ADV7180WBSTZ datasheet(30/120 Pages) AD

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ADV7180

Data Sheet

Rev. G | Page 30 of 120

DEF_VAL_AUTO_EN, Default Value Automatic Enable,

Address 0x0C[1]

This bit enables the automatic use of the default values for Y, Cr,

and Cb when the ADV7180 cannot lock to the video signal.

Setting DEF_VAL_AUTO_EN to 0 disables free-run mode. If

the decoder is unlocked, it outputs noise.

Setting DEF_VAL_EN to 1 (default) enables free-run mode, and

a colored screen set by user-programmable Y, Cr, and Cb values

is displayed when the decoder loses lock.

CLAMP OPERATION

The input video is ac-coupled into the ADV7180. Therefore, its dc

value needs to be restored. This process is referred to as clamping

the video. This section explains the general process of clamping

on the ADV7180 and shows the different ways in which a user

can configure its behavior.

The ADV7180 uses a combination of current sources and a

digital processing block for clamping, as shown in Figure 20.

The analog processing channel shown is replicated three times

inside the IC. While only one single channel is needed for a

CVBS signal, two independent channels are needed for Y/C

(SVHS) type signals, and three independent channels are

needed to allow component signals (YPrPb) to be processed.

The clamping can be divided into two sections:



Clamping before the ADC (analog domain): current

sources.



Clamping after the ADC (digital domain): digital

processing block.

The ADC can digitize an input signal only if it resides within

the ADC 1.0 V input voltage range. An input signal with a dc

level that is too large or too small is clipped at the top or bottom

of the ADC range.

The primary task of the analog clamping circuits is to ensure that

the video signal stays within the valid ADC input window so that

the analog-to-digital conversion can take place. It is not necessary

to clamp the input signal with a very high accuracy in the analog

domain as long as the video signal fits within the ADC range.

After digitization, the digital fine clamp block corrects for any

remaining variations in dc level. Because the dc level of an input

video signal refers directly to the brightness of the picture

transmitted, it is important to perform a fine clamp with high

accuracy; otherwise, brightness variations may occur. Further-

more, dynamic changes in the dc level almost certainly lead to

visually objectionable artifacts and must, therefore, be prohibited.

The clamping scheme has to complete two tasks. It must acquire

a newly connected video signal with a completely unknown dc

level, and it must maintain the dc level during normal operation.

To acquire an unknown video signal quickly, the large current

clamps should be activated. It is assumed that the amplitude of

the video signal at this point is of a nominal value. Control of

the coarse and fine current clamp parameters is performed

automatically by the decoder.

Standard definition video signals may have excessive noise on

them. In particular, CVBS signals transmitted by terrestrial

broadcast and demodulated using a tuner usually show very

large levels of noise (>100 mV). A voltage clamp is unsuitable

for this type of video signal. Instead, the ADV7180 employs a

set of four current sources that can cause coarse (>0.5 mA) and

fine (<0.1 mA) currents to flow into and away from the high

impedance node that carries the video signal (see Figure 20).

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[4]

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

current sources in the analog front end altogether. This may be

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.

COARSE CURRENT SOURCES

FINE CURRENT SOURCES

DATA

PRE-

PROCESSOR

(DPP)

ADC

VIDEO PROCESSOR

WITH DIGITAL

FINE CLAMP

CLAMP CONTROL

ANALOG

VIDEO

INPUT

Figure 20. Clamping Overview

ADV7180WBSTZ Datasheet(PDF) 30 Page - Analog Devices

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