The Patterns
Pluge Low
This is a very simple pattern used to set the brightness
control. There are four vertical bars on a background that is
reference black (code value 16 in 8-bit video). There is are
two bars set to levels below reference black and two set to
levels above reference black. In a system where 100%
represents reference white, and 0% represents reference black,
the four bars are set to -4%, -2%, 2%, and 4%, counting left
to right. The idea of the pattern is to set the brightness
control until the two left bars are not visible and the two
right bars are visible.
The original PLUGE generator used only -4% and +4% bars on a
black background, but that left a fairly wide range of
settings that would look correct. Absolute precision was not
considered as important as having a pattern that could be used
quickly and easily. Adding a -2% and +2% bar makes the pattern
more precise, but a little harder to set, because when the -2%
bar disappears the +2% is often just barely visible. Obviously
you can just use the outer +/-4% bars and get the same
precision as the older pattern.
There is also a background
checkboard of rectangles that are at digital levels 16 and 17.
Level 16 is reference black, so level 17 is just barely gray.
When everything is set properly, the checkboard should be
either invisible or just barely visible, and then only after
letting your eyes get completely adjusted to the dark. On a
single-chip DLP projector, if you look closely at the screen
when everything is set correctly, you’ll see faint moving
noise in the 17-level blocks, and nothing in the 16-level
blocks. That’s a good thing, and tells you your black level is
right on the money.
This pattern (and others on the disc) has the edges of the
bars aligned to compression block boundaries. Compression
blocks (actually “macroblocks” for the picky) are 16x16 pixels
for all common video compression formats. We do this for all
the patterns on the disc where it makes sense to do so. It’s
worth taking a moment to talk about why.
One of the hardest things for a video compressor to encode
properly is a hard edge. Video compression is designed to be
most efficient at compressing smooth areas of the image. When
a compression block contains a hard edge, it requires many
bits for the encoder to ensure that the edge continues to be
clean and sharp, not soft, and has no extraneous “mosquito
noise” around it. Whereas an edge that is on the boundary of
two blocks is effectively “free.” It costs nothing to encode,
and can’t be ruined by multiple encode/decode passes, as long
as the image isn’t shifted or cropped. It’s not strictly
necessary to align edges on block boundaries; it is possible
to get clean edges anywhere in the image by assigning enough
bits to the frame. But this lets us build in a little extra
insurance, so no matter how the pattern gets encoded,
transcoded, etc. it should stay clean.
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Pluge High
This pattern is the same as PLUGE Low, but with a bright
window on the screen. This lets you check whether your display
will hold the black level constant when it is showing a bright
image. Not all displays will. Note that because of the bright
reference, your eyes may have more trouble seeing the PLUGE
bars. Block out the bright area with your hands and let your
eyes get used to the ambient level before trying to evaluate
the display’s black level retention.
The edges in this pattern are aligned to compression block
boundaries.
Contrast
This pattern is one
of several on the disc that are useful for multiple purposes.
We call this a contrast-setting pattern, and it’s quite useful
for setting contrast properly, but there are other handy
things about it.
In the days when CRTs were the dominant display, contrast was
usually set to the highest level that didn’t cause geometric
distortion of the image or “blooming,” where bright areas
would visibly get larger on screen. Clipping was not a common
problem, because CRTs just don’t clip until wildly overdriven.
With modern fixed-pixel displays, neither geometric distortion
nor blooming is a problem. The principal problem is clipping,
because they have a hard limit on the brightest level they can
display.
The common advice is to set the display so that values above
reference white are clipped, similar to the way the black
level is set so values below reference black are clipped. This
is wrong. As mentioned earlier, CRTs never clip under normal
circumstances, so all the values above reference white will be
reproduced on a CRT, and CRTs are still the standard reference
used by professional video technicians. So even though it
makes the image just a little bit darker overall, the best
advice is to set contrast so all of the above-reference bright
values are visible.
When contrast is set correctly, all of the light-gray numbered
bars near the bottom of the screen should be visible except
the rightmost one or two. Theoretically the last one should be
near-impossible to distinguish from the background, but in
practice often the next-to-last will also be impossible to
distinguish from the background, even when everything is set
properly. It’s best to turn contrast down until as many of the
bars as possible are visible, then turn it up until one of the
rightmost bars disappears, then turn it down one notch. You
want the contrast to be set to the highest level possible that
doesn’t clip any of the picture information.
