ADV7180
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.
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.
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.
CLAMP 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.
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.
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 would be
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 17).
The ADV7180 uses a combination of current sources and a
digital processing block for clamping, as shown in Figure 17.
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
(S-VHS) type signals, and three independent channels are
needed to allow component signals (YPrPb) to be processed.
The clamping can be divided into two sections:
The following sections describe the I2C signals that can be used
to influence the behavior of the clamping block.
•
Clamping before the ADC (analog domain): current
sources.
CCLEN, Current Clamp Enable, Address 0x14 [4]
•
Clamping after the ADC (digital domain): digital
processing block.
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.
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.
When CCLEN is 0, the current sources are switched off.
When CCLEN is 1 (default), the current sources are enabled.
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.
FINE CURRENT SOURCES
COARSE CURRENT SOURCES
DATA
ANALOG
VIDEO
INPUT
VIDEO PROCESSOR
PRE-
ADC
WITH DIGITAL
PROCESSOR
FINE CLAMP
(DPP)
CLAMP CONTROL
Figure 17. Clamping Overview
Rev. A | Page 27 of 112
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