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Chapter: Television and Video Engineering - Essentials of Color Television

Dynamic Convergence Adjustments

The display unit consisting of picture tube and deflection unit is inherently self-converging. However, small adjustments become necessary and are provided. For this purpose two types of four-pole dynamic magnetic fields are used.

DYNAMIC CONVERGENCE ADJUSTMENTS

 

The display unit consisting of picture tube and deflection unit is inherently self-converging. However, small adjustments become necessary and are provided. For this purpose two types of four-pole dynamic magnetic fields are used.

 

One is generated by additional windings on the yoke ring of the deflection unit. It is energized by adjustable sawtooth currents synchronized with scanning. The other type of dynamic field is generated by sawtooth and parabolic currents which are synchronized with scanning and flow through the deflection coils. (i) Line symmetry. Figure (a) shows a situation in which the plane where the beams are converged automatically is slightly tilted with respect to the screen plane due to some small left-right asymmetry in the distribution of the horizontal deflection field.

 

 As a result, horizontal convergence errors of opposite signs occur at the sides of the screen. The same type of error can be caused by a horizontal deviation of the un deflected beams from the screen center. As shown in Fig. (b) such an error can be corrected by a 4-pole field aligned diagonally with respect to the deflection fields. This field is generated by driving a sawtooth current at line frequency through an additional four-pole winding provided around the core of the deflection yoke.

 

The sawtooth current is obtained directly from the line deflection circuit. (ii) Field symmetry. As illustrated in Fig. vertical displacement of the plane of the beams with respect to the center of the vertical deflection causes horizontal convergence errors during vertical deflection.

 

These errors can be corrected by feeding a rectified sawtooth current at field frequency through the additional 4-pole winding on the deflection unit. (iii) Line balance. Vertical displacement of the plane of beams with respect to center of the horizontal deflection field causes cross-over of the horizontal red and blue lines.

 

This illustrated in Fig. The same type of error can also be caused by top-bottom asymmetry of the horizontal deflection field. It can be corrected by a four-pole field which is aligned orthogonally with respect to the deflection fields.

 

Such a field is generated by unbalancing (see corresponding figures) the line deflection current through the two halves of the horizontal deflection coils acted with respect to its normal orientation, a parabolic vertical convergence error occurs during both horizontal and vertical deflection (Fig.).

 

This error can be corrected by feeding a parabolic current at line frequency through the line deflection coils.(v) Field balance at top and bottom. Left-right asymmetry of the vertical deflection field or horizontal deviation of the un-deflected beams from the screen center causes vertical convergence errors during vertical deflection.

 

This is illustrated (Fig) separately for top and bottom of the screen. The correction at the top is made by unbalancing the field deflection coils during the first-half of the field scan. Similarly correction at the bottom is made by unbalancing the field deflection coils during the second-half of the field scan.













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