Raster Scan - Theory and History

Theory and History

In a cathode ray tube (CRT) display, when the electron beams are unblanked, the horizontal deflection component of the magnetic field created by the deflection yoke makes the beams scan "forward" from left to right at a constant rate. The data for consecutive pixels goes (at the pixel clock rate) to the digital-to-analog converters for each of the three primary colors. (For modern flat-panel displays, however, the pixel data remains digital.) As the scan line is drawn, at the right edge of the display, all beams are blanked, but the magnetic field continues to increase in magnitude for a short while after blanking.

To clear up possible confusion: Referring to the magnetic deflection fields, if there were none, all beams would hit the screen near the center. The farther away from the center, the greater the strength of the field needed. Fields of one polarity move the beam up and left, and those of the opposite polarity move it down and right. At some point near the center, the magnetic deflection field is zero. So, therefore, a scan begins as the field decreases. Midway, it passes through zero, and smoothly increases again to complete the scan.

After one line has been created on the screen and the beams are blanked, the magnetic field reaches its designed maximum. Relative to the time required for a forward scan, it then changes back relatively quickly to what's required to position the beam beyond the left edge of the visible (unblanked) area. This process occurs with all beams blanked, and is called the retrace. At the left edge, the field steadily decreases in magnitude to start another forward scan, and soon after the start, the beams unblank to start a new visible scan line.

A similar process occurs for the vertical scan, but at the display refresh rate (typically 50 to 75 Hz). A complete field starts with a polarity that would place the beams beyond the top of the visible area, with the vertical component of the deflection field at maximum,. After some tens of horizontal scans (but with the beams blanked), the vertical component of the unblank, combined with the horizontal unblank, permits the beams to show the first scan line. Once the last scan line is written, the vertical component of the magnetic field continues to increase by the equivalent of a few percent of the total height before the vertical retrace takes place. Vertical retrace is comparatively slow, occurring over a span of time required for several tens of horizontal scans. (In old analog TVs, typically, setting brightness to maximum made the vertical retrace visible.)

In analog TV, originally it was too costly* to create a simple sequential raster scan of the type just described with a fast-enough refresh rate and sufficient horizontal resolution, although the French 819-line system had better definition than other standards of its time. To obtain a flicker-free display, analog TV used a variant of the scheme in moving-picture film projectors, in which each frame of the film is shown twice or three times. To do that, the shutter closes and opens again to increase the flicker rate, but not the data update rate. *Practical video (similar to digital dot clock) bandwidth, including transmission bandwidth