Data Sheet
ADV7180
DCT[1:0], Digital Clamp Timing, Address 0x15[6:5]
•
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 clamp timing register determines the time constant of the
digital fine clamp circuitry. It is important to 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.
The ADV7180 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.
Table 34. DCT Function
DCT[1:0]
Description
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.
Digital resampling filter. This 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.
00 (default)
Slow (TC = 1 sec)
Medium (TC = 0.5 sec)
Fast (TC = 0.1 sec)
Determined by ADV7180, depending on the
input video parameters
01
10
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.
Figure 22 through Figure 25 show the overall response of all filters
together. Unless otherwise noted, the filters are set into a typical
wideband mode.
Y Shaping Filter
When DCFE is 0 (default), the digital clamp is operational.
When DCFE is 1, the digital clamp loop is frozen.
LUMA 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 ADV7180. 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.
Data from the digital fine clamp block is processed by the three
sets of filters that follow. Note that 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 ADV7180 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 ADV7180 is always 28.6363 MHz.)
The ITU-R BT.601 recommends a sampling frequency of
13.5 MHz. The luma antialias filter decimates the
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.
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.
Rev. G | Page 31 of 120
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