ADV7180
DCT[1:0], Digital Clamp Timing, Address 0x15 [6:5]
•
Luma Shaping Filters (YSH).
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 very quickly because it is supposed to
immediately correct 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.
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.
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 S-VHS type sources, and a second for nonstandard
CVBS signals.
Table 30. DCT Function
DCT[1:0]
Description
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
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.
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 19 through Figure 22 show the overall response of all
filters together. Unless otherwise noted, the filters are set into a
typical wideband mode.
When DCFE to 0 (default), the digital clamp is operational.
When DCFE is 1, the digital clamp loop is frozen.
LUMA FILTER
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. 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 three sets
of filters. 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 27 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 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.
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.
Rev. A | Page 28 of 112
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