Nf-ID: #N:nemesis:-62134:000:5406
Nf-From: nemesis.UUCP!uhclem    Nov 15 12:07:00 1992


<>
[0]  I have a 27" Toshiba TV, and I've noticed a green spot on the right
[0]side of my screen, and a red spot on the left side of my screen, after
[0]playing my Super Nintendo for a period of time.
[0]
[0]It is especially noticable after playing Street Fighter II.
[0]
[0]  This isn't caused by leaving the same image on the screen, since SF2
[0]changes the backgrounds pretty often.

The big question is, if you switch to another source, (broadcast TV, VCR,
or LD), does the "blotch" go away after a while?  If so, I have the answer.

In developing a TV-based entertainment system over the past year, we came
across a similar problem that only showed up on certain titles and TVs.
After a while, portions of the screen would begin to show incorrect colors.
For example, a yellow area would develop a purple or green blotch, a white
area may develop a blue-ish blotch, and so on.

Changing to other material, such as over the air broadcast or even just
turning the TV off for a while, made the problem go away.

Unfortunately, nearly all the TV models we planned to market with this
system exhibited this artifact (same manufacturer).  So the TV maker was
brought in and came up with an interesting explanation.   We expected to hear
that the problem was some sort of effect caused by static screen and the
magnetic field of the built-in speakers, but they said that wasn't the
primary cause.

The problem is caused by overheating of the shadow mask, particularly those
tubes with a steel shadow mask, rather than some other material.  When an
extremely bright image falls on the same part of the mask long enough, it
distorts, resulting in a greater or lesser distance between the mask and the
phospher.  This means the electrons from the various guns don't hit the
desired phosphers, and spill over onto adjacent phosphers, resulting in
incorrect colors.

In nearly all cases, the shadow mask straightens itself out the moment the
excessive bombardment is reduced (brightness changes), so going to more
natural images (non computer-generated) will usually correct the problem.

Tubes with dot triads and a steel shadow mask can have more problems than
a steel stripe shadow mask, as the shadow mask absorbs more of the beam
in the dot configuration.  Also, since the red gun is usually turned up
higher than the other guns to compensate for a less-bright red (ok, NEAR red)
phospher, images with more reds & oranges & whites can heat things up faster
than other parts of the color spectrum.

Stationary images are usually worse, but if the brightness levels for all
colors are too high, nearly any color can be used to exhibit the problem.

The steel shadow mask can also be affected by the speakers in the cabinet,
but I am told that there is usually sufficient shielding to prevent the
speakers affecting beam deflection, and that is more than enough to avoid
interaction with the shadow mask.  Whatever.


[0]
[0]  Turning down the brightness on my TV helps somewhat.  The factory
[0]settings for the TV have the brightness all the way up.

This is the instant fix.  Setting the TV brightness, sharpness, color and
contrast settings close to "correct" helps a great deal, and eliminates
the issue on most TVs.  Note that many TV makers (including ----) send their
units out with the brightness and contrast set excessively high, so their unit
will "stand out" on the showroom floor.  (Sorta like moths attracted to light
across the darkened showroom.)  It is assumed the customer will readjust the
settings once they get the unit home.  HA!

On the other hand, not everyone wants to adjust the settings on the TV just
because they want to play a game, and then set them back later so that the
news anchorperson won't look nuclear orange.  So it would be nice if all
devices generated comparable signal levels.

The harder fix is to change the device generating the signal so that the
luminance levels don't come out so "hot".  We discovered that even if you are
right on NTSC spec (100% of what it allows), some TVs still can't cope with
large areas of the screen that are brightly lit for extended periods of
time.   Very little of what comes off the air hits 100% all the time, just
the occasional peak.  But if you lower all levels to avoid 100%, the result
looks muddy.  It would be nice if there was a "nominal" spec, but I haven't
found one.

As an aside, we also discovered many TVs can't deal with these brightness
anyway, as their power supplies are so cheap and don't have enough reserve
capacity.  The result is massive pincushioning and trapezoidal sweep patterns.


[0]  Is this causing any lasting damage to my TV?

From what we have seen, no, but your picture tube mileage may vary.  We went
to a TV showroom and plugged one of the notorious titles into the building
TV system.  Within minutes, every brand you ever thought of had at least one
model that started acting up.  But once the video was back on Donahue, they
all straighted back up.  Needless to say, the offending title was altered.

As with all tubes, don't leave a static image up there for hours at a time,
or you will get phospher burn, and that is permanent.

Frank Durda IV <uhclem@nemesis.lonestar.org>|"The Knights who say "LETNi"
...utacfd!nemesis!uhclem (nearest internet) | demand...  A SEGMENT REGISTER!!!"
...letni!rwsys!nemesis!uhclem	            |"A what?"
				            |"LETNi! LETNi! LETNi!"  - 1983