HD Monitor Calibration - SMPTE, ARIB, and White Balance — NegativeSpaces - Ben Cain
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Transcript of HD Monitor Calibration - SMPTE, ARIB, and White Balance — NegativeSpaces - Ben Cain
ABOUT CONTACT BLOG STILLS VIDEO FAQ
monitor calibration -smpte, arib, and whitebalance
I haven't spent a whole lot of time revisting old
entries on this site. The most widely read post here
was one written January 8, 2008 about HD Monitor
Calibration using ARIB Bars. It's one of the only
articles on the net that specifically addresses
working with ARIB which is the reason for the traffic.
Because of its continued use, I feel an overhaul
probing a little deeper into the topic is beneficial to
anyone referencing it.
In the original article, the emphasis was on using
ARIB or SMPTE color bars to properly set your HD
display's Brightness (black level), Contrast (white
level), and Chroma (color saturation). I made the
mistake of labeling this "calbration". What this
assumes is that the most important component of
display calibration, White Balance / Color
Temperature, has already been set correctly. Color
bars and color fields do nothing to tell you if your
display's whites are truly white. White Balance or
setting your monitor's Red, Blue, and Green Gain
levels to the correct outputs to create a pure,
chroma-free white is the most critical aspect of
monitor calibration. The main reason I'm re-
addressing this topic is that if your display's white is
off, "calibrating" it with a test signal such as color
bars doesn't mean a whole lot. Color bars will help
you find the correct white, black, and color saturation
levels but if 100% white input isn't output as 100%
chroma-free white, your display is out of alignment.
First, a re-cap:
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This post is in regards to HD monitor calibration
only. There are several issues relating to standard
definition video and monitoring that do not apply to
HD:
1. NTSC Setup, or 7.5 IRE (%) Black Level. Setup is
for standard definition only. The black portion of HD
test signals hit 0 IRE (%) on the waveform.
2. Phase: There is no Phase control for digital HD
monitoring. Only CHROMA (saturation) affects
picture as Phase relating to monitoring is an analog
issue only.
COLOR BAR TEST SIGNALS:
Many cameras and recording decks generate color
bars - either the HD SMPTE (Society of Motion
Picture and Television Engineers) version, which is
this:
Or the newer HD specific version, ARIB (Association
of Radio Industries and Businesses):
Correct use of these test signals will help you set
your monitor's brightness, contrast, and choma
levels. You can use either one of these
interchangeably and one is not necessarily more
accurate than the other. The process of using either
of these color fields is virtually identical.
WHITE BALANCE AND GAMMA:
This post is only concerned with display calibration
for Rec709, which is for now, the standard
colorspace for HD broadcast. Rec709 uses a
defacto gamma of 2.2 and a white point of D65
(Daylight 6500 kelvin). Very recently the ITU
released Rec1886 which specifies a gamma of 2.4.
The topic of which gamma is the "correct" one is
much larger than this post so for simplicity, let's
assume we're all using the widely implemented
gamma of 2.2. Many monitors have preset choices
for both gamma and white balance. In this standard
Rec709 calibration scenario, you're going to set your
gamma to 2.2. For white balance, you may find
choices such as D50, D65, and D93 but not all
monitors will actually allow you to manually adjust
Red, Blue, and Green Gain to create your own
custom white point. If you do have a monitor that
allows for custom alignment, the only way to do this
is with an external probe that will measure the output
levels and help you find the correct RGB mix to
display a pure, chroma-free white. Additionally some
displays, namely OLED's and newer LCD's, also
require an adjustment for Bias which is similar to
Black Balance to produce neutral, chroma-free dark
tones. If your display does not allow for a manual
RGB White and Bias, set White Balance to D65 and
move on to the next step. Your monitor cannot be
truly aligned beyond what was deemed "calibrated"
at the factory.
AN ANALYSIS OF THE HD SMPTE COLOR FIELD:
The components of this test signal are 75% Contrast
Color Bars (Yellow, Cyan, Green, Magenta, Red,
Blue), 20% Blue Chip, 10% Purple Chip, 75%
Contrast White Chip, 100% Contrast White Chip, 0%
Black chips, and The Pluge. When using this test
signal to set Brightness, Contrast, and Chroma, all
you really need to concern yourself with are the 75%
Color Bars, 100% White Chip, and Pluge. The 20%
Blue Chip and 10% Purple Chip fall along the IQ Line
on the Vectorscope for verification that the color
information in the test signal is accurately centered
on the scope.
