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Moiré
The textbook definition
Moiré patterns are a natural interference phenomena that appear in all color CRT
displays. Moiré patterns appear as ripples, waves, and wisps of intensity variations that
are superimposed on the screen image. They are most noticeable on high resolution displays
that have finely focused beams.
Moiré
Moiré patterns are a double-edged sword because you need to worry about them whether
or not you see them. If you don't see them, it's possible that your display is producing a
fuzzy image, which is the case in most improperly adjusted or poor quality displays. If
you do see them, they're annoying.
A Moiré pattern arises because the pixels that are generated by the video board cannot
be perfectly aligned with the phosphor dots or stripes on the screen. For some pixels the
CRT beam hits the screen phosphors dead center, and produces a bright pixel, and for
others it hits off-center and produces a dimmer pixel. The Moiré pattern is actually a
map of their alignment over the screen. For color televisions, a Moiré pattern can arise
from the color subcarrier.
Moiré patterns are not a defect in the monitor, but rather result from a practical
limitation in display technology. In order to completely eliminate Moiré patterns, the
dot or stripe pitch on the monitor would have to be significantly smaller than the size of
a pixel, which is generally not possible.
How do you get rid of Moiré
- If your system has a Moiré Reduction Control similar to those in computer monitors,
then you can use it to reduce or eliminate the patterns. Just make sure, you write
your default settings before you change it.
- Some folks have gotten electronic fixes (firmware or software) from TV manufacturers but
results have been spotty at best.
- Slightly defocusing the image via the mechanical focus of your set appears to be the
best bet. This has been proven effective in various TVs. Be careful not to
over do-it though so that you don't significantly degrade image sharpness. See below why/how this works.
The Simpler Explanation
Tip from Don Landis: "Moire is a
phenomena related to the physical distance between the scan lines in the raster (fixed by
your screen size) to the picture's image lines such as pin striped clothing worn by the
talent on camera.
When I shoot a particular person on camera that is wearing pinstripped clothing and I
know this will moire on certain viewer's sets I sometimes elect to shoot the scene just
ever so slightly out of focus to reduce the moire.
Besides moire there is one other phenomena that can be cured with defocusing the
picture and that is where you have a thin horizontal line in the picture of contrasting
color, like a thin horizontal black line on a very light background. This line will
vibrate as it tries to find the odd or even scan line to display itself. Defocusing will
cure this too.
Defocus can be done either in the camera or at the monitor. Doing it at the camera
eliminates it from the get-go.
Another trick some cameramen will use is a tiffen soft effects filter on their lens
when shooting talent wearing clothing that will moire. This will work most of the
time."
What? Defocus
the Picture to rid of Moire?
In responding to a Toshiba cinema series owner's question, video specialist Michael TLV gives a great explanation on why this
fix works:
"The fix is kind of funny in that it is and it isn't a fix. The moire problem
shows up not just on Toshiba sets, but many other tube sets as well including RCA/Hitachi
...
It is a function of the tube and its operating characteristics. The fix is to ...
locate the focus control and slightly defocus the picture. I'm talking about a marginal
defocusing ... nothing major.
Now before one jumps up and down screaming bloody murder about defocusing ... let me
get back to the thing about the tube's operating
characteristics.
If we image the focus of the set on a scale of 1 to 100 where 100 is max focus and 1 is
min focus ... the optimal operating point of the TV is actually not at 100, but something
less ... more like 96 ...
The problem is ... when the set is at 100% focus, other flaws in the system start to
show up ... like the moire pattern. So what the manufacturer does is to slightly defocus
the set while on the assembly line ... so that it goes into the box at the "96"
point and no moire is visible.
You open up the box, plug in the TV and the image looks great. The fact that the tube
is at 96 is completely transparent to the end user.
The problem here is that the focus adjustment is done by people on the assembly line
and sometimes someone might set the TV at 100 by mistake instead of 96. So when you take
the set and watch it, you see the moire pattern and wonder if there is something wrong
with the set.
Now whether you reset the tv to 96 or the factory does it, the result is the same. Tube
design is not perfect and probably never will be. Every set has its trade offs.
The fix for the cinema series tubes is to defocus the tube. If you actually get down to
it, you will see that it really is such a minor defocusing. But the moire will show up on
other tubes too.
If you choose to go the defocus route, make sure you do not wear any dangling metal
chains while poking around in the TV ...
Total fix time ... 30 sec. after you take the cover off.
Another Possibility
I experienced this very phenomenon at one time. But it wasn't moire. It
was radio frequency (rf) interference!
The cables I was using at the time was not shielded or was not shielded well. It
caused the wavelike patterns on the screen.
How did I find out about this? Our resident expert, Michael TLV, suggested I
remove all components attached to the set. I did. Next, I connected the OTA
antenna. No problem. I then reconnected the DVD using my "home-made"
component cables. Checked out the set and right away the wavelike patterns
returned. Michael suggested I replace the cables with the thin/flimsy cables that
came with the player. I did and guess what? The patterns were gone.
Hence, it was not moire at all. It was
interference due to unshielded or poorly shielded cables.
BTW, moire technically doesn't affect projection sets. It only applies to direct
view sets per Michael TLV. I can try to explain why that is but it is best that you
contact Michael TLV instead. He can
articulate it better. In fact, he is to be credited for making a manufacturer aware
of moire problems with their line of directview sets.
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