23
FN6760.0
September 12, 2008
SOG
For component YPbPr signals, the sync signal is embedded
on the Y channel’s video, which is connected to the green
input, hence the name SOG (Sync on Green). The horizontal
sync information is encoded onto the video input by adding
the sync tip during the blanking interval. The sync tip level is
typically 0.3V below the video black level.
To minimize the loading on the green channel, the SOG input
for each of the green channels should be AC-coupled to the
ISL98003 through a series combination of a 10nF capacitor
and a 500Ω resistor.
SOG Slicer (Figure 2)
The SOG input has programmable threshold, 40mV of
hysteresis, and an optional low pass filter than can be used
to remove high frequency video spikes (generated by
overzealous video peaking in a DVD player, for example)
that can cause false SOG triggers. The SOG threshold sets
the comparator threshold relative to the sync tip (the bottom
of the SOG pulse).
Inside the ISL98003, a 1µA pull-down ensures that each sync
tip triggers the clamp circuit causing the tip to be clamped to a
600mV level. A comparator compares the SOG signal with an
internal 4-bit programmable threshold level reference ranging
from 0mV to 300mV above the sync clamp level. The SOG
threshold level, hysteresis, and low-pass filter is programmed
via registers 0x30and 0x31. If the Sync-On-Green function is
not needed, the SOG
IN
pin(s) may be left unconnected.
SYNC Processing
The ISL98003 can process sync signals from 3 different
sources: discrete HSYNC and VSYNC, composite sync on
the HSYNC input, or composite sync from a Sync-On-Green
(SOG) signal embedded on the Green video input. The
ISL98003 has SYNC activity detect functions to help the
firmware determine which sync source is available.
Macrovision
The ISL98003 automatically detects the presence of
Macrovision-encoded video. When Macrovision is detected,
it generates a mask signal that is ANDed with the incoming
SOG CSYNC signal to remove the Macrovision before the
HSYNC goes to the PLL. No additional programming is
required to support Macrovision.
The mask signal is also applied to the HSYNC
OUT
signal.
When Sync Mask Disable = 0, any Macrovision present on
the incoming sync will not be visible on HSYNC
OUT
. If the
application requires the Macrovision pulses to be visible on
HSYNC
OUT
, set the HSYNC
OUT
Mask Disable bit (register
0x7A bit 4).
Headswitching from Analog Videotape Signals
Occasionally this AFE may be used to digitize signals
coming from analog videotape sources. The most common
example of this is a Digital VCR (which for best signal quality
would be connected to this AFE with a component YPbPr
connection). If the digital VCR is playing an older analog
VHS tape, the sync signals from the VCR may contain the
worst of the traditional analog tape artifacts: headswitching.
Headswitching is traditionally the enemy of PLLs with large
capture ranges, because a headswitch can cause the
HSYNC period to change by as much as ±90%. To the PLL,
this can look like a frequency change of -50% to +900%,
causing errors in the output frequency (and obviously the
phase) to change. Subsequent HSYNCs have the correct,
original period, but most analog PLLs will take dozens of
lines to settle back to the correct frequency and phase after
a headswitch disturbance. This causes the top of the image
to “tear” during normal playback. In “trick modes” (fast
forward and rewind), the HSYNC signal has multiple
headswitch-like discontinuities, and many PLLs never settle
to the correct value before the next headswitch, rendering
the image completely unintelligible.
R(GB)
IN
0
CLAMP
GENERATION
R(GB)
GND
0
R(GB)
IN
1
R(GB)
GND
1
VGA0
VGA1
V
IN
+
V
IN
-
DC Restore
Clamp DAC
V
CLAMP
8 bit ADC
Offset
DAC
Fixed
Offset
ABLC™
ABLC™
Offset
Control
Registers
ABLC™
Fixed
Offset
0x000
To
ABLC
Block
To Output
Formatter
10
10
10
8 8
8
10
Automatic Black Level
Compensation (ABLC™) Loop
DC Restoration
Input
Bandwidth
PGA
Bandwidth
Control
10
Vref
FIGURE 1. VIDEO FLOW (INCLUDING ABLC™)
ISL98003
