1.0 Functional Description
CX25870/871
1.3 Device Description
Flicker-Free Video Encoder with Ultrascale Technology
Figures A-1 through A-8 illustrate many of the allowable overscan
compensation percentage pairs for the major desktop resolutions and the most
popular video outputs. These figures illustrate the minimum horizontal blanking
times the data master must possess along with overscan compensation plots for
pixel based data masters as well as 8-and 9-cycle character clock based graphics
controllers.
1.3.18 Standard Flicker Filtering
To understand what flicker filtering is, one must understand two of the primary
differences between the analog video standards used by TVs and the technology
used in today’s computer monitors.
First of all, computer monitors receive their video signal in a more basic,
pristine form than TVs do. As discussed earlier, the video signal sent by a
computer to its monitor is broken into multiple electrical components (red, green,
blue and sync) while a TV signal has all necessary information combined into a
single composite signal or separate Luma and Chroma analog channels
(S-Video). In order to process this composite signal, a TV must break it up into its
original components, inevitably degrading the picture quality and creating
distortions.
A second factor contributing to the decreased quality of images displayed on
TV monitors is interlacing, a technique by which a complete TV picture is drawn
in two passes from top to bottom on the picture tube. In interlacing, the first pass
paints all the “odd” lines and the second pass paints the “even” lines. Noticeable
flicker occurs when the images in the odd lines are very different from the images
in the even lines. As the odd and even lines are alternately displayed, the eye
perceives the quick appearing and disappearing of visual information. This results
the in the irritation called flicker. Flicker is especially noticeable when viewing
thin horizontal lines that only take up a single row within the odd or even field. If,
for example, the line happens to be on an odd row, it totally disappears every time
the even rows are displayed resulting in that item appearing and disappearing at
the field rate on the TV.
Unlike TV monitors, computer monitors paint an entire image in one pass
from top to bottom, in a display format called noninterlaced or progressive.
Images displayed in a noninterlaced format do not suffer from the same flicker
problems.
For improved image quality and reduced flickering, the CX25870 contains a
5-tap or 5-line flicker filter for both the Luma (F_SELY[2:0]) channel and
Chroma (F_SELC[2:0]) channel. The Conexant standard flicker-filter works by
applying a mathematically weighted averaging algorithm to the incoming pixels
of data from different lines. This slightly alters the digital information that is
processed and eventually converted to the odd and even lines of a TV picture so
that the alternating lines are more similar to each other. This way, when they
appear and disappear in the interlacing process, the flicker is less noticeable. The
more similar the lines are made to appear, the less flicker is visible.
1-36
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