The Pluge will help you set your Brightness (Black
Level). It consists of a -3.5% (IRE) chip on the left,
0% chip in the middle, and 3% chip on the right.
Here are SMPTE Color Bars with lifted blacks so you
can see the pluge better:
Here's the same signal's luma waveform. This helps
to see where everything in the field is hitting in terms
of level, particularly the pluge.
And Vectorscope. You can see that each of the color
bars lines up perfectly with thier targets indicating
that these are pure, undiluted primary and
secondary video colors. If this was a 100% contrast
color field, the vectors would land perfectly in their
little targets; R = Red, Mg = Magneta, B = Blue, Cy =
Cyan, G = Green, Yl = Yellow.
SETTING BRIGHTNESS, CONTRAST, AND COLOR
WITH HD SMPTE BARS:
1. Set your gamma to 2.2 and White Balance to D65.
Set your Brightness, Contrast, and Chroma to their
default levels. If you have a display that can be
custom white balanced, make sure that you're
aligned before starting this. If you're working with a
facility on a project, they can send a technician with
a probe to you and he can do it for you. Or you can
talk to you manufacturer and get a list of
recommended probes for use with your display along
with x,y (Color Temperature), and Y (Luminance)
targets so you can do it yourself. Once you're there,
send the HD SMPTE test signal to your monitor via
HD-SDI. This is best done in a dark environment so if
you're outside or in an unshielded location, try and
keep as much ambient light and direct light off the
display as you can.
2. Everyone has their own way of doing this. All I can
say is that I feel that I've had pretty good results with
this method which involves jumping back and forth
between the different adjustments to find just the
right levels. I start with Contrast which is the most
subjective. Looking at the 100% white chip, turn the
contrast up until it visually stops getting any brighter.
Now back it off a little bit. This will be different on
every display and really the smart way to do is with a
probe that reads Luminance level.
3. By default, increasing Contrast will also somewhat
raise the black level. Now use the pluge and set your
Brightness so that the left (-3%) pluge chip
disappears into the surrounding 0% black field. The
right chip which reads 3.5% on the waveform should
be just barely visible.
4. Now check your contrast again. Is the 100% white
chip still hitting peak white? If you need to adjust,
make sure to go back and check your pluge again.
By going back and forth between these 2
adjustments, you should be able to arrive at a
satisfactory black and white level.
5. Now check Chroma. Your monitor most likely has
some sort of Blue Only feature. This is used to help
you correctly set your color saturation level with
Chroma. Turn it on and have a look.
If it looks something like this, you're in good shape.
If you're looking at something like this:
Adjust the Chroma level on the display until you're
looking at solid, alternating bars of equal value. The
larger top portion of each individual bar needs to
blend into the smaller section beneath it.
6. If you find that you have to make some
adjustments to Chroma, this could up slightly
changing your overall contrast so turn Blue Only off
and check the pluge and 100% white chip again. By
tweaking back and forth between all of these
adjustments, you will be able to find the most
accurate settings your monitor can produce. Please
note that some monitors don't have Blue Only but
can display in Monochrome. The Chroma calibration
process with Monochrome is identical to Blue Only.
At the end of the process, you should be looking at
something like this:
AN ANALYSIS OF THE ARIB COLOR FIELD:
This test signal has a greater variety of components
than it's SMPTE relative. It contains the same 75%
Contrast Color Bars, with the addition of neutral gray
chips at various luma levels, and pluge with more
steps (-2%, 2%, and 4%) that I suppose offer a bit
more finesse for setting black level with Brightness.
Here's a handy diagram detailing what's what in the
ARIB:
Here's the luma waveform of the signal. Note the
0%-100% gradient that ramps through the middle of
the field and the pluge at -2, 2, and 4%.
SETTING BRIGHTNESS, CONTRAST, AND COLOR
WITH ARIB BARS:
Though it looks radically different, on principle it's
the same as the SMPTE. Follow the exact same
steps outlined above when using this signal to arrive
at correct Brightness, Contrast and Chroma levels.
The biggest difference is with the pluge. When using
this signal, the middle (2%) and right (4%) pluge
chips should be barely visible with the the -2% chip
blending into the surrounding 0% black field.