In addition to the contrast-setting bars at the bottom, there
are also bars at the top that can be used to check the
brightness setting, as well as check that your player is
passing all the below-reference black information. If you turn
up brightness, you should be able to make all the black bars
visible. Once you’ve verified that, you’ll want to turn the
brightness back down until only bars 18 and higher are
visible, or go back to the PLUGE pattern and reset brightness.
The ramps are good for checking that your brightness and
contrast settings are being accomplished with sufficient
precision to avoid posterization. As you adjust contrast
and/or brightness up and down, the ramps should stay smooth
and clean and shouldn’t ripple, sparkle, or show any real
change. If they do, or they start to look like distinct
vertical bands instead of smooth gradients, that’s a sign that
the display is doing integer arithmetic to apply the
brightness, contrast, and other adjustments, and not using
sufficient precision. On some displays, different settings for
brightness and contrast produce different levels of
posterization. If the ramps look best with the brightness or
contrast a notch or two above or below the theoretically
perfect setting, our advice is to use that setting, because
posterization is much more visible than small amounts of
clipping.
The edges in this pattern are aligned to compression block
boundaries.
Color Bars
This is a classic
pattern used since the earliest days of color TV. It’s used to
adjust the color and hue settings to compensate for any
differential in amplitude caused by analog circuitry between
the broadcaster and the TV. Nowadays with digital video (and
better controls on analog components) that essentially doesn’t
happen, so it’s rare that you even need to adjust these
controls. But this pattern can also be used to verify that
your display is properly converting YCbCr to RGB without any
fudge factors. It used to be common to modify the color
decoder (the component that converts from YCbCr to RGB) so
reds would get redder. This makes skin tones “pop” on screen.
This is less common on modern TVs, but if you want to check
such things you need a color bar pattern.
To check color decoding, you need to be able to turn off
individual RGB channels, or a very good set of colored
filters. Colored filters must be chosen carefully and must
block out the majority of light from the unwanted channels. In
general it’s not recommended to use filters to adjust or check
color decoding unless you have high confidence that the
filters are excellent.
If you are using colored filters,
you can check that each filter is good by looking at the bars
that turn dark when looking through the filter. All the dark
bars should be completely black. If they show any color at all
or are not the same brightness level, the filter is not
blocking the two color channels it’s supposed to block, and
that filter should not be used for fine-tuning color or hue.
If the color or hue settings are wildly wrong, then such a
filter will allow you to improve on the factory settings, but
they will not get you the very best possible settings.
When looking at the color bar pattern with only the red,
green, or blue channels turned on (or through red, green, or
blue filters), you should see that some bars in the top and
second row disappear (different bars for each channel) and
some bars turn solid red, green, or blue. The brightness of
the remaining bars (just in those two rows) should be
identical. Bars that touch each other should look identical,
though there may be a thin line or border between the two
bars. To adjust color and hue, you can use any single channel,
though blue is traditional. To check color decoding, you first
need to adjust color and hue using one channel, then check
that everything looks correct for the other two. If you can’t
get all three channels to look correct at the same time, then
the color decoder is “fudged.”
The edges in this pattern are aligned to compression block
boundaries.
HD Color Bars
This is a newer
color bar pattern with an extra ramp to check for
posterization, and a white reference bar below the main bars
to use as a reference for color and hue checking. There is
also a more precise PLUGE pattern in the lower right (see our
notes on the PLUGE pattern for the reasoning).
The edges in this pattern are aligned to compression block
boundaries.
Sharpness
Sharpness is
important. Having just the right amount of sharpness in a
display is essential for getting the most pleasing picture,
but oversharpening is just as bad as undersharpening. Unlike
brightness, contrast, color, and hue, there is no “perfect”
level for sharpness, because it is at least partially
perceptual and the perfect amount of sharpness in an image is
entirely dependent on a host of factors including display
technology, screen size, and how far you normally sit from the
screen. The perfect sharpening is the level that makes real,
normal edges look clean and sharp without adding visible halos
or stairstepping.
One problem with some sharpness patterns out there is that
they have a certain level of jaggies or halos because of
processing artifacts in the software used to produce the
pattern. It’s hard to tell the difference between artifacts
added by the display and artifacts inherent in the image. Our
sharpness pattern has perfectly smooth, artifact-free edges.
The edges were antialiased using the best oversampling we
could generate, so on a display with no extra sharpening they
should look smooth and clean even up close to the screen. As
you turn up the sharpness control on the display, you should
start to see faint halos around the lines. The diagonals will
start to look jagged. Turn sharpness up until the screen looks
harsh and oversharpened when viewed from your regular seating
position, then turn it down again just until all the lines and
curves look smooth again.
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Clipping
“Clipping” is when
the display (or player) shows the brightest parts of the image
as a single uniform brightness. So if you had pixels at levels
252, 253, and 254, they would all look like the same color
and/or brightness instead of three different colors or
brightnesses. Clipping can happen in the luma (brightness)
channel, in video color space, or it can happen in the red,
green, or blue channels in display color space. This pattern
allows you to see at a glance if there is any clipping going
on in any of the four spaces, and in some cases will tell you
which device is doing the clipping.
When the display is not clipped, you should see concentric
squares of different brightness. If you see a solid rectangle,
or a few concentric squares with a large square in the center,
that shows clipping.
If luma is not clipped, but R, G, or B is clipped, that tells
you that the display is receiving the entire video signal, but
is converting it to out-of-range red, green, and/or blue
levels. You can attempt to remedy this problem by turning down
contrast until all of the channels are not clipped. If
reducing contrast doesn’t resolve the clipping, then the
display is fundamentally incapable of displaying the full
range of all the color channels.
If luma is clipped, but R, G, and B aren’t clipped, your first
assumption should be that contrast is set too high. Turn down
contrast and see if the clipping goes away. If it doesn’t go
away, then the player or display is clipping the high luma
range. To figure out whether it’s the player or the display
generally requires either test equipment or temporarily
substituting a different player and/or display.
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Image Cropping
Not every
player or display actually shows you every pixel that is
encoded on the disc. This pattern will show you how much of
the image is missing. It also allows you make sure the image
you’re seeing is centered on the screen. And as a bonus it has
a quick gamma check in the center. The gamma check can only be
used effectively if you are using a 1080p display with no
clipping.
If the display is clipping no pixels, you should
see a white line at the outermost edge of all of the numbered
boxes around the edge of the screen. If some of the boxes are
missing the white line near the edge of the screen, that tells
you that the edge of the image is clipped off. The
largest-numbered box that is missing a white line tells you
how many pixels are missing on that edge of the screen.
If there are an uneven number of pixels clipped between the
left and right, or between the top and bottom, you may want to
use the shift controls on your display (if any) to move the
image so there is equal clipping on both sides and equal
clipping on the top and bottom.
Assuming your display is 1080p, and there is no clipping of
the image and the image stretches completely from edge to edge
of the screen, and the black and white checkerboard in the
middle of the screen looks clean and moiré-free, you can check
the gamma of your display by checking if the checkerboard
pattern in the center of the screen has the same apparent
brightness as the gray background of the rest of the screen.
If the pattern is brighter or darker than the background, your
display is not set to 2.5 gamma.
If your display is not 1080p, or the image is scaled in any
way, (which would show as moiré and stripes in the
checkerboard), then you can’t check the gamma using this
pattern.
Chroma Alignment
This pattern
is useful for checking all of the following:
• Horizontal alignment of the luma and chroma channels
•
Vertical alignment of the luma and chroma channels
•
Horizontal alignment of the red, green, and blue channels
•
Vertical alignment of the red, green, and blue channels
•
Chromatic aberration of the display lens (if the display has a
lens)
The problem with most chroma alignment patterns (sometimes
called Y/C delay patterns) is that they usually ask you to
visually line up a colored edge and a black-and-white edge,
which is nearly impossible to do accurately. Chroma and luma
have different precision because of 4:2:0 color encoding, so
the chroma channel doesn’t necessarily have a sharply defined
edge. Depending on the specific upconversion technique used,
the chroma edge may vary visually as much as a pixel while
still being “correct.” There is no standard for chroma
upsampling, so “correct” upsampling is not something that can
be judged completely objectively.
In trying to solve this problem we hit on the idea of using
symmetry. In this pattern, when chroma and luma are aligned
the left and right sides of every object in the pattern will
be a perfect mirror image of each other. If the chroma is
shifted up, down, left, or right relative to the luma, the
various picture elements will no longer look symmetrical. This
effect works no matter what chroma upsampling approach is
used.
