PCVisionplus
Hardware Reference
Theory of Operation
2–33
Rev 02; February 8, 2002
VOFF
V Active
HSYNC
VSYNC
VBLANK
Vertical Active Region
Vertical valid
video window
VACTIVE
Figure 2–32. PWG Vertical Window Timing
In Figure 2–32, VACT and VOFF are programmed to crop the active video region. The PTG Vsync, stripped Vsync,
or FEN input starts the vertical counter, depending on the acquisition and timing mode selection. The Vertical Active
region is determined by VOFF and VACT. The region of active lines that fall within the VACT time are loaded into
the memory. The remainder of the video is cropped, creating a vertical AOI.
2.7.6 Field Shift Mode
Field shift mode changes the alignment of the first line of video with respect to vertical blank. Field shift mode allows
the PWG to crop the half lines in the beginning and end of the even and odd fields of interlaced video.
When the PWG is in interlaced mode, every setting in the vertical values VOFF and VACT affect each field equally.
This means if the first line of a field is a half line. To crop this, the VOFF value is increased by one; but normally this
also crops the first line of the odd field. Instead of cropping only one line, two lines get cropped; the first line of the
even field (which is the 1/2 line targeted to crop) and the first line of the odd field which is valid data, but is lost due to
the nature of the interlaced video and how the PWG does cropping.
The same thing happens on the last line of the field (in this case the odd field has the half line and the even field has a
full line of video). Programming the VACT to chop the last line in the odd field also crops the last line in the even
field. To crop the two half lines in an image, four lines get cropped.
For example, using 576 line CCIR format, two of those lines are half lines. Programming the PWG to crop the first
half line, the result is 574 lines. If the PWG is programmed to also crop the last half line, the result becomes 572.
The FLDSHIFT bit changes the alignment of the first line of each field so the first 1/2 line of one field corresponds to
a non-video line on the second field. Now programming the PWG to crop the half line, it cuts a non-video line from
the second fields blank region. Therefore the half lines from the top and bottom of the video can be cropped without
cutting an extra line of real video from the opposite field.
Field Shift mode changes which line is the first line in a frame (in non-field shift mode, the first line of the even field
is the first line in the frame and in field shift mode, the first line of video is from the odd field). The field polarity is
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