Wednesday, April 19, 2006

Since When Is Black and White Green?

I think I've just figured out something that's been bugging me since I succumbed to temptation and bought a couple of HDTVs a couple of years ago. Especially on my Hitachi plasma in the basement, and less so on my Samsung DLP rear-projector in my living room, black and white programs can look distractingly greenish.

Such anomalies can often be laid at the doorstep of an inaccurate "grayscale"; indeed, that's the first reasonable explanation. Grayscale calibration means using test signals and expensive instruments to eliminate the inaccuracy by balancing the three primary colors — red, green, and blue — at every possible brightness level of the TV picture, so that the entire grayscale of the TV is pleasingly neutral in color.

Beware: grayscale calibration, if needed, is best done by a professional technician using state-of-the-art instruments.

For reasons I won't go into here, I had a bad experience trying to get professional grayscale calibration for my TVs. It never happened.

At any rate, recently I chanced to watch perhaps the first program I've ever seen on a high-definition channel in black and white: "Roy Orbison and Friends: A Black and White Night." (The show was recorded perhaps a year before the great singer-songwriter's untimely death from a heart attack in the late 1980s. The "friends" included a young Bruce Springsteen, Elvis Costello, Bonnie Raitt, k. d. lang, and a number of other music biz luminaries. Apparently it was taped in an early hi-def format. Great stuff technically, musically, and nostalgically.)

So this piece of hi-def, if B&W, gold showed up on my Samