adv7189b Analog Devices, Inc., adv7189b Datasheet - Page 26

• Manufacturer Part Number: adv7189b
• Description: Multiformat Sdtv Video Decoder
• Manufacturer: Analog Devices, Inc.

ADV7189B
The following sections describe the I
used to influence the behavior of the clamping block on the
ADV7189B.
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 can 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.
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 realize that the
digital fine clamp reacts very quickly because it is supposed to
immediately correct any residual dc level error for the active
line. The time constant of 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 29. DCT Function
DCT[1:0]
00
01
10 (default)
11
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 three sets
of filters. Note: The data format at this point is CVBS for CVBS
input or luma only for Y/C and YPrPb input formats.
x
Luma antialias filter (YAA). The ADV7189B receives video
at a rate of 27 MHz. (In the case of 4× oversampled video,
the ADCs sample at 54 MHz, and the first decimation is
performed inside the DPP filters. Therefore, the data rate
into the ADV7189B is always 27 MHz.) The ITU-R BT.601
recommends a sampling frequency of 13.5 MHz.
Fast (TC = 0.1 sec)
Description
Slow (TC = 1 sec)
Medium (TC = 0.5 sec)
Determined by the ADV7189B, depending on
the I/P video parameters
2
C signals that can be
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x
x
F igure 12 through
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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. YC separation must aim for the 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 ADV7189B. Comb filtering, however, relies on the
frequency relationship of the luma component (multiples of the
video line rate) and the color subcarrier (F
CVBS signals, this relationship is known; the comb filter
algorithms can be used to separate out luma and chroma with
high accuracy.
With 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.
The luma antialias filter decimates the oversampled video
using a high quality, linear phase, low-pass filter that pre-
serves 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 can work more efficiently if the video
is low-pass filtered.
Digital resampling filter. This block is used to allow 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.
The ADV7189B has two responses for the shaping filter:
one that is used for good quality CVBS, component, and
S-VHS type sources, and a second for nonstandard CVBS
signals.
The YSH filter responses also include a set of notches for
PAL and NTSC. However, it is recommended to use the
comb filters for YC separation.
F igure 15 show the overall response of all
1 7 4 H
SC
). For good quality