1.0 Functional Description
CX25870/871
1.3 Device Description
Flicker-Free Video Encoder with Ultrascale Technology
Table 1-15 summarizes the direction of the BLANK* encoder in each
interface. For more information refer to Section 1.3.8.
Table 1-15. Summary of Allowable BLANK* Signal Directions by Interface
Interface
Allowable Direction of BLANK*
Input or Output
Master
Pseudo-master
Slave
Input
Input
1.3.17 Overscan Compensation
Overscan compensation is the process by which the encoder converts the digital
input lines to the appropriate number of output lines for producing a full-screen
image on the television receiver. This conversion is done in accordance with the
Vertical Scaling Ratio (VSR). VSR is the ratio of the number of input lines
received to number of output lines generated by the CX25870 (i.e., 262.5
lines/field for NTSC and 312.5 lines/field for PAL-BDGHI and SECAM). Using
the correct amount of compensation in both the horizontal and vertical
dimensions (at least 10 percent) will ensure that the entire digital image normally
seen on the PC monitor is satisfactorily mapped to the analog television without
any pixels or lines hidden in unviewable areas.
Increasing the Horizontal Overscan Compensation (HOC) percentage while
keeping the Vertical Overscan Compensation (VOC) percentage the same will
have several effects on the VGA Encoder. First, the number of output clocks per
line (H_CLKO) will increase. Correspondingly, the clock frequencies shared
between the data master and CX25870 (i.e., CLKO = CLKI) will increase.
Therefore, the original number of active pixels will be squeezed into a smaller
analog video display region because the frequency at which input data is clocked
into the CX25870 has increased. Since the CX25870 now processes active data at
a faster rate than CCIR601-only compatible encoders, the graphics controller will
need to transmit more blank pixels per line (i.e., HTOTAL must increase to match
CX25870’s H_CLKI) to make up the difference.
Increasing the (VOC) percentage while keeping the Horizontal Overscan
Compensation percentage the same will have several different effects on the VGA
Encoder. First, the H_CLKO total will stay the same as will the pixel rate (i.e.,
CLKI = CLKO). These parameters are dictated by the HOC value only. Second,
the number of total vertical input lines (V_LINESI = data master’s VTOTAL) will
increase, which will increase the internal VSR. The net result is that more active
pixels and more active lines will be used to generate each output line. The only
way for the graphics controller to transmit these additional input lines with the
same clock frequency as before is to decrease the amount of blanked pixels per
line.
1-32
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