One other problem with judging chroma alignment is that if the
R, G, and/or B alignment is off it throws off the chroma
measurements. Similarly, any visible chromatic aberration in
the lens can make the chroma look misaligned. So we added the
thin crosshairs in the center and at the corners.
• If any of the crosshairs have colored fringes rather than
looking solid white, that shows that there is either an RGB
convergence issue or there is a chromatic aberration issue.
• If the display doesn’t have a lens, chromatic aberration
cannot be a factor. Only rear- and front-projector displays
can have chromatic aberration.
• If the display is a
single-chip DLP, there cannot be convergence issues, so it
must be chromatic aberration.
• If the fringes are not
present in the center, but are present in the corners, and the
colors are spread out diagonally toward the corners of the
screen (i.e. along imaginary lines radiating from the center
of the screen) that suggests chromatic aberration in the lens.
• If the display is a three-panel LCD projector, and the
colored fringes are spread out diagonally in a clockwise or
counterclockwise direction (i.e. perpendicular to imaginary
lines that radiate from the center), that suggests that one of
the panels is slightly rotated relative to the other two.
•
If the fringes are spread out in the same direction on all
five crosshairs, that suggests that one or more of the panels
are shifted up, down, left, and/or right relative to the
others.
• On a flat-panel LCD or plasma display, if you get
very close to the display the crosshairs will appear to have
blue fringes on the left and red on the right, or vice-versa.
This is normal and is just the way the panel is designed.
If your display has aberration or convergence issues, then
only chroma shifts that are larger than the underlying
convergence or aberration issues in that part of the screen
are clearly and unambiguously luma/chroma misalignment.
Dynamic Range High
This is
essentially just the high range of the Contrast pattern. It’s
handy when you’re validating dynamic range with a waveform
monitor, because there are fewer distracting elements. For
setting and validating contrast visually, this pattern and the
Contrast pattern are equally useful.
Dynamic Range Low
As with the
other Dynamic Range pattern, this pattern is here primarily
for use with a waveform monitor.
11 Step Crossed Gray Scale
This pattern is useful for checking output levels using a
waveform monitor. For visual setting and validating of
brightness and contrast, the Contrast and PLUGE Low patterns
are better.
Luma Multiburst
The multiburst
pattern was originally used in analog broadcast to
characterize and measure the high-frequency rolloff that
happens in all analog transmission and processing. With modern
all-digital signal paths, it’s rare that the multiburst is
useful. But it does come in handy when you know your display
is scaling the image, and you want to see how the scaling
affects the image.
Each burst has a wavelength that is an even number of pixels.
The thinnest lines are a 2-pixel wavelength, meaning that the
sine wave takes two pixels to go through a single cycle. The
next thinnest are 4 pixels wide, and the next are 6 pixels,
and so forth.
If the display is scaling the image down
from 1080p to 720p (or a similar resolution like 768p), you
should expect that the highest frequency multibursts (the ones
with the thinnest lines) should turn an even gray shade. The
next highest should still be visible, though they may have
some visible variation in line thickness. All the rest of the
bursts should look clean.
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Chroma Multiburst
This pattern
is the equivalent to the Luma Multiburst pattern, but for
chroma. Because chroma is upsampled when converting 4:2:0
YCbCr to RGB, in a sense every display scales this pattern,
even a native 1080p display. In some cases the player scales
the chroma vertically to produce 4:2:2 YCbCr, and then the
display scales it horizontally while converting to RGB. Or the
player may output RGB, in which case the player is doing the
chroma scaling. For players that have different output options
over HDMI, you can change them while viewing this pattern and
see the differences in how the player and the display handle
the chroma channel.
The individual sine waves in the center
of each burst should look even, with no color shifts or
intensity shifts on either side of the peaks. The waves should
look smooth and not posterized.
If the display is a 720p display, it should be possible for it
to show all of the bursts cleanly, though the thinnest lines
may look somewhat uneven. If the last burst turns gray, it
tells you that the display is scaling the image down in YCbCr,
which produces a slightly lower quality image than converting
the 1080p signal to RGB and then scaling it to the display
resolution.
Luma Zone Plate
This pattern
is in a sense a variation on the multiburst. The entire screen
is turned into a large frequency sweep, covering every angle
and frequency of interest. It’s a good way to check at a
glance overall luma reproduction, and it’s not a bad pattern
to check focus with.
