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New-type CR Tube
The RCA colour-television system operated by a sampling process. The three-colour pictures are broken up into a series of dots by sampling them at appropriate intervals. They are then mixed and the combined signal is a form of time-division pulse modulation. At the receiving end the three groups of pulses are sorted out by gating circuits and so allocated to three separate colour channels.
In the apparatus previously described the three signals were applied to three separate CRTs to produce pictures in the three colours, which were then superimposed optically to give the full colour picture. This presents considerable difficulty and special single tubes are now being developed.
In the first of these there are three separate guns to which the colour signals are applied, but a common deflection system operates to deflect all three beams together. The screen is made up of a mosaic of fluorescent material in the three colours, arranged as groups of three dots - red, green and blue. At the back of this there is a perforated disc with as many holes as there are groups of three dots on the screen.
In order to reach the screen each electron beam must pass through a hole and the disc and screen are so aligned that each beam can land only on the dot of its own beam. Thus, a blue-signal beam passes through a hole to land on a blue dot. The red-signal beam will pass through the same hole to land on the red dot because it approaches at a different angle. Similarly, with the green beam.
A second type of tube has a single gun only but the same form of colour dot fluorescent screen and perforated disc. Here it is not necessary to separate the colour signals and the receiver output can be applied directly to the tube to modulate its electron beam. The sorting out process is done by so deflecting the beam that, at the appropriate times, it is directed on to the correctly coloured dots of the screen. This is done by a small circular deflection of the beam additional to the normal scanning deflection. This circular deflection produces at the appropriate times a change in the angle by which the beam approaches the perforated disc and so simulates with one gun the effect obtained by three in the other.
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