RIVA128ZX Datasheet, PDF (62/85 Pages) STMicroelectronics – 128-BIT 3D MULTIMEDIA ACCELERATOR

RIVA128ZX
Figure 68. Interface to monitor or television
TV RGB
Encoder
R Y ⁷⁵
G C ⁷⁵
B Y/C ⁷⁵
R
RIVA128ZX G
B
75 75 75
128-BIT 3D MULTIMEDIA ACCELERATOR
PAL/NTSC
Television
75 75 75
Monitor
Monitor detection
Figure 68 shows the typical connection of a televi-
sion or computer monitor to the RIVA128ZXs’
DAC outputs. The RIVA128ZX expects only one
output display device to be connected at a time
and does not support simultaneous output to both
the monitor and television.
During system initialization, the BIOS detects if a
monitor is connected by sensing the doubly-termi-
nated 75Ω load (net 37.5Ω). When no monitor is
connected, only the local 75Ω load is detected and
the RIVA128ZX switches to television output
mode. The BIOS sets the CRTC registers to gen-
erate the appropriate timing for the local television
standard and the DACs are adjusted to compen-
sate for the single 75Ω load.
Monitor mode is always selected if a monitor is de-
tected since it is assumed to be the output device
of choice, having a higher display fidelity than tele-
vision.
Timing generation
Televisions contain two Phase-Locked Loops
(PLLs). One PLL locks the horizontal frequency
and is used to synchronize horizontal and vertical
flyback, and to keep the active video region stable
and centered. The second PLL locks the color
subcarrier frequency (NTSC 3.5794545MHz or
PAL 4.43361875MHz). The color subcarrier is
used as a phase reference to extract the color in-
formation from the television signal.
The RIVA128ZX encodes horizontal and vertical
timing on a composite sync signal. Using a
13.5000MHz reference clock, the RIVA128ZX tim-
ing generator creates ITU-R-601 NTSC and PAL
compliant horizontal timing with only ppm (parts
per million) error. The RIVA128ZX does not use
the color subcarrier clock internally. The reference
clock source can be located on the television up-
grade module with the video encoder and TV out-
put connectors, thus lowering the base system
cost.
Flicker filter
RIVA128ZX provides an optional flicker filtering
feature for TV and interlaced displays.
Without flicker filtering, elements of an image
present on either the odd or the even field, but not
both, are seen to flicker or shimmer obtrusively.
This is a problem especially with 1-pixel-wide hor-
izontal lines often originating from computer gen-
erated GUI displays.
Flicker filtering causes a slight smearing of pixels
in the vertical direction. This trades off image qual-
ity versus flicker. The displayed pixel contains a
proportion of the data for the pixel on that line, plus
a smaller proportion of the data of the equivalent
pixel on the line above and on the line below.
Overall, the proportions add up to 1 so that the
brightness of the screen does not alter and the pix-
el data does not get clipped.
Flicker filtering only takes place on pixel data from
the framestore - the pattern written into the cursor
already has flicker removed. No flicker removal is
performed on video images.
Overscan and underscan
The RIVA128ZX supports overscan and under-
scan in the horizontal and vertical directions using
hardware scaling. Underscan allows 640x480 res-
olution to fit onto NTSC displays and 800x600 res-
olution to fit onto PAL displays. Scaling can be ad-
justed and controlled by software to suit specific
TV requirements. The TV output image position is
also software controllable.
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