for a parabolic waveform generation is parabola ax 2 bx c where variables a b c and z are calculated as follows from the user defined values for v1 v2 and v3 where vgd vertical grid distance represents a vertical image dimension measured in scan lines which may have values between 11 and 63 during setup the parabola amplitude coefficients v1 v2 and v3 are adjusted in conjunction with focus control r f to achieve optimum overall crt focus data representing coefficient v4 is read from ram 115 and formed into digital word v4 by compensation data generator 140 data switch 150 provides selection between parabola data from generator 130 and compensation data representative of a fixed or dc value from generator 140 switch 150 is controlled by a vertical rate signal vrt to select dc compensation data during the vertical retrace period and parabolic waveform data for the active part of the field period the function coefficient v4 value will be explained with reference to fig2 a 2 b and 2 c in fig2 a parabolic signal vpar is illustrated with coefficient v4 having two different values namely v 4 and v 4 u2032 shown with a dashed line fig2 b and 2c depict signal vpar of fig2 a coupled via summing amplifier 180 and capacitor c 3.