BT848 ETC, BT848 Datasheet - Page 40

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

F
Video Scaling, Cropping, and Temporal Decimation
30
UNCTIONAL
Cropping Registers
D
ESCRIPTION
The Horizontal Delay Register (HDELAY) is programmed with the delay be-
tween the falling edge of HRESET and the rising edge of ACTIVE. The count is
programmed with respect to the scaled frequency clock. Note that HDELAY
should always be an even number.
number of active pixels per line of video. This is equivalent to the number of scaled
pixels that the Bt848 should output on a line. For example, if this register contained
90, and HSCALE was programmed to downscale by 4:1, then 90 active pixels
would be output. The 90 pixels would be a 4:1 scaled image of the 360 pixels (at
CLKx1) starting at count HDELAY. HACTIVE is restricted in the following man-
ner:
cluding blanking, sync and active regions. Therefore:
tween HRESET and the first active pixel. Because the front porch is defined as the
distance between the last active pixel and the next horizontal sync, the video line
can be considered in three components: HDELAY, HACTIVE and the front porch.
See Figure 16. When cropping is not implemented, the number of clocks at the 4x
sample rate (the CLKx1 rate) in each of these regions is shown below:
HDELAY = [(CLKx1_HDELAY / CLKx1_HACTIVE) * HACTIVE] & 0x3FE
becomes:
NTSC
PAL/SECAM
The Horizontal Active Register (HACTIVE) is programmed with the actual
For example, in the NTSC square pixel format, there is a total of 780 pixels, in-
When scaled by 2:1 for CIF, the total number of active pixels is 390. Therefore:
The HDELAY register is programmed with the number of scaled pixels be-
The value for HDELAY is calculated using the following formula:
CLKx1_HDELAY and CLKx1_HACTIVE are constant values, so the equation
In this equation, the HACTIVE value cannot be cropped.
HACTIVE + HDELAY Total Number of Scaled Pixels.
HACTIVE + HDELAY 780.
HACTIVE +HDELAY 390.
NTSC:HDELAY = [(135 / 754) * HACTIVE] & 0x3FE
PAL/SECAM:HDELAY = [(186 / 922) * HACTIVE] & 0x3FE
CLKx1
Front Porch
21
27
L848A_A
CLKx1
HDELAY
135
186
CLKx1
HACTIVE
754
922
Single-Chip Video Capture for PCI
Bt848/848A/849A
CLKx1
Total
910
1135
Brooktree
®