CRT Gurus: how does Gain/Bias/Drive/etc. affect light output? Formula needed.

Hi,

I have finished the first draft of my blend calibration software and I need to know how G2 and DRIVE (in Marquee terms) affect light output.

I understand generally speaking light output is:

L = K * (b + c *x) ^ G

Where:

L -> Light output
K -> Max value (would be a factor a Marquee's CONTRAST, I guess)
b -> Brightness (black level offset)
c -> Contrast
x -> IRE (from 0..1)
G -> Display gamma

What I need to know is how G2/DRIVE affect the above formula.

The reason I need this is b/c I am writing an automatic calibration software for CRT blends and I've found that the regular mode to do grayscale tracking on a Marquee (iterate thru 30/70 IRE readings) does not yield a very good BLUE curve - and I want to further correct this by using the ability to individually control SUB-GAIN bias introduced in latest marquee versions.

My software currently does:

- Measure light output for all 3 channels (R G B) from ~10 IRE to 100 IRE taking 20 samples in total (configurable). With this it calculates the gamma curves for R G B. Measuring low-IRE output is possible using a digital camera. I wrote the software to do this using a DSLR camera in RAW mode, so I read 16-bit readings off the camera's CMOS sensors.. Each reading takes ~30 secs, but the process is fully automated (you need two computers, the one measuring/controlling and a pattern generator server which I also wrote).

- It synthesizes a gamma curve assuming different low/mid/high gamma, and it creates an ideal blend curve based on that. I am using my Blendzilla right now, and I've gotten the best blend I've managed to get.

I'll post some screenshots of the software if you are interested. I also contacted TV-ONE in case they are interested in this software working for their units.

rgs - Andres

Athanasios - right now I am just measuring curves and synthesizing a blend curves. I have already a prototype proof of concept which communicates with the PJs via RS232. For the sub-brite to work I have to do some hacking on the RS232 protocol (sniff it, basically) as the sub-brite controls are not documented. I *think* sub-brite controls are available beyond 5.0.

Tim - thanks for the info - what I am looking forward is a formula that relates G2 and drive.

I think I've read that current intensity in the cathode is:

Ic = k * Vcut ^ e * Vd ^ Y

where e+Y = 3/2

Vcut is the cut-off voltage which is related to G2, Vd may just be the drive, but i am not sure.

Next weekend I will take some measurements changing G2/drive, then changing BRITE to see how they affect.

I think BRITE is just the DC offset, but I want to know how G2 and drive interact, and how G2 and drive are related to BRITE and CONTRAST.

At this point I don't. One of the benefits of having this system is that it would allow the recalibration of the CRTs as often as you'd like - so if there is aging and G2 needs to be upped as a result... the SW will do it for you.

I don't - but it is not needed. I would expect the equations for have some parameters that vary with aging, and I would expect those parameters to have a range.

What I am looking for is a general idea to determine if:

- G2 affects the slope of the gamma curve, if so, how - or does G2 just OFFSET the gamma curve.
- Drive affects low end, or is a multiplier to the overall curve.

I think I am just going to:

Set G2 to say 60. Measure from 10..100
Get G2 to say 65. Measure again, see how the curves relate

Same for drive.

Thanks Mark.

Andy - yes, you are right in that it could also be used with digitals - especially with regards to calibration.

I am in talks with TVONE to see if they are interested in adding gamma correction on all three R G B channels using (uploaded) gamma curves. Having R G B gamma correction is a very powerful feature but honestly, without a way of automatically setting it up, calibrating it manually using manual touching of the curves would make the offering relevant to hard-core tuners, whereas the automatic reading/calibration would make grayscale calibration accessible to a broader consumer audience.

Those gamma correction curves would be generated using the luminance readings for each channel... and now that I think of it, it could also be used to achieve perfect grayscale tracking for digitals & CRTs, using 256-point gamma correction in an automated way, for any projector.

PM and email sent.

I asked this to tse and he's confirming that G2 ~ offset, DRIVE ~ multiplier:

--
Perhaps overly simplified:

DRIVE x signal = light output

G-2 + signal = light output

BUT, DRIVE is scaled such that minimum is not "0" nor is 50% one half 100%. The green DRIVE only makes a few percent change in green output when changed from min to max. Red and blue DRIVE have alot more effect.

The sub brite adjustments do the same as G-2. Sub-brite + signal = light output. BUT the sub-brite controls can crush the video at the black end or the bright end. Unless you have a scope to check the video crush it is best to leave the controls at midrange. Use G-2 for offset adjustments.

Hope that helps.
--

Which is good news in that at least now we know more, but bads in that you cannot really control the SHAPE too much with these parameters, hence you need an X-point curve correction (a Lumagen or better yet R G B gamma compensation in the blend box) to achieve PERFECT grayscale tracking and hence perfect blend.

That's one reason I started this. The calibration process is manual and time-consuming. I want to streamline it, so if I need to recalibrate it due to aging, I'd just leave the software & PJs doing the auto-calibration.

Athanasios - I agree re: B defocus.

Stupid question: when you do electronic defocus to gain light output on blue do you need to change static focus UP or DOWN? (well... I guess the question is mostly for the stock yokes as people with modded frankenyokes may have reversed polarity of static focus inadvertently).

Thanks William.

BTW:


sounds SO FUNNY! :-)