cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 67

• Manufacturer Part Number: cx25840
• Description: Video Decoder And Broadcast Audio
• Manufacturer: Conexant Systems, Inc.

CX25840/1/2/3 Data Sheet
3.5.6
3.5.7
3.5.8
102284B
8/3/05
Vertical Cropping
Scaling and Cropping Effects on SPI
Horizontal Cropping
To horizontally crop an image, the values of HBLANK (0x470 and 0x471) and
HACTIVE (0x471 and 0x472) are adjusted. To maintain proper image location, the
cropped values of HBLANK and HACTIVE should sum to less than or equal to the
value of HBLANK + HACTIVE without cropping. To crop the left side of an image,
increase HBLANK by the number of pixels desired. To crop the right side of the
image, decrease HACTIVE by the number of pixels desired.
To vertically crop an image, the values of VBLANK (0x474 and 0x475) and
VACTIVE (0x475 and 0x476) are adjusted. To maintain proper image location, the
cropped value of VACTIVE plus (VDELAY) should be less than or equal to the value
without cropping.
The units of VBLANK and VACTIVE are half lines. To crop the top of the image,
increase VBLANK by the number of desired lines multiplied by two. To crop the
bottom of the image, decrease VACTIVE by the number of desired lines multiplied by
two.
The decoder has the following external timing signals available, which are affected by
scaling and cropping as described below:
Vertical Sync: Not affected by scaling or cropping
Horizontal Sync: Occurs at the beginning of each line and is not affected by
horizontal scaling
Vertical Active: This signal is used to determine when the analog active video is
valid. When vertical scaling occurs, this signal goes to its inactive state for the
lines that should not be stored into the video memory. When image cropping is
taking place, the length of Vertical Active is adjusted to provide fewer lines per
field.
Horizontal Active: This signal is used to determine when the analog active video is
valid. Since horizontal video is always scaled, Horizontal Active contains more
clocks than the pixels programmed into the Horizontal Active register. This signal
is affected by the value programmed into the scaling register. When image
cropping is taking place, the length of Horizontal Active is adjusted to provide
fewer valid pixels per line.
Valid Pixel Indicator: To determine which clocks are valid for storage into a video
buffer, the Valid Pixel signal is high for valid pixels and low for invalid pixels. The
Valid Pixel Indicator toggles throughout the entire line time to include blanking
and synchronizing intervals even with cropping. This signal is affected by the
value programmed into the scaling register.
Odd/Even Field Indicator: Not affected by scaling or cropping
Cb/Cr Indicator: Informs the user whether the pixel is Cb or Cr. When invalid
pixels are encountered, the Cr, Cb order is therefore maintained from valid pixel to
valid pixel, as flagged by this signal pin. This signal is affected by the value
programmed into the scaling register.
Conexant
Detailed Functional Description
3-41