CX25870/871
3.0 PC Board Considerations
Flicker-Free Video Encoder with Ultrascale Technology
3.6 Applications Information
Now, assume the end-user clicked on the Down button once. This action
dictates that more blanking will exist before the active region is displayed. This
operation requires decrementing the graphics controller’s new VSYNC_START
register value to (VSYNC_STARTdefault - 5 lines). As the timing master, this
would force the controller to issue its VSYNC* digital signal’s leading edge five
lines earlier in time than before. The software engineer must also subtract five
lines to the controller’s VSYNC_END register to maintain the same VSYNC*
pulse duration (nominally two-to-six lines). Within the encoder, the
V_BLANKI[7:0] register must be incremented by 5 lines so the encoder can
accommodate the five more lines of blanking required to start the field and still
display the original active area of the frame.
For an explanation of the Left and Up buttons, simply apply the opposite
offsets to the values explained for the Right and Down operations. Remember that
SYNC_START/END always works in the opposite direction of picture
movement. If the Position control works correctly, the end user should see a
gradual change to either the X and Y position of the active image after each
corresponding mouse click.
3.6.2.9 Size
This control pad is used by the end-user to change the active X and Y dimensions
of the TV Out picture. This is done by modifying the amount of horizontal (X
dimension) and vertical (Y dimension) overscan compensation. Ideally, there
should be four directional controls included: two for horizontal adjustment and
two for vertical adjustment. For practical usage, the maximum amounts of
Horizontal Overscan Compensation (HOC) and Vertical Overscan Compensation
(VOC) should be limited to 25 percent (or three mouse clicks in any direction).
The minimal amounts of HOC and VOC should be capped at 10 percent since
percentages smaller than this often make the TV image so large that all edges are
behind the bezel of the TV, rendering the outer regions of the Windows desktop
useless.
Based on testing, Conexant recommends changing the HOC percentage by ~ 3
percent from its previous value for each‘+ or – horizontal mouse click within the
GUI. The + symbol denotes a larger picture size in that direction (and a decrease
in the amount of horizontal blanking or HOC percent) and a - sign corresponds to
smaller picture size.
In addition, TV Out software designers should vary the VOC percentage by ~
3 percent from its previous value for each + or – vertical mouse click within the
GUI. The + symbol denotes a larger picture size in that direction and a – sign
corresponds to smaller picture size (and an increase in the amount of vertical
blanking or VOC percent).
The overscan percentages horizontally and vertically are independent of each
other. However, the TV Out picture looks best when HOC and VOC are equal or
within 2 percent of each other. Having realized this fact, Conexant has
incorporated many autoconfiguration modes that have a minimal difference (i.e.,
Delta) between the HOC and VOC ratios.
100381B
Conexant
3-21
CONEXANT [ CONEXANT SYSTEMS, INC ]
