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SpyderTV Pro: gray scale versus the color decoder

post #1 of 15
1/15/07 at 12:29pm
Thread Starter 
I have a Hitachi director series rear projection LCD TV which I recently calibrated with SpiderTV Pro. I used the service menu to adjust the RGB gains and cuts, and everything seems to have worked well. The TV came with all the cuts adjusted to the minimum value, so I had to boost them all to give myself some wiggle room. In the end I was able to get Blue and Green to 100% for both gains and cuts and Red within 1% of them. Double checking color with the Digital Video Essentials color bars and filter looks perfect.

My question involves a completely different set of controls that the Hitachi gives me access to - the color decoder. I can adjust the phase and gain of red, green, blue, magenta, and cyan. I don't really understand the interaction between gray scale and color decoder, but I'm wondering about the linearity of the colors in the gray scale. The SpyderTV Pro measures low and high gray using one to set the gains and the other the cuts. Two points make a line, but as far as I can tell it's an unproven assumption that the colors are linear between those two points.

Looking at what the color decoder seems to do, this uneducated observer suspects that changing the phase would change the linearity of the color components of the gray scale outside of the two measured points. Am I completely off base here? If I'm not, is there any way to adjust the color decoder to improve the linearity of the gray scale?
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post #2 of 15
1/15/07 at 2:49pm
Greyscale is calibrating the greyscale of your panel to not have too much of any one color - they are supposed to be balanced to make grey.

Color Decoder is calibrating your video's colors to not have too much of any one color. You do not want your blues to be blue or your cyans to be too blue.

Entirely different things - and the controls for one do not work for the other. Understand that video is nothing more than colorized B&W. This is where it gets confusing because any colorpush in the B&W will show up as underlaying the colorizing. The colorizing can be correct - but the colors will still be wrong.

Spyder TVPRO is not a tool for adjusting your color decoder, and the color decoder is not the way you adjust your greyscale. You would need to upgrade to better software to know if the grayscale at the other points on the line are correct - the SpyderTVPRO is assuming that calibrating those two points will get things in line. This is not always the case.
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post #3 of 15
1/15/07 at 3:07pm
Color decoding and gray scale are two completely separate things. Changing the color decoder settings does not (should not) affect the gray scale.

When calibrating the gray scale, you are adjusting the three primary colors to achieve a color of white (the gray scale is varying intensities of white) that matches a standard, usually D6500. Picture color information is applied on top of the gray scale, therefore, while the gray scale affects all of the colors in the picture, color decoding does not affect the gray scale. Think of it as using crayons on a piece of paper. The color of the paper will affect the apparent colors generated by the crayons, but the crayons do not change the underlying color of the paper. If the gray scale is wrong, you cannot generate the correct (i.e. originally composed) colors in a picture. However, you CAN have a correct gray scale, and still have the wrong colors displayed.

By adjusting the color decoding, you are adjusting saturation of the three primary colors (red, green and blue) and balancing their mix to generate the correct secondary colors (magenta, yellow and cyan). This makes the displayed colors as close to the intended colors as possible, limited only by the displays inherent color gamut.

Regards,
Steve

Edit: Kras beat me to it, dang two finger typing! Anyway, ditto what he said.
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post #4 of 15
1/15/07 at 9:44pm
Thread Starter 
Thanks. If I'm interpreting what you're saying correctly, you're sort of corroborating what I said.

So to restate in a somewhat different manner: what's the intrinsic value of calibrating gray scale ala SpyderTV Pro if you don't calibrate the color decoder?
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post #5 of 15
1/16/07 at 10:50am
Again, the color decoder does not affect the gray scale. When adjusting the gray scale, you are setting the "color" of white, attempting to eliminate any pinkish, greenish or redish tint. It's very difficult to do this correctly by eye, so instrumentation is necessary.

Adjusting the color decoder can be done by eye using standard color bars and optical filters (e.g. the red, green and blue filters that come with Avia and DVE). The color decoding is correct when all color bars containing a primary color (red, green, or blue) are the same color and intensity when viewed through their respective filters. That is, when viewed through the red filter, the white, yellow, magenta and red bars should all look the same, and so-on. Calibrating the color decoder adjusts the displays color pallet so that it displays the intended colors corrcectly (i.e. so that red, purple, brown, etc. on the display match the actual or intended colors of the source material). For most newer displays, properly adjusting the COLOR and TINT settings in the user menu will usually yield correct (or very nearly correct) color decoding.

If the gray scale is wrong, then the displayed colors will never be correct, regardless of the color decoder accuracy.

Make sense?

Regards,
Steve
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post #6 of 15
1/16/07 at 1:24pm
Thread Starter 
One of the suggestions I've seen for using the SpyderTV Pro is to move the color setting to zero before setting the gray scale. If I understand the impact of doing that to be taking the color decoder out of the equation, then I guess I can understand how gray scale is independent of color decoder because that setting change makes it so.

If you leave the color turned up, then I don't understand how it can be independent. The gray scale is manipulated with RGB cuts and gains, and the color decoder is manipulated with phase and gain (on my set). With two sets of controls increasing and decreasing the intensity of the three primary colors that make up white, I don't see how they could be adjusted independently unless you do as I described in the previous paragraph. I think it's curious that the calibration instructions with the SpyderTV Pro don't even suggest this.

