Background information 23
APPENDIX A:BACKGROUND INFORMATION
Clamping Inside the display, the video signal from the video
input is connected to the video amplifier. This
circuit amplifies the signal, paying special
attention to two signal levels: the amplitude and
the dc-level. The dc-level of the video signal in
the video amplifier determines the eventual black
level of the image on the picture tube (CRT).
The circuits of the video amplifier can pick up
interferences from different ambient sources.
These interferences can change the dc-level of
the video signal. In the end, the black level of the
image on the CRT will be incorrect and unstable.
This is avoided by special clamping circuits inside
the video amplifier. They clamp a particular part of
the video signal to a constant dc-voltage. In this
way, the video dc-level and thus the image black
level is kept constant.
In a color display, clamping is normally done on
the backporch. This is the part of the video signal
that follows the sync pulse. But if the video
signal's backporch is too small for clamping,
another part of the signal has to be chosen. In
that case, sync tip clamping has to be selected.
In the MGD 2621 P, clampig is best (and default)
done on the sync tip.

Luminance Uniformity

Correction

A characteristic (or limitation) of every picture
tube (CRT) is that the luminance decreases
towards the edge of the screen surface. The
decrease is normally 20 to 30 %.
This is caused by the shape of the picture tube.
Inside the CRT, a so-called electron gun shoots
an electron beam towards the front (the glass
panel). Because this panel is rather flat instead of
having a spheric shape, the electron beam has to
'travel' a longer distance in the corners than in the
center. So the intensity is higher in the center.
This phenomenon is even increased by the
irregular distribution of the phosphor and
aluminium layer on the glass panel. These tend to