[argyllcms] Using Argyll to calibrate a Rec.709 HDTV CRT monitor

  • From: Technical Operations <tech@xxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 4 Oct 2012 18:11:14 -0700

Hello! First postâ after some 50 hours of reading on CMS in general and Argyll 
in particular.

I've been using dispcalGUI and ArgyllCMS and love what they offer, from Rec.709 
curves with ambient light compensation to the handy HTML reports.  I was 
wondering if I could use a similar workflow to calibrate our HD CRT video 
monitor.


Equipment Setup
================
Sony PVM-20L5 HD CRT with HD-SDI input
Blackmagic Design Decklink HD Extreme 3D+ card sending 4:2:2 10-bit YUV in 
"extended desktop" mode
Mac OSX 10.6.8
ArgyllCMS 1.4.0 with dispcalGUI 1.1.0.0
XRite i1 Display Pro
XRite DTP94 (MoncaoOptix)


1. The goal was to measure the conformity of the monitor to Rec.709, and if 
needed improve accuracy via the built-in hardware controls.  Profiling or any 
outboard calibration was out of the question, since if anything in the world is 
designed to conform to Rec.709, it's this kind of device. (And Rec.709 was 
created for this kind of device.) If it can't be calibrated in the box we need 
to throw it out and get something elseâ

Being a proper professional CRT, the monitor has legit brightness, contrast, 
chroma and phase controls. It also has true gain and bias RGB controls, in 
1,000 steps each. Fun :-)


2. First I wanted to evaluate how accurate that "extended desktop" mode was.  I 
fed a test pattern to an external scope, and compared it to a proper native 
feed (from software that's designed to output Rec.709 YUV via the card).  The 
white and black points were correct, but the gamma was different.


3. I used dispcalGUI's hardware calibration interface, doing Iterative 
adjustments of the white point and black point. dispcalGUI was set to D65 white 
point, 120 cd/m^2 white level, Rec.709 tone curve. Measured ambient light was 4 
lux.
White point and brightness were a joy to adjust easily to within 0.4dE.
Black point was much less stable, with the readout cycling between a dE of 1 
and 3.

4. Verification command:
dispcal  -v2 -d3 -c1 -yc   "-P0.48762541806,0.512977099237,1.49647887324"   -X 
CRT.ccss  -t6500.0 -b120.0 -g709 -f1.0 -a4.0 -k1.0 -E


Test Results
================

i1DP
1. Factory D65 preset
Brightness is 108.510325 cd/m^2, should be 120.000
White point error = 9.206717 deltaE
Maximum neutral error (@ 1.000000) = 9.203313 deltaE
Average neutral error = 5.441553 deltaE

2. User preset (after calibration)
Brightness error = 0.356962 cd/m^2 (is 120.356962, should be 120.000000)
White point error = 1.599242 deltaE
Maximum neutral error (@ 0.149337) = 9.059615 deltaE
Average neutral error = 3.322566 deltaE

3. User preset w/ CRT CCSS file
Brightness error = 0.612392 cd/m^2 (is 120.612392, should be 120.000000)
White point error = 4.643833 deltaE
Maximum neutral error (@ 0.149337) = 9.074183 deltaE
Average neutral error = 4.120487 deltaE

4. User preset w/ CRT CCSS file, recalibrated
Brightness error = 2.711093 cd/m^2 (is 122.711093, should be 120.000000)
White point error = 0.244133 deltaE
Maximum neutral error (@ 0.149337) = 8.585793 deltaE
Average neutral error = 3.418107 deltaE

5. Same as #4, with Auto Color
The monitor features an auto chroma&phase setup function, which calibrates 
color when you feed SMPTE bars to the monitor. This feature moved the white 
point to 4.4dE and the average dE to 4.46 (measured with i1DP).

6. Same as #4, with DTP94 probe
Brightness error = 4.850000 cd/m^2 (is 124.850000, should be 120.000000)
White point error = 1.073658 deltaE
Maximum neutral error (@ 0.149337) = 8.666491 deltaE
Average neutral error = 3.784474 deltaE
Number of measurements taken = 100


Questions
===========

1. Applying the CRT CCSS correction changed the white point target by 3dE. Is 
this expected?

2. Black level calibration read at 0.58cd/m^2 while the calibration interface 
called for 1.07. Why 1.07 when I specified "native"? (BTW, the target black 
level for Rec.709 is under 0.05cd/m^2).

3. Without additional hardware, should I use this calibration at all - can I 
trust the result to any degree - or am I better off doing it the way it's 
always been done - by eye (with a SMPTE chart and blue-only mode)?

4. EBU Tech paper 3320 specifies tolerances for reference video monitors, based 
on dE*uv in CIE1976. (The main formula is âE*uv = â(âL*Â + âu*Â + âv*Â)  if it 
passes through the mailing list correctlyâ).  Comparing it to Argyll's 
measurements would be handy, the math is a bit beyond me.
http://tech.ebu.ch/webdav/site/tech/shared/tech/tech3320.pdf


Thanks in advance,

Drew Lahat
Precision Post
Los Angeles, CA


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