Raster Scan - Description - Scanning Pattern

Scanning Pattern

In raster scanning, the beam sweeps horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line. During this time, the vertical position is also steadily increasing (downward), but much more slowly – there is one vertical sweep per image frame, but one horizontal sweep per line of resolution. Thus each scan line is sloped slightly "downhill" (towards the lower right), with a slope of approximately –1/horizontal resolution, while the sweep back to the left (retrace) is significantly faster than the forward scan, and essentially horizontal. The resulting tilt in the scan lines is very small, and is dwarfed in effect by screen convexity and other modest geometrical imperfections.

There is a misconception that once a scan line is complete, a CRT display in effect suddenly jumps internally, by analogy with a typewriter or printer's paper advance or line feed, before creating the next scan line. As discussed above, this does not exactly happen: the vertical sweep continues at a steady rate over a scan line, creating a small tilt. Steady-rate sweep is done, instead of a stairstep of advancing every row, because steps are hard to implement technically, while steady-rate is much easier. The resulting tilt is compensated in most CRTs by the tilt and parallelogram adjustments, which impose a small vertical deflection as the beam sweeps across the screen. When properly adjusted, this deflection exactly cancels the downward slope of the scanlines. The horizontal retrace, in turn, slants smoothly downward as the tilt deflection is removed; there's no jump at either end of the retrace. In detail, scanning of CRTs is done by magnetic deflection, by changing the current in the coils of the deflection yoke. Rapidly changing the deflection (a jump) requires a voltage spike to be applied to the yoke, and the deflection can only react as fast as the inductance and spike magnitude permit. Electronically, the inductance of the deflection yoke's vertical windings is relatively high, and thus the current in the yoke, and therefore the vertical part of the magnetic deflection field, can change only slowly.

In fact, spikes do occur, both horizontally and vertically, and the corresponding horizontal blanking interval and vertical blanking interval give the deflection currents settle time to retrace and settle to their new value. This happens during the blanking interval.

In electronics, these (usually steady-rate) movements of the beam are called "sweeps", and the circuits that create the currents for the deflection yoke (or voltages for the horizontal deflection plates in an oscilloscope) are called the sweep circuits. These create a sawtooth wave: steady movement across the screen, then a typically rapid move back to the other side, and likewise for the vertical sweep.

Furthermore, wide-deflection-angle CRTs need horizontal sweeps with current that changes proportionally faster toward the center, because the center of the screen is closer to the deflection yoke than the edges. A linear change in current would swing the beams at a constant rate angularly; this would cause horizontal compression toward the center.