Bt860/861
2.0 Inputs and Timing
2.2 Digital Video Ports
Multiport YCrCb to NTSC/PAL /SECAM
2.2.5 Alpha Pin Blending
The ALPHA[1:0] pins are used to select the amount of blending per pixel when
BLENDMODE = 1. The pins are sampled at the system clock rate and samples
during both luma and chroma components may be captured to create 1-, 2-, or
4-bit blend factors. For 1- and 2-bit blend modes, the multiplier LUT (in registers
ALPHA_LUT_0 through ALPHA_LUT_3 is programmed with user-defined
multiplier values.
In 1-bit blend mode, the ALPHA[0] pin indexes registers ALPHA_LUT_0
and ALPHA_LUT_3 to generate the multiplier value. In 2-bit blend mode, the
ALPHA[1:0] pins are used as a 2-bit index for registers ALPHA_LUT_0 through
ALPHA_LUT_3.
In 4-bit blend mode, the four bits required are captured in successive load
clocks from ALPHA[1:0]. The two LSBs of the 4-bit value are latched during the
luma portion of the overlay data load, and the two MSBs are latched during the
chroma component load. These four bits provide a direct multiplier for the
blending module. Figure 2-2 illustrates the alpha blending timing diagram.
Figure 2-2. Alpha Blending Timing Diagram
System Clock
8-bit Overlay
Data
Cb
Y
Cr
Y
Cb
Y
Cr
Y
3
0
0
0
1
2
2
2
OSD[7:0]/P[7:0]
A[3]
A[2]
A[1]
A[0]
A[3]
A[2]
A[1]
A[0]
A[3]
A[2]
A[1]
A[0]
A[3]
A[2]
A[1]
A[0]
ALPHA[1]
ALPHA[0]
0
0
0
0
1
1
1
1
2
2
2
2
3
3
3
3
4-bit Alpha
2-bit Alpha
A[1]
A[1]
A[1]
A[1]
ALPHA[1]
ALPHA[0]
0
0
1
1
2
2
3
3
A[0]
A[0]
A[0]
A[0]
A[0]
A[0]
A[0]
A[0]
ALPHA[1]
ALPHA[0]
1-bit Alpha
0
1
2
3
A[1]
A[0]
A[1]
A[0]
A[1]
A[0]
A[1]
A[0]
OSD[1]/P[1]
OSD[0]/P[0]
2-bit Content-
based Alpha
0
0
1
1
2
2
3
3
NOTE(S):
1. Shaded areas indicate which video components are affected by each multiplier or index.
2. A blank data packet means this data carries no alpha information.
861_026
2.2.6 Content-based Blending
Content-based blending uses the two LSBs of the overlay byte associated with the luma
pixel to address the multiplier lookup table (registers ALPHA_LUT_0 through
ALPHA_LUT_3). This method is selected by setting BLENDMODE = 0, and is a
convenient means of using blending when no alpha pins exist from the overlay device.
D860DSA
Conexant
2-5
CONEXANT [ CONEXANT SYSTEMS, INC ]
