Sencore VP300 - Manual (Page 10)

Page 9
contrast/picture control is adjusted for a higher white level than the display device is capable of
producing properly, one of two distortion effects will be observed; blooming or raster distortion.
Blooming is especially prevalant in CRT displays and results in light from very bright pixels
(phosphors) spilling over to adjacent pixels. This causes bright picture areas to become
defocused and slightly larger than they should be. If a display blooms at high luminance levels,
as the contrast control is adjusted to maximum, the top, white block in this pattern’s gray scale
will become slightly wider than the gray blocks below it.
Raster distortion at high luminance levels are caused by high voltage power supply regulation
problems. If the contrast/picture control is adjusted for a higher white level than the display
power supply is capable of fully supporting on a short-term basis, the black lines on the bottom
of the pattern will hook or bend outward.
Ideally, for an acccurate picture, the contrast/picture control should be adjusted just below the
point at which either blooming or raster distortion is evident. This may result in an unacceptably
low white level for the picture, however, and a compromise white level adjustment may have to
be made between an accurate picture and a bright picture.
Staircase
Pattern Description: Eleven equal-width vertical bars step from black at the left (7.5 IRE for
NTSC, 0 IRE for all other formats) to 100 IRE white at the right.
Pattern Usage: Useful for visually checking gray-scale tracking performance of a display. Poor
performance is seen as a primary color tint (red, green, or blue), especially at the bright or dark
end of the pattern. Also useful for adjusting gray-scale tracking by eye, when a color analyzer is
not available. Adjust the display cutoff/bias controls for a neutral dark gray tone at the dark end
of the pattern. Adjust the display drive/gain controls for a neutral bright gray tone at the bright
end of the pattern. Also good as a reference input signal when troubleshooting and signal tracing
luminance or color amplifier problems within a display device.
Horizontal Staircase (H Stair)