EVAL-ADV7183BEB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-ADV7183BEB' PDF : 104 Pages View PDF
ADV7183A
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 fast since it is supposed to immed-
iately 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 57. DCT Function
DCT[1:0] Description
00
Slow (TC = 1 sec).
01
Medium (TC = 0.5 sec).
10 (default) Fast (TC = 0.1 sec).
11
Determined by ADV7183A dependent on 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.
Table 58. DCFE Function
DCFE
Description
0 (default) Digital clamp operational.
1
Digital clamp loop frozen.
LUMA FILTER
Data from the digital fine clamp block is processed by three sets
of filters. 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 ADV7183A receives
video at a rate of 27 MHz. (In 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 ADV7183A is always 27 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 ADV7183A allows selection of 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.
• 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.
Figure 12 through Figure 15 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. YC separation must aim for 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 ADV7183A. 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 out 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 an optimum fashion
without the assistance of the shaping filter block.
An automatic mode is provided. Here, the ADV7183A evaluates
the quality of the incoming video signal and selects 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.
Rev. B | Page 28 of 104
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