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Gamma Correction: What is it? Why is it needed?

 
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kal
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PostLink    Posted: Mon Jun 04, 2007 5:18 pm    Post subject: Gamma Correction: What is it? Why is it needed? Reply with quote


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Gamma Correction: What is it? Why is it needed?


More and more home theater devices are offering gamma correction circuitry. Many of Moome's products now feature a Gamma Correction Circuit (v1.01).

Gamma correction is an essential component to reproducing the best possible image possible. Without gamma correction, dark details (ie: shadow detail) in your projected image will lack detail or simply be missing.


What's this gamma correction add-on card? What does it do? Why would I need it?
Gamma correction is a manipulation to the contrast of the display signal to correct for the fact that the intensity displayed on a CRT is not linearly related to the input voltage. In other words: Without gamma correction, dark areas in picture will simply look too dark or black. A gamma boost corrects for this by increasing the contrast in the darker areas with affecting the overall black level of the picture.

I still don't understand why gamma correction is useful or needed. Can't I just increase the brightness setting on my projector?
No. Your brightness setting should be set such that a true black signal looks black. If you increase your projector's brightness setting you will see more dark area detail but blacks will also look grey. Gamma correction doesn't affect the low end 'true black' signal.



Some information on what the add-on gamma correction card does and how it works:

Why is gamma correction needed?

A CRT's response is non-linear. Without gamma correction, the low level black details in a CRT image will be degraded and detail will be lost. By adding a non-linear gamma boost that is the opposite to the CRT curve, the result is a linear response as seen below:




In the real world however, it is very difficult to get a perfect flat linear response. Most responses usually end up something like we see below:



This is still considerably better than an uncompensated curve.


There are basically 3 ways to add a gamma boost to the signal. In the source device (1), as an add-on box or card of some sort (2), or a CRT projector that has gamma correction circuitry built in (3) (see below):




To get a near-perfect linear response, it's possible to apply gamma correction at multiple points along the signal chain as seen below:




Sample video of Moome Gamma correction V1.01

Click here for the sample video (2mb, AVI format)

In this video a gray scale step pattern is displayed and the gamma circuit is switched off and on twice. The difference is very obvious. With the gamma correction on, black level is still correct but more of the dark details of the image can be seen.

Gamma Tuning Guide for Moome products


So what does gamma boost actually look like with source material?

Here are some real life examples of the Crescendo-Systems RTC2200 box performing a gamma boost (Moome's gamma boost looks similar):






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Last edited by kal on Tue Mar 11, 2008 7:18 pm; edited 2 times in total
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rsh




Joined: 01 Aug 2007
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PostLink    Posted: Wed Aug 01, 2007 11:42 pm    Post subject: Gamma Correction Reply with quote

Kal,

Excuse my ignorance, but if I have the Anthem D2 processor that does all de-interlacing, upscaling and also has gamma correction feature as well as HDMI switching, would I still need to do a gamma correction with the momme card or it will only serve as D-A conversion box with signal amplification for the let's say VGA break out cable from the box to the PJ?

Will this be a correct solution? Or should I get an internat card and run HDMI cable to the PJ (I have the Sony G90)?

OF course it would be much easier for me just to use the cable that I already have (it is VGA->5 BNC break out cable).

Thank you in advance
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Moose




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PostLink    Posted: Thu Aug 02, 2007 12:48 am    Post subject: Reply with quote

I have the Crescendo RTC200 but I haven't inserted it into my system yet. My question is - how to calibrate? Is this done simply by eye, is there a display screen that can be used for calibration, or what?
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kal
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PostLink    Posted: Thu Aug 02, 2007 2:08 am    Post subject: Re: Gamma Correction Reply with quote

rsh wrote:
Kal,

Excuse my ignorance, but if I have the Anthem D2 processor that does all de-interlacing, upscaling and also has gamma correction feature as well as HDMI switching, would I still need to do a gamma correction with the momme card or it will only serve as D-A conversion box with signal amplification for the let's say VGA break out cable from the box to the PJ?

