3.0 PC Board Considerations
CX25870/871
3.6 Applications Information
Flicker-Free Video Encoder with Ultrascale Technology
3.6.2.1 Contrast
Contrast is a video quality that refers to how far the whitest whites are from the
blackest blacks in an analog video waveform. If the peak white is far away from
the peak black, the image is said to have high contrast. With high contrast, the
image is very pure like a black and white tile floor. If the two parameters are very
close together, the image is said to have poor, or low, contrast. With low amounts
of contrast, an image may be referred to as being washed-out. Instead of easily
recognized black portions of the image versus white parts, the image with low
contrast looks gray.
Register MY[7:0] in conjunction with register Y_ATTENUATE[2:0] controls
adjustment of contrast. Y_ATTENUATE has 8 possible values ranging from 1.0
gain (No attenuation) to 0 gain (Force Luminance to 0). Conexant recommends
inclusion of an 8-level slider to control the Contrast level. Each single movement
of the slider should reprogram this bit field to a different fractional value. Lab
testing has shown that values from ¾ gain (Y_ATTENUATE=011) to 15/16 gain
(i.e., 001) yield the crispest TV picture.
Register MY modifies the luminance multiplier allowing for a larger or
smaller luminance range. For more drastic changes in the Contrast, change MY.
For more subtle changes, shifting the Y_OFF register as the end-user moves the
slider should be sufficient.
Since the difference between contrast and brightness is usually understood by
video professionals only, Conexant recommends the designer increment or
decrement the YATTENUATE[2:0] field for either brightness or contrast
adjustments.
3.6.2.2 Saturation
Saturation is the amount of color present. For example, a lightly saturated green
looks olive-green to gray while a fully saturated green looks like the color of a
pine tree. Saturation does not mean the brightness of a color, just how much
pigment is used to make the color itself. The less pigment, the less saturated the
color is, effectively adding white to the pure color.
The amount of Saturation is controlled by the bit field named
CATTENUATE[2:0]. CATTENUATE has eight possible values ranging from 1.0
gain (No attenuation) to 0 gain (Force Chrominance to 0). Conexant recommends
inclusion of an 8-level slider to control Saturation level. Each single movement of
the slider should reprogram this bit field to a different fractional value. Lab
testing has shown that values from ¾ gain (CATTENUATE=011) to 1.0 gain (i.e.,
000) yield the crispest TV picture.
3.6.2.3 Brightness
Brightness is defined to be the intensity of the video level and refers to how much
light is emitted from the display. The amount of Brightness is controlled by the
register named Y_OFF[7:0].
Y_OFF[7:0] is a 2s complement number, such that a value of 0x00 is 0 IRE
offset, a value of 0x7F is an increase of 22.14 IRE above black level. Active video
will then be added to the offset level set by the Y_OFF value.
Since the difference between contrast and brightness is usually understood by
video professionals only, Conexant recommends the designer increment or
decrement the YATTENUATE[2:0] field for either brightness or contrast
adjustments.
3-16
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