It is not possible to reproduce all of the highest frequencies
cleanly at every angle, but the centermost 1/3 of the screen
or so should look smooth and clean, with the contours getting
more and more stairstepped and less smooth as you move toward
the edges of the screen. There will be false circular contours
toward the edges of the screen looking like ghostly echoes of
the center of the screen – if these contours look egg-shaped
or oval, or if the false contours take up more than 1/8 of the
screen height, something bad is happening to the image. The
far edges of the screen should have a very small, almost
unnoticeable falloff in brightness. If the brightness falloff
is large and easily seen, that’s not as good.
If the display is a 720p display, the outer edges of the
screen should turn gray, and the transition should be smooth.
If there are large bright or dark patches near where the sweep
turns gray, that’s bad.
Chroma Zone Plate
Like the
chroma multiburst, this is a good pattern to check overall
chroma reproduction and scaling. The center third or so of the
screen should be clean and smooth, and you should still see
full color all the way out to the edges (at least on a 1080p
display). On a 720p display, it is possible to get full color
all the way to the edge. If the edges of the screen turn gray,
that suggests that the 1080p->720p scaling is happening early,
before the conversion to RGB, which is less optimal.
Chroma Upsampling Error
This
pattern is designed to make it easy to check if the chroma
upsampling being used in the MPEG decoder has the (in)famous
chroma upsampling error, or “chroma bug.” For more details
about the error and the underlying causes, read the article we
wrote about it on the Secrets of Home Theater and High
Fidelity web site:
http://www.hometheaterhifi.com/volume_8_2/dvd-benchmark-special-report-chroma-bug-4-2001.html
If the player is upsampling the chroma properly, the diagonal
lines should look smooth and clean. If it isn’t, the diagonals
will be jagged and have visible zigzags when viewed up close.
You can also use the chroma zone plate to look for chroma
error, but most people find it easier to spot on this pattern.
Geometry & PIP Geometry
These
patterns are designed to check that the display is properly
set to 16x9 ratio, and that 4x3 content is being rendered
without distortion in the center of the 16x9 area. The circles
and squares should be the same height and width everywhere in
the image. If they are not, you’ll want to adjust the image
scaling, which may be in the service menu of the display.
Deinterlacing Clips
Source Adaptive
These clips
test the ability of the deinterlacer in the display and/or
player to detect film-sourced content and reconstruct the
original film frames. Both the wedge pattern and the
real-world content were chosen to show clearly when the
deinterlacer is in film-reconstruction mode and when it is in
video-deinterlacing mode.
In film-reconstruction mode, the wedge should have minimal
moiré, and you should be able to see individual black and
white pixels at the tip of the horizontal wedge. In the
racecar clip, the bleachers should have no distracting moiré.
When the deinterlacer drops to video mode, you’ll see lots of
moiré in the wedge and/or the bleachers, and the tip of the
horizontal wedge will either turn gray or flicker.
All of the pulldown cadences are real cadences that come up in
real film-sourced content, and all of them are “fair” in the
sense that there is enough information for the deinterlacer to
detect that the frames are originally from a film source. The
very best deinterlacers will stay in film mode continuously
through all of the test sequences.
To create the various film-to-video pulldown patterns, we
created our own pulldown generating software that we fed
perfect progressive content into on one end, and got back the
exact cadences we needed on the other. We used the same
software to generate unusual edits where the pulldown sequence
is interrupted, or a section of it is skipped over as a result
of an edit in the video domain.
Edge Adaptive
These clips are
primarily for testing the ability of a deinterlacing chip to
upconvert video content to progressive while minimizing
jaggies and stairstepping. The very best chipsets use an
edge-sensitive scaling method when they need to take a single
field and scale it to a full frame. These patterns will tell
you if your deinterlacing chip is doing so, and how well it
manages to reduce jaggies on edges without adding extra
artifacts that weren’t there in the original image.
The mixed film and video test clip is designed to test how
well the deinterlacer handles content where video material
(the scrolling text) is overlaid over film-sourced content.
Some deinterlacers will detect the 2-3 pulldown pattern of the
film material and use film-reconstruction mode, which will
cause combing artifacts in the scrolling text. The best
deinterlacers will recognize that portions of the image are
not film-sourced and correctly use video deinterlacing
techniques for the whole frame.
