CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
To support a custom overscan ratio, an entire set of overscan compensation
calculations is required. This results in as many as 25 new register values for the
CX25870. For ease of use, these equations are embedded into Conexant’s
programming application called Super Cockpit. Each computation is somewhat
interdependent on the others but the basic overscan equations are as follows:
(*) VSR = (V_LINESI) / (# of total output lines per field)
and
(**) # Blanked Pixels = {[H_CLKO / VSR] – H_ACTIVE}
For illustrative purposes, the calculations used to generate the 13.785 percent
HOC percentage for Autoconfiguration Mode 0–640x480 RGB in, H_CLKO =
1792, NTSC output, are shown below:
From Appendix C (CX25870/871 Data Sheet):
Number of clocks necessary to latch in the V.S.R. # of input lines for every 1
analog output line = 1792 CLKs [i.e., H_CLKO]
CX25870 must ensure input is 2X upsampled.
Therefore:
# active CLKs per analog line = 2*(H_ACTIVE)
# active CLKs per analog line = 1280 active CLKs per analog line
percent of input used to create active video area = {1280 active CLKs / 1792
total CLKs} = 71.4286 percent
Therefore:
(x) = active region percent of analog output line = 71.4286 percent
(y) = active region percent of typical analog video for NTSC = 52.65556 µs /
63.55556 µs = (y) = 82.4945 percent of line is active
Ratio of [x/y] = {71.4286 percent / 82.4945 percent} = 0.862147
HOC percentage = 1–{Ratio of [x/y]}
HOC percent = 1–0.862147 = 13.785 percent = HOC percentage for
Autoconfiguration Mode 0
As a result, 13.785 percent of the horizontal active region within each line of
an NTSC signal will be forcibly blanked by the CX25870. For most TVs, this will
resize the upsampled digital image properly so all of the pixels fit horizontally
within the bezeled area of North American or Japanese TVs. The 13.785 percent
overscan percentage is equally distributed on either side of the horizontal active
region (i.e., 13.785 percent / 2 = 6.89 percent extra blanking for the beginning and
end of the line). The original 640 active pixels (i.e., H_ACTIVE) will then be
‘squeezed’ into the remaining analog active region due to the faster pixel rate.
The explanation of the vertical overscan percentage value is similar. For
autoconfiguration mode #0, V_ACTIVEO is 212, which means there are 210 full
active lines per field. The first and last lines are filtered lines that assist in
smoothing the transitions into and out of the active region to avoid flickering and
are not counted. Any NTSC standard calls for 243 active lines per field, so
210/243 = 0.864198 of the vertical active region is used. This calculation yields a
vertical overscan compensation percentage of 100–86.4198 = 13.5802 percent.
100381B
Conexant
1-33
CONEXANT [ CONEXANT SYSTEMS, INC ]