When setting Chroma with Blue Only, this is what
you should be looking at with the correct level:
And when you're all done, this is what you should be
looking at:
SMPTE 100% COLOR FIELD:
Some recording decks will output these but very few
cameras will. I don't think this is as useful a
calibration signal as there's no pluge to help you set
black level. It's good for checking saturation on
displays or if for some reason you needed to check
a 100% signal in a video system. This field contains
the 3 primary video colors (Red, Green, and Blue)
and 3 secondary colors (Yellow, Cyan, and
Magenta) at 100% contrast along with a 0% Black
Chip and 100% White Chip.
On the vectorscope, you can see the colors hit their
100% targets spot on.
When looking at this color field with Blue Only and if
your Chroma level is set correctly, you should be
seeing something like this - bars of equal and
alternating value:
PROBES, MANUAL WHITE BALANCE, AND BIAS:
And now the meat - using a probe to do this right.
The process of manual alignment with a probe is
pretty similar monitor to monitor and there are many
different varieties of probes out there. Some are
incredibly expensive, some are pretty affordable.
Cheaper probes like the X-Rite i1 Display Pro and
Display 2 aren't of any use to us because they
calbrate by building an icc profile that can't actually
be loaded into any professional broadcast monitor.
Some displays like the Cinemage series can use
these probes to automatically align by running a
series of calibration images. That's handy but it
seems to be the exception. Most displays require this
be done manually. To do it, we need to get in there
and adjust our RGB gains to produce a chroma-free
white and the only way to do this is to input a 100%
white test signal into the display and objectively
measure it there. If you're using an OLED monitor
(some LCD's), you'll need to adjust RGB Bias as well
but this process is very similar to adjusting White,
using 20% neutral gray instead of 100% white.
Here's a relatively affordable probe that I like -
Sony i1 Pro EOPRHWK
Sony has released a freeware that lets the i1 Pro be
used as a manual probe which means that it can
calibrate any display that has the functionality. This
isn't the most accurate probe available but it's also
not 10 grand. I've had very good results matching
displays with it. Please note, older probes for use
with CRT's cannot be used on LCD/OLED. All you
need do is contact your display's manufacturer and
get the x, y (Color Temperature), and Y (Luminance)
targets for White and Bias. Most of the LCD's I've
encountered have these targets:
x .313
y.329
Y 100
The x and y points should be the same for both Gain
and Bias. The Y level for Bias will change with the
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3D Circus LUT Translator
February 21, 2012 Ben Cain 4 Comments
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gamma setting be it 2.2, 2.4, or 2.6. Consult your
manufacturer!
The probe can be ordered from B&H and the
software is available through Sony Business along
with instructions on how to use it if you want to try
this yourself. Or contact me and I'll either rent you
the probe or come to you along with the necessary
calibration images and align your monitor for you as
a service.
Update
on 2012-03-01 03:35 by Ben Cain
Please note that there is a real science behind
display profiling that is far beyond the scope of this
article. The information provided here is a very "bare
bones" calibration and not necessarily suited to all
projects. Every display has its own characteristics
and even if a probe is telling you that you're spot on,
results could vary wildly from display to display.
Update
on 2012-03-10 17:50 by Ben Cain
Fixed a typo regarding vectorscope gain. It read
Gain x2 for monitoring, should be Gain x1. Added
some clarification regarding x,y, Y targets for Gain
and Bias.
Thanks, Steve! I've been referencing your
site for years. It's a treasure trove of great
information.
For calibration today a good CMS system -
like our LightSpace CMS - is a good tool to
use, combined with a good probe. The latest
i1 Display Pro for X-Rite is a good one, and a
lot cheaper than previous versions.
Lots of information on calibration on the light
Illusion website - www.lightillusion.com
Obviously I do have a vested interest in the
software as we manufacture it, but we also
provide a lot of support and information for
free.
I have to say the info here is very good as
well!
Preview Post Comment…
Ben Cain
about a year ago
Steve Shaw
about a year ago
By my understanding, Rec709 encodes
gamma at .45 and it's display gamma is 2.2.
.45 x 2.2 = .99. In order to linearize an
image, the target gamma is always a
theoretical value of 1. This of course can be
adapted to emulate different displays which
is why many monitors have selectable
gamma levels such as 2.4.
I was under the impression that 2.2 is the
input gamma for Rec.709, and that there is
no specified output gamma. I've seen it
recommended to use 2.4 to more closely
match the CRT monitors of yore. As
worthwhile as that is in the age of flat
panels...
Ben Cain
about a year ago
Pippy about a year ago
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