The Williams-Kilburn Principle

This description of the dot-dash method for a Williams-Kilburn CRT Store, is an edited version of one provided by Prof. D.B.G. Edwards.

The principle of the CRT store can be summarised as follows. Bits of information are stored on the insulating screen of a CRT as small areas of electronic charge in an array of isolated dot/dashes. If a metal pick-up plate is placed close to the outer surface of the CRT screen then voltage pulses will be detected when the charge is changed, due to the capacitive coupling. These changes occur successively in time when the storage areas in the array are bombarded by the CRT electron beam in a regular time sequence in each full screen refresh cycle. Their nature is now described.

Suitable X and Y deflection voltages and a short repetitive bright-up signal Z applied to the CRT grid will cause an array of dots to be generated on the screen at a set of fixed positions. If a particular fixed dot position is to store a 0, then the bright-up signal Z is applied precisely to that position, and the screen acquires a positive voltage when it is bombarded with a high energy electron beam due to the fact that the secondary emission ratio of the screen material is greater than 1. Provided that the dot position is bombarded again before the positive potential has had a chance to decay significantly, then there will be no change of charge at the dot position, and the screen holds a 0 in this position.

If a dot position is to hold a 1, the bright-up signal is lengthened so that the dot is extended into a dash. Now the initial positive potential in the precise dot position attracts secondary electrons from the bombardment occurring close to it as the beam moves along the dash, and is rapidly lost. This means that when that dot position is bombarded again on the next refresh cycle it will have to acquire a positive potential and that change of charge from the storage area will provide a positive signal at the pick-up plate during the dot period. So a 1 has been stored and detected, and if it is to be preserved then the Z bright-up signal must be extended to cause a dash to be rewritten.

In practice a transient signal is observed on the pick-up plate when the electron beam is turned on for a dot position and again when it is turned off. These signals are due to the cloud of secondary electrons introduced in the vicinity of the screen when the electron beam is switched and is negative going at switch-on and positive going at switch-off. Therefore both a "screen size" pick-up plate and "dot size" pick-up plate are used, to detect whether the signals are whole-screen related rather than single dot related. If they are whole-screen related then there is no signal due to potential change at the dot position, so the value is a 0, but if they are dot related the potential has changed, so a 1 was stored previously and the beam must be extended along the dash.

Normal leakage from the screen would destroy the storage charges in tenths of a second but refresh rates significantly faster than this read the information and replenish the charges so that information can be retained indefinitely. It also means that long term drifts in the voltage supply, which cause slight movement of the array positions, are not critical to the storage operation.