CX25870/871
3.0 PC Board Considerations
Flicker-Free Video Encoder with Ultrascale Technology
3.6 Applications Information
3.6.2.4 Hue
Hue refers to the wavelength of the color. That means that hue is the term used to
represent the base color–red, green, magenta, yellow, and so forth. Hue is
completely separate from the intensity or the saturation of the color. For example,
a red hue could look brown at low saturation, fire-engine red at a higher level of
saturation, or pink at a high brightness level. All three colors have the same hue
however.
Occasionally, the end user may need to alter the hue. The method for adjusting
this parameter with the CX25870/871 is to program a different value to the
HUE_ADJ register. This method changes the hue in the composite and S-Video
signals for NTSC, PAL, and SECAM waveforms according to the following
equation:
Desired Phase Offset (in degrees) = [360° / 256] * (HUE_ADJ)
A slider labeled ‘HUE’ should be included in the GUI so minor alterations
( 20°) in this parameter are possible. Major alterations(>20°) in the phase offset
are not recommended since dramatic hue shifts will result in different colors than
the original.
3.6.2.5 Sharpness
Occasionally, drastic phase shifts occur at the borders of dialog boxes within
applications programs and with certain combinations of text and background
colors. This is due to the primary and secondary colors being at opposite ends of
the UV hue spectrum. The result of these phase differences is that the edges or
text look blurry to the observer.
The CX25870/871 has a bit field available named PKFIL_SEL[1:0] to
sharpen these edges so they look crisper on the television. Four choices are
available, each of which enables a different type of peaking filter. The 0 dB
(Bypass) filter is the defaulted level while gains of 1 dB, 2 dB, and 3.5 dB are also
possible.
3.6.2.6 Dot Crawl
Dot crawl refers to a specific image artifact that is the result of the NTSC
standard. When some computer generated text shows up on top of a video clip
being shown, close viewing of the TV will show some pixels or jaggies rolling up
or down the picture in the area of a dialog box’s edges. Another term for this
phenomenon is creepy-crawlies or the zipper effect.
Conexant has derived software code to minimize the dot crawl. This is not a
register or bit within the CX25870/871 but rather a complicated software
algorithm that modifies the 90-degree color subcarrier shift exhibited in four
consecutive NTSC fields. To obtain this code, file a request with your local
Conexant sales office. The algorithm/function for dot crawl should be enabled
with the NTSC Composite output only. It will have no effect for PAL or SECAM
outputs.
100381B
Conexant
3-17
CONEXANT [ CONEXANT SYSTEMS, INC ]
