Chapter 3
Hardware Overview
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National Instruments Corporation
3-7
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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).
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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.
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Field—For an interlaced video signal, a field is half the number of
horizontal lines needed to represent a frame, as shown in Figure 3-3.
The first field of a frame contains all the odd-numbered lines. The
second field contains all of the even-numbered lines.
Figure 3-3.
Video Field
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Frame—A frame is a complete image. In interlaced formats, a frame is
composed of two fields.
Programming Video Parameters
You can program all of these video parameters on the 1408 device:
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Programmable pixel and line count—When generating an internal
PCLK, the 1408 device uses a programmable clock generator with an
advanced VCO circuit. This circuitry generates a PCLK frequency
from 11 to 16.5 MHz, depending on the horizontal count. The standard
sampling rate for RS-170/NTSC video signals is 12.3 MHz, and the
standard sampling rate for CCIR/PAL is 14.75 MHz. To correct for
external pixel aspect ratio errors of up to ±9%, the VCO covers the
range from approximately 11.0 to 16.5 MHz.
Vertical Sync
Line Region
Horizontal
Sync
Video
Line
