BT848 ETC, BT848 Datasheet - Page 43

• Manufacturer Part Number: BT848
• Description: Single-Chip Video Capture for PCI
• Manufacturer: ETC

Bt848/848A/849A
Single-Chip Video Capture for PCI
Brooktree
The Hue Adjust Register
The Brightness Register
Register (CONTRAST)
The Saturation Adjust
The Contrast Adjust
Registers (SAT_U,
®
(BRIGHT)
SAT_V)
(HUE)
Video Adjustments
The Bt848 provides programmable hue, contrast, saturation, and brightness.
The Hue Adjust Register is used to offset the hue of the decoded signal. In NTSC,
the hue of the video signal is defined as the phase of the subcarrier with reference
to the burst. The value programmed in this register is added to or subtracted from
the phase of the subcarrier, which effectively changes the hue of the video. The hue
can be shifted by plus or minus 90 degrees. Because of the nature of PAL/SECAM
encoding, hue adjustments can not be made when decoding PAL/SECAM.
The Contrast Adjust Register (also called the luma gain) provides the ability to
change the contrast from approximately 0% to 200% of the original value. The de-
coded luma value is multiplied by the 9-bit coefficient loaded into this register.
The Saturation Adjust Registers are additional color adjustment registers. It is a
multiplicative gain of the U and V signals. The value programmed in these regis-
ters are the coefficients for the multiplication. The saturation range is from approx-
imately 0% to 200% of the original value.
The Brightness Register is simply an offset for the decoded luma value. The pro-
grammed value is added to or subtracted from the original luma value which
changes the brightness of the video output. The luma output is in the range of 0 to
255. Brightness adjustment can be made over a range of –128 to +127.
Automatic Chrominance Gain Control
The Automatic Chrominance Gain Control compensates for reduced chrominance
and color-burst amplitudes. Here, the color-burst amplitude is calculated and com-
pared to nominal. The color-difference signals are then increased or decreased in
amplitude according to the color-burst amplitude difference from nominal. The
range of chrominance gain is 0.5–2 times the original amplitude. This compensa-
tion coefficient is then multiplied by the Saturation Adjust value for a total chromi-
nance gain range of 0–2 times the original signal. Automatic chrominance gain
control may be disabled.
L848A_A
F
UNCTIONAL
Video Adjustments
D
ESCRIPTION
33