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Raster, Image Area, and Internal Test Pattern Centering on Tube Faces
Centering of the image on the tube phosphor surface is done by centering the
raster on tube phosphor and then centering of the actual image within the
raster. You should begin by hooking up and displaying a video source.
Raster:
The area of the tube phosphor that is painted by the electron beam.
Normally,
only part of the raster is actually used to produce the image. The raster can be
seen by peering through the lens after turning down contrast nearly all the way
and then raising brightness to make the normally black raster light up. This
makes the entire raster light up dimly. It may be necessary to open the left,
right, top, and bottom blanking controls to allow visualization of the entire
raster. Size of the raster is adjusted using vertical and horizontal size (aka
amplitude) controls. The rasters are best kept small enough to ensure active
video image is never extended beyond or near the phosphor edges. Keeping at
least 7 to 10 millimeters of unused phosphor on all edges helps prevent the
catastrophic tube failure that will occur if active image is projected beyond or
too close to the phosphor edges. Since the raster is usually larger than the
actual image area and it is actually the energy of the image area that can cause
damage, some installers will allow the raster to extend beyond the edges while
still maintaining image area within the safe portion of the phosphor.
When viewed on screen, the leftmost portion of the raster is drawn first after
the electron beam completes horizontal retrace. Looking into the projection lens
the orientation is backwards. During the first portion of the horizontal
movement the beam has not settled completely and one may see some waviness in
the image if the extreme left edge of the raster is used. Some installers will
intentionally displace the raster slightly leftward so the active image is
displayed on the later, more stable portion of the raster.
Centering the raster is performed by use of centering magnets on the CRT necks
just behind the deflection yokes. The centering magnets are a pair of rings with
small tab handles. By rotating the rings relative to each other and also around
the neck of the tube one can shift the raster about the phosphor surface. Most
projectors also have electronic static position controls for fine adjustment of
the raster centering. It is best to use minimal electronic correction to reduce
strain on the convergence circuitry. One can do so by centering the electronic
controls prior to centering the raster with the centering magnets. On some
projectors (such as NEC XG's), there are no centering magnets and raster
centering is carried out purely with electronic controls for raster centering
hidden in a service menu. Even on such machines it is reasonable to first center
the user centering controls prior to setting the service menu raster centering
controls.
Image Area: The portion of the raster which is actually used to display
the video image
Within the raster, the active video image is displayed. The projector often has
"position" or "image shift" controls which allow movement of the image within
the raster. The name of this control varies from brand to brand. You can verify
you have the correct control by making the raster visible and seeing if the
image is moving about within the raster but the raster is not moving as you use
the control. If raster moves as well as the picture, you are adjusting the
raster position and not the image position within the raster.
Ideally, the image area is centered both vertically and horizontally on the
phosphor surface. You can achieve this by first neutralizing the linearity
controls and then centering the raster relative to the phosphor edges. Then
center the image area within the raster edges. Once both are done, display a
white field pattern from a calibration disc (AVIA, S&V Home Theater Tune-Up) and
verify that the active is centered in the phosphor. We'll cover this in greater
detail later.
BTW, Don't use an internal test pattern for checking centering as they are often
not themselves centered relative to external signals unless you have also taken
the steps described next in this note.
Internal Test Pattern Centering
Built in test pattern generators do not necessarily coincide with actual video
signals in timing. They tend to be off center and not exactly scan locked to
match a real signal. This means that making an internal test pattern look
correct does not necessarily optimize the machine for a real signal. However,
internal test patterns are handy and some machines like NEC's actually require
you to set their timing relative to the video signal and convergence systems to
prior to other geometry and convergence adjustments. If you have a machine which
allows adjustment of internal test pattern timing, it is possible to center the
internal test patterns to coincide with the center of actual video signals. This
is much easier to do if mechanical gun aim has already been accomplished. In the
next section, mechanical gun aim will be covered so skip forward and do that.
Then set internal test pattern centering (phase) as described below.
For example, the phase control of NEC projectors MUST be set before geometry and
convergence adjustments when creating a new input memory. The phase control
changes the timing of the internal test pattern generator and also timing of the
convergence system relative to the video signal. Since you have already centered
the gun mechanical aim and centered the image on the screen and phosphor
surfaces, adjusting phase to make the internal generator centered on screen also
centers it with the video image center. Adjust phase to make the center vertical
line of the internal generator centered left/right on screen. Then adjust phase
to make the deflection blip centered around the center vertical line. The blip
shows the centering of the convergence system. If it makes it easier for you,
display the internal center cross pattern rather than the crosshatch while
adjusting phase. Ignore the odd shape of the lines, just concentrate on the
position of the middle of the center vertical line and the blip.
Since there is no control to shift the internal test pattern vertically, the
only way to achieve vertical coincidence of the image and internal pattern
centers is the image position control. Once that is done, you may need to
readjust projector tilt to get things centered on screen. Going through all
gains you the option of using either internal or external test patterns with
good accuracy.
One should strive to keep the image centered within the phosphor surface and
leave at least 7 to 10 mm of unused phosphor all around the active image.
Mechanical aim of the guns is easier to do when the image and rasters are
vertically and horizontally centered on the tube faces. Further fine tuning of
raster and image centering can be performed as well as centering of the internal
test pattern generator once gun aim is accomplished.
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