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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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