The deflection yoke’s electromagnets are used to alter the path of the electron beam in a CRT. The job of synchronising the influence exerted by each of the yoke’s two magnets, and so controlling the directing of the beam towards specific parts of the screen, is handled by some electronics known as the deflection circuit.
The deflection circuit has to make complex adjustments to the amount by which the electrons are deflected, depending upon whereabouts on the screen the beam is being pointed. One reason for this is that the distance the beam travels before it hits a spot on the screen increases the further the spot is from the screen’s centre. The further the beam has to travel, the longer the magnets have to maintain their influence on it; the longer they maintain their influence, the further the beam is pulled away from the centre. The deflection circuit has to make allowance for this or the result would be heavy pincushion distortion of the on-screen image.
There are other variables which also have to be catered for by the deflection circuit, such as the varying time for which the beam is influenced by the anode and the curvature of the inside of the tube’s faceplate, which affects the length of the beam’s path. These same factors also make necessary other dynamic adjustments which can’t be handled by the deflection circuit, which is looked at in beam focusing.