But if I take the color decoder out of the equation, doesn't that prevent any gamma curve that my TV has from coming into play? If so, wouldn't that mean that linearity of the gray scale would be changed as I modify color decoder section? How can you possibly set the color response of the TV from only two points for the gray scale and two colors for the decoder?
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post #7 of 15
1/16/07 at 2:12pm
When adjusting the gray scale, only gray test patterns are used, there isn't supposed to be any color being displayed. If a test pattern has an error that introduces some coloration (this is a well known problem with the gray scale patterns on the Avia calibration DVD), it can often be corrected for by reducing the color setting to zero. However, if the gray scale test pattern is encoded correctly, it does not have any coloration, thus any coloration that is exhibited is caused by the display, hence the need for calibration. Adjusting the RGB cuts and gains to correct the white point (color of gray) has no effect on color decoding. Color information (chrominance data) and gray scale (luminance data) are processed separately before being combined to generate the RGB values that drive the display. Again, think of the picture coloration as being overlays placed on top of a black and white picture. Calibrating the gray scale eliminates coloration in the black and white part of the image while calibrating the color decoder adjusts the color of the overlays.

FYI, for digital displays, the gamma curve is generated by a look-up table and is generally not adjustable except for specific values provided in a user menu setting on some displays.

There is a wealth of information available on-line the explains this much better than I have, try searching on "gray scale calibration".

Regards,
Steve
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post #8 of 15
1/16/07 at 7:15pm
You will never understand it until you take the time to learn some video engineering. Poynton has some excellent texts that explain the video chain and signals - once you understand that you will understand why they are calibrated seperately.

The paint on paper analogy is the best one (though in my version I use water colors - not crayons! )

I think you are just getting yourself confused because your displays controls have some confusing related names for unrelated functions.

It would be unreasonable for a calibration wizard to do more complex than it is - the two points on greyscale are the ones that the two greyscale controls most impact - yes you are ignoring other points on the assumption that it will track - even though it may not and you need the rest of the data to compromise (something computers are bad at doing). The assumption about using the two colors to align the video is the same - if the color decoder is correct - then you only need to make sure that the two are right as the rest will be aligned. Again this is usually not the case - but for a computer wizard to cover the complexity of video decoder controls that follow no known standard that half the time gives the ISF tech a migraine? Not quite there yet on the AI systems.
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post #9 of 15
1/16/07 at 7:28pm
How does the color decoder come into play for RGB video signals, there is nothing to decode" as far as I can see.

I can understand how a decoder is required for NTSC or PAL video signals, but not RGB.

PC monitors run on RGB and don't have a color decoder as far as I know.
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post #10 of 15
1/17/07 at 11:15am
Color decoding is not required for RGB, however, gray scale calibration is. FYI, color matching on computer monitors is accomplished through software, not by monitor adjustments.

-Steve
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post #11 of 15
1/17/07 at 11:31am
Some display run the RGB back thru a video encoder so that the video decoder controls are available - in response to consumer demand wondering where the tint control went over their DVI connection. Yes this means the signal can get doubly screwed up...

RGB is the data format used by cameras and displays - video is the transport format (media/broadcast) because it is easily compressed in analog and digital domains. If RGB was used as the transport format - then the transport would potentially screw up the grayscale.
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post #12 of 15
1/17/07 at 3:29pm
Actually the higher grade PC monitors have RGB gain and cut controls for calibration of gray scale.

Manipulation of the video card LUT via software is not an ideal solution, and best avoided if possible IMHO.

Digital TV is compressed and transmitted in component color space, same as DVD and other digital video sources, which needs to be converted back to RGB by the display.
Is the component to RGB converter classed as a decoder and is it normally adjustable? I cant see why it would be as component to RGB conversion is a standard process and only varies between SD and HD video formats.

Analogue NTSC and PAL video signals need decoding, but only in NTSC does the user have a tint control to compensate for deficiencies in the Never The Same Color system.
When receiving a PAL video signal all the displays I have seen disable the tint control and the same seems to apply to many displays when receiving a HD video signal.
I am therefore confused as to how a color decoder relates to HD displays, since HD is in component color space.
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post #13 of 15
1/17/07 at 4:21pm
For component video, the three signals are luminance, and two chroma channels which are then decoded into RGB. Component video was invented so that the color information could be subsampled, thus allowing for more data compression (i.e. MPEG 2, JPEG, etc.). This is generally acceptable because the human eye is much more sensitive to luminance than color.
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post #14 of 15
1/17/07 at 4:39pm
Quote:
Originally Posted by slb View Post

For component video, the three signals are luminance, and two chroma channels which are then decoded into RGB. Component video was invented so that the color information could be subsampled, thus allowing for more data compression (i.e. MPEG 2, JPEG, etc.). This is generally acceptable because the human eye is much more sensitive to luminance than color.

Yes but is the component to RGB decoder the one that is being referred to with regard to calibration?
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post #15 of 15
1/17/07 at 4:44pm
Yes the color decoder is the one that does YCbCr to RGB.....this is considered video calibration. The minimal set of controls are contrast, brightness, and color to account for analog variation - tint is not an inherent control but is usually there because Svideo and Composite need the control. But if those controls are unbalanced with a bad video decoder (red pushed faces) then you need the service menu that accesses the color decoder. There are different standard decoders used for SD, HD, EBU, PAL, NTSC...

The display assigns RGB to the display gamut - with the white point defined by the mixture of RGB for that display gamut (many standards here but only D65 is used in video) then the secondary CMY results from that white point and opposing primary. This is considered display calibration since it is dependent on having RGB data.

You will not have correct colors unless you have a reference REC709 video decoder and a reference D65 calibration and a reference REC709 display gamut. GIGO - unless you have correct RGB data it matters not that you calibrated the display. With VGA, RGBHV, DVI and HDMI/RGB interfaces - the video decoder is in the source not the display - with caveat that some display may reencode the data back to video...and some sources/displays may not have video decoder controls to ensure conversion to RGB is done properly.
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