Hey! Smart minds think alike! I'm in the market for a new prepro and decided on the D2 (haven't ordered yet) after a few months of research... so you're asking the right guy. Smile

What you've written is exactly right: The D2 will do all this for you. (Plus a whole bunch of other nice things of course).

I haven't seen how well the D2 does gamma but it seems to be incredibly flexible and better than any of the single adjustment solutions like Kim's RTC-2200 or Moome's cards. The D2 gives you multiple points along the graph so I can only presume that you'd not need the Moome gamma at all.

If you're curious, I started a thread about the D2 and how it fits in with CRT projectors.

Quote:
Will this be a correct solution? Or should I get an internat card and run HDMI cable to the PJ (I have the Sony G90)?


Run all sources to the D2 either by HDMI and component (component's sometimes best for satellite/cable boxes that drop signal every time you change channels - every annoying with HDMI). The have D2 do all the deinterlacing/scaling/gamma correction/picture moving-shaping-cropping/etc.

One option then is to then feed that out over HDMI to the projector into something like Moome's internal card .

Quote:
OF course it would be much easier for me just to use the cable that I already have (it is VGA->5 BNC break out cable).


This is the other option.... Using something like Moome's upcoming EXT-HD external solution with a VGA output would work for this. This is likely what I will do since I already have a good quality 25' VGA -> 5BNC cable and Moome doesn't make an internal card solution for my Barco anyway.

If you use the EXT-HD you'd completely disable the gamma circuit by turning it down/off since the D2 will do the gamma adjustment for you.

As long as your RGBHV cable is of good quality, there really shouldn't be any difference between these two methods.

Kal

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Last edited by kal on Sun Nov 28, 2010 5:19 pm; edited 1 time in total
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kal
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PostLink    Posted: Thu Aug 02, 2007 2:14 am    Post subject: Reply with quote

Moose wrote:
I have the Crescendo RTC200 but I haven't inserted it into my system yet. My question is - how to calibrate? Is this done simply by eye, is there a display screen that can be used for calibration, or what?

There are complicated ways of measuring gamma curves but for most cases just doing it by eye is sufficient. You want to give enough boost to the near-black things without washing out the picture or making it look flat.

Most say the RTC-2200 looks best somewhere around 9-11 turns from 'off'. (That's what I use on mine).

Kal

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rsh




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PostLink    Posted: Thu Aug 02, 2007 2:23 am    Post subject: Reply with quote

Kal.

Thank you for your response. That was exactly what I though I would do. Do you have an idea when the external boxes will be available?

FYI. Contact me if you need the D2. I may help Smile
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kal
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PostLink    Posted: Thu Aug 02, 2007 2:25 am    Post subject: Reply with quote

rsh wrote:
Kal.

Thank you for your response. That was exactly what I though I would do. Do you have an idea when the external boxes will be available?

My guess is within a month or so. Hopefully sooner.

Quote:
FYI. Contact me if you need the D2. I may help Smile

I'll send you a PM!

Kal

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Frostbite




Joined: 22 Apr 2007
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PostLink    Posted: Fri Aug 31, 2007 12:47 pm    Post subject: Reply with quote

I have a Barco 1209S/E and I have difficulty getting the shadow details to show properly, while maintaining inky blacks. If I turn up brightness they do show satisfactorily, but then I also have to live with the dreaded mist of grayness, which is not an option for me. Fiddling with the projector's settings for gain and cut-off hasn't solved the problem either. I assume using this box with its gamma correction circuitry would be a simple solution to this problem? As far as I've understood, using gamma correction does not influence the base black level of the displayed image.
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PostLink    Posted: Fri Aug 31, 2007 1:37 pm    Post subject: Reply with quote

You must remember that Kim's or Moome's gamma correction is not a true gamma correction circuit, it only corrects/helps the very low end of the gamma curve and not the full curve. True gamma curve correction circuits do exist in pjs like the Barco cine series and S series and the Zenith 1200x also I believe in the G90 too.
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kal
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PostLink    Posted: Fri Aug 31, 2007 1:51 pm    Post subject: Reply with quote

Ok, you're right Walter: Moome's and Kim's do low IRE gamma boost which is what most people (incorrectly) refer to when they mention "gamma correction".

