Datasheet

ADV7188
Rev. A | Page 31 of 112
DCT [1:0], Digital Clamp Timing, Address 0x15 [6:5]
The clamp timing register determines the time constant of the
digital fine-current clamp circuitry. It is important to realize
that the digital fine-current clamp reacts quickly, correcting any
residual dc level error for the active line immediately. Therefore,
the time constant of the digital fine clamp must be much
quicker than the one for the analog blocks.
By default, the time constant of the digital fine-current clamp is
adjusted dynamically to suit the currently connected input signal.
Table 35. DCT [1:0] Function
DCT [1:0] Description
00 (default) Slow (TC = 1 sec)
01 Medium (TC = 0.5 sec)
10 Fast (TC = 0.1 sec)
11 TC determined by the input 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 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.
0 (default)—The digital clamp is operational.
1—The digital clamp loop is 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
format or luma only for Y/C and YPrPb input formats.
• Luma Antialias (YAA) Filter. The ADV7188 receives video
at a rate of 27 MHz. (For 4× oversampled video, the ADCs
are sample at 54 MHz, and the first decimation is performed
inside the DPP filters. Therefore, the data rate into the
ADV7188 is always 27 MHz.) The ITU-R BT.601 standard
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 and simultaneously attenuates out-of-band
components. The luma antialias filter has a fixed response.
• Luma-Shaping (YSH) Filters. The shaping filter block is a
programmable low-pass filter with a wide variety of re-
sponses. 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 ADV7188 has two responses for the shaping filter: one
that is used for good quality composite, component, and
S-video type sources, and a second for nonstandard composite
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 Y/C separation.
• 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 without requiring user intervention.
Figure 16 through Figure 19 show the responses of all luma
filters. Unless otherwise noted, the filters are set in 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. Y/C separation must aim for best possible
crosstalk reduction while 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 ADV7188. Comb filtering, however, relies on the
frequency relationship of the luma component (multiples of the
video line rate) and the color subcarrier (F
SC
). For good quality
CVBS signals, this relationship is known and therefore the
comb filter algorithms can be used to separate luma and
chroma with high accuracy.
For nonstandard video signals, the frequency relationship may
have an offset, and the comb filters may not be able to remove
all crosstalk artifacts in an optimum fashion without the
assistance of a shaping filter.
An automatic mode is provided. In this mode, the ADV7188
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 override the automatic decisions manually in part or in full
(see
Figure 15).
The luma-shaping filter has three sets of control bits:
• YSFM [4:0] (Address 0x17) allow the user to manually
select a shaping filter mode (applied to all video signals) or
to enable an automatic selection (dependent on video
quality and video standard).
• WYSFMOVR (Address 0x18) allows the user to override
the automatic WYSFM filter selection and enable manual
selection of the WYSFM filter via WYSFM [4:0].
• WYSFM [4:0] (Address 0x18) allow the user to select a
different shaping filter mode for good quality composite
(CVBS), component (YPrPb), and S-video (Y/C) input
signals.