is clamped externally.

When CCLEN is 0, the current sources are switched off. When

CCLEN is 1 (default), the current sources are enabled.

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

The clamp timing register determines the time constant of the

digital fine clamp circuitry. 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 36. DCT Function

DCT[1:0]

Description

00 (default)

Slow; time constant (TC) = 1 sec

Medium; TC = 0.5 sec

Fast; TC = 0.1 sec

Determined by ADV7182A, depending on the

input video parameters

DCFE, Digital Clamp Freeze Enable—Address 0x15, Bit 4

This bit allows the user to freeze the digital clamp loop at any

time to perform manual clamping. The user can 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 set to 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. The data format at this point is CVBS

for CVBS input or luma only for Y/C and YPrPb input formats.

The ADV7182A 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 ADV7182A is

always 28.6363 MHz.) ITU-R BT.601 recommends a sampling

frequency of 13.5 MHz. The luma antialias filter (YAA) 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.

The luma shaping filter (YSH) block is a programmable low-pass

filter with a wide variety of responses. It can be used to reduce

selectively 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. If the video is low-pass filtered,

a follow on video compression stage can work more efficiently.

The ADV7182A 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.

The digital resampling filter 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 18 through Figure 21 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 ADV7182A. 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.

An automatic mode is provided that allows the ADV7182A to

evaluate the quality of the incoming video signal and select 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 the following control bits.



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

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

automatic selection (depending 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 composite (CVBS), component

(YPrPb), and SVHS (Y/C) input signals.

In automatic mode, the system preserves the maximum possible

bandwidth for good CVBS sources (because they can be successfully

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