CRT Scanning: Cathode Ray Tubes work by rapidly painting a phosphorescent screen with a tightly focused electron beam. The screen can be evenly plated with a single color phosphor in a monochrome monitor or can be plated with tiny color dots which are scanned simultaneously by separate electron beams. (See Dot Pitch article).

All CRTs used as computer monitors have some common properties including a Horizontal Scan frequency, a Vertical scan frequency, a Refresh Rate, and interlacing. Horizontal scanning is done very quickly as the electron beam is swept across the screen thousands of time thousands of times a second. The Vertical scan is much slower -- hundreds of horizontal scans. Vertical scan frequency is typically a few dozen times a second. Refresh time is the time required to completely repaint the screen.

Interlace is a technique used in older and less expensive CRTs to reduce cost and power requirements by painting alternate lines on alternate scans. Interlace was developed in the mid 20th Century to allow NTSC television to produce acceptable displays of movies and sports despite the fact that available vacuum tube technology couldn't support sixty full repaintings of the screen per second with reasonably priced equipment. The solution was to interlace 2:1 with a 60Hz vertical scan rate and a 30Hz refresh rate. Most people do not perceive enough flicker in the resulting display to complain. A full image with all the lines is a Frame. A single vertical scan is a Field.

Although most display time is used to actually shoot electrons at the screen, some time has to be allowed for the electron beam to move back at the end of the scan. The electron beam is turned off during these "blanking" intervals. The signal used to drive the monitor typically uses the refresh intervals to send synchronization pulses that keep the scanning beam matched to the desired modulation. Horizontal and Vertical synchronization are discriminated by the length of the synch pulse. In the case of Television, the Vertical synchronization interval is far longer than is really needed. Additional undisplayed information has been hidden in that timeframe. Monitors may send synchronization on different wires than the signal or may only include synch pulses on one color.

Return To Index Copyright 1994-2002 by Donald Kenney.