Figure 8-16.--Effects of dynamic pincushion corrections.

"anti-pincushion" correction yokes. These yokes

The color convergence circuits provide a

secondary control over the electron beam of each gun.

produce a somewhat distorted magnetic field that

Convergence of the three electron beams to exact

compensates for any pincushion error caused by a flat

locations on the face of the three-gun CRT is necessary

screen surface. Another method of correcting

to produce good monochrome and color images.

pincushion error in monochrome receivers is to place

permanent magnets in the drift space between the

Other differences, such as automatic color control,

tuning indicators, and color reception indicators, serve

deflection yoke and the screen. The magnetic field

to simplify the operation of front-panel controls.

produced by each magnet "bucks" pincushion error,

and an undistorted raster is obtained. Although these

Pincushioning

methods work well for single-gun CRTs, more

sophisticated means are required to correct distortion

Modem color receiver design calls for the use of

in three-gun tubes. For example, the magnetic fields

a wide-angle deflection yoke. Wider deflection angles

produced by the permanent magnets would not exert

allows the CRT to be shortened. This allows the

equal force on each gun, due to the triangular gun

cabinet to be made smaller. In an attempt to provide a

construction.

larger viewing area, receivers are made with flat-

Modem color receivers use dynamic correction

surface rectangular CRTs. Unfortunately wide-angle

circuits to modify the height and width of the raster.

deflection with flat-surfaced CRTs causes some

Pincushioning along each side of the raster is

problems. These are bowed scan lines and elongated

corrected by subtracting from the horizontal

corners at the edge of the raster. This distortion,

referred to as "pincushioning," is caused by projecting

deflection width at the beginning and end of the

the raster onto a flat surface and using wide deflection

vertical scan (fig. 8-16, view A). In contrast,

angles.

pincushion error is corrected at the top and bottom of

the raster by adding to the vertical sweep. This

Figure 8-15 illustrates the effects of pincushioning

when a cross-hatch pattern is projected onto the

addition occurs at the center of each line near the top

screen. Notice that the vertical and horizontal lines

and bottom of the raster. When the sweep pulses are

passing through the center of the raster are not

modified in this manner, the pincushion error is

noticeably distorted. As the scan goes away from the

corrected along both sides and the top and bottom of

center, distortion occurs along the top, bottom, and

the raster (fig. 8-16, view B).

both sides of the raster. In effect, the raster becomes

Although many circuits are presently used to

"stretched" at the corners. This stretching is due to the

provide dynamic pincushion correction, the end result

greater distance the electron beams have to travel at

is always the same; that is, predistortion of the sweep

the outer edges.

current waveform. Basically, this involves decreasing

In monochrome receivers, where a single-gun

the change rate of the sweep current as it approaches

CRT is used, geometric distortion can be corrected by

its maximum values.

Figure 8-15.--The effect of pincushion distortion on a

Figure 8-16.--Effects of dynamic pincushion corrections.

cross-hatch pattern.

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