True "gamma correction" like you and I have in our Cine series projectors does not boost low level IRE.

Kal

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kal
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PostLink    Posted: Fri Aug 31, 2007 1:54 pm    Post subject: Reply with quote

Frostbite wrote:
I have a Barco 1209S/E and I have difficulty getting the shadow details to show properly, while maintaining inky blacks. If I turn up brightness they do show satisfactorily, but then I also have to live with the dreaded mist of grayness, which is not an option for me. Fiddling with the projector's settings for gain and cut-off hasn't solved the problem either. I assume using this box with its gamma correction circuitry would be a simple solution to this problem? As far as I've understood, using gamma correction does not influence the base black level of the displayed image.


Correct. Using this gamma correction would do exactly what you're looking for: It allows you to come out of black faster without affecting the true base black level. Both Kim's transcoder and Moome's box do this (as do HTPC's and many scalers).

Kal

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Frostbite




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PostLink    Posted: Fri Aug 31, 2007 2:39 pm    Post subject: Reply with quote

I see. That sounds reassuring. Thank you! Now, for that pre-order...
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JonFo




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PostLink    Posted: Sat Sep 01, 2007 12:03 pm    Post subject: Reply with quote

I posted a guide on how to set the Gamma with these new cards and boxes in this thread: http://www.curtpalme.com/forum/viewtopic.php?p=72411
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kal
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PostLink    Posted: Sat Sep 01, 2007 12:42 pm    Post subject: Reply with quote

Excellent JonFo! I'll link to that thread from all of his products. Thanks for putting this together!

Kal

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dutchlincoln79
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PostLink    Posted: Sun Jan 18, 2009 9:16 pm    Post subject: Reply with quote

Kal,

Thanks for the writeup. It explains in a easy way what the use of gamma correction is.
Now i wonder: You say that HTpc also do this.
I've seen it coming somewhere in my ATI catalist settings, but how do i CORRECTLY set these levels?
Do i need a hardware calibration tool, wich measures the screen?
Can you shine a little light on this?
I have the AMD 9950BE and the Gigabyte board with the AMD G790 chipset (ATI radeon 3300) and catalist control center.
Thanks.
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kal
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PostLink    Posted: Sun Jan 18, 2009 9:43 pm    Post subject: Reply with quote

dutchlincoln79 wrote:
Thanks for the writeup. It explains in a easy way what the use of gamma correction is.
Now i wonder: You say that HTpc also do this.
I've seen it coming somewhere in my ATI catalist settings, but how do i CORRECTLY set these levels?
Do i need a hardware calibration tool, wich measures the screen?
Can you shine a little light on this?
I have the AMD 9950BE and the Gigabyte board with the AMD G790 chipset (ATI radeon 3300) and catalist control center.
Thanks.


Yes. You need to use a meter and measure it. I have a guide here that walks you through the whole process from start to end: http://www.curtpalme.com/forum/viewtopic.php?t=10457

Kal

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loribates




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PostLink    Posted: Sat Apr 11, 2009 6:32 am    Post subject: Reply with quote

This is an older thread, I know, but I've been reading in anticipation of my Moome box's arrival (tracking says it's inbound customs for the last couple of days) When adjusting the gamma, has everyone had t go back and do another greyscale and color calibration? Which came first, the chicken or the egg? Set the gamma on the box or do a greyscale and color calibration, then adjust the gamma or back and forth between the box and the pj during a calibration? I'm SOOO looking forward to breaking out the Spyder again.....
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