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BT861KRF

Multiport YCrCb to NTSC / PAL / SECAM Digital Video Encoder

Conexant Systems 

Bt860/861

Multiport YCrCb to NTSC/PAL /SECAM

2.0 Inputs and Timing

2.2 Digital Video Ports

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 OSD[7:0]/P[7:0]

Data

4-bit Alpha

ALPHA[1]

ALPHA[0]

2-bit Alpha

ALPHA[1]

ALPHA[0]

1-bit Alpha

ALPHA[1]

ALPHA[0]

2-bit Content-

based Alpha

OSD[1]/P[1]

OSD[0]/P[0]

Cb0 Y0 Cr0 Y1 Cb2 Y2 Cr2 Y3

A[3]0 A[1]0 A[3]1 A[1]1 A[3]2 A[1]2 A[3]3 A[1]3

A[2]0 A[0]0 A[2]1 A[0]1 A[2]2 A[0]2 A[2]3 A[0]3

A[1]0

A[0]0

A[1]1

A[0]1

A[1]2

A[0]2

A[1]3

A[0]3

A[0]0

A[1]0

A[0]0

A[0]1

A[1]1

A[0]1

A[0]2

A[1]2

A[0]2

A[0]3

A[1]3

A[0]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

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