can be selected at different price points for higher speed
interfaces or more complex functions but also for lower
cost feature sets used in other highly specific applications.
IP cores, such as microprocessors and DSPs, extend the
capabilities far beyond those of traditional FPGA logic.
As the library of video processing function grows, the
value of the IP is realized again and again. As newer and
more capable FPGAs are introduced, the existing code
can be ported to take advantage of speed, power or cost
CRT form, fit, and function interface (F3I) LCD
replacement is expected to continue due to ongoing
military budget constraints that result in service life
extension rather than total replacement. When the
legacy replacement calls for the development of an in-
house LCD controller, the developer should embrace the
challenge and work to reap the benefits of reuse and add
to the organization’s intellectual property portfolio.
FPGA video processing forms the basis for legacy
CRT replacement and provides the IP for low-cost
variants. Coupled with the appropriate front end
(analog, DVI, etc.), this approach can result in a
full-featured, integrated video processing capability
embedded within existing display hardware without
the need for a separate LCD controller. The ensuing
design provides flexibility to address future customized
applications and offers a range of cost and lifecycle
Raster/Stroke Signal Characteristics
Most legacy CRTs use either a raster or stroke video input, and some can switch between modes or even use
both at once. Most engineers are familiar with these increasingly rare signals.
Monochrome raster video uses a constantly moving electron beam for which the X-Y target location on the
display surface is always known. The beam moves horizontally from one edge to the other (raster line) and then
moves quickly back to the next starting position (horizontal retrace). When the beam gets to the last position, it
moves quickly back to the upper starting position (vertical retrace). Depending on the system, the beam either
paints every line or every other line. During the raster line periods the intensity of the beam determines the
brightness, and in many cases, color, of the display at that point. Regardless of the intensity, starting location,
Monochrome stroke video does not use the
horizontal and vertical line and retrace signals,
but instead controls the X and Y location, and
beam intensity, with separate signals. This
allows the control circuit to essentially write to
any part of the display at any time. Common
examples include oscilloscopes and radar/IFF
Plan Position Indicators (PPI).
The vertical refresh interval of the raster signal
is relatively long, and some legacy displays
take advantage of part of the vertical retrace
blanking time to allow stroke signals to control
the electron beam. This hybrid approach allows
the use of stroke graphics overlaid on the raster
picture and is typically used for amplifying
information overlays, such as symbols and text.
The notional timing diagram for a raster/
stroke mode is shown in Figure 3. In this
example, the X and Y deflection, gated by the
stroke intensity, paint a box-shaped object.
Figure 3: Raster/stroke timing diagram and
sample image. (Photo courtesy of IEE, Inc.)