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Chapter 3
Hardware Overview
©
National Instruments Corporation
3-7
PCI/PXI-1408 and NI-IMAQ for Win95/NT
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Active pixel region—The active pixel region is the region of pixels
actively being stored. The active pixel region is defined by a pixel
start (relative to HSYNC) and a pixel count.
•
Horizontal count—The horizontal count is the total number of
pixels between two HSYNC signals. The horizontal count
determines the frequency of the pixel clock.
•
Active line region—The active line region is the region of lines
actively being stored. The active line region is defined by a line
start (relative to VSYNC) and a line count.
•
Line count—The line count is the total number of horizontal lines
in the picture.
•
Video line—A video line consists of an HSYNC, back porch, active
pixel region, and a front porch, as shown in Figure 3-2.
Figure 3-2. Video Line
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Picture aspect ratio—The picture aspect ratio is the ratio of the
active pixel region to the active line region. For standard video
signals like RS-170 or CCIR, the full-size picture aspect ratio
normally is 4/3 (1.33).
•
Pixel aspect ratio—The pixel aspect ratio is the ratio between the
physical horizontal size and the vertical size of the region covered
by the pixel. An acquired pixel should optimally be square, thus the
optimal value is 1.0, but typically it falls between 0.95 and 1.05,
depending on camera quality.
Horizontal Sync
Pixel Region
Back Porch
Front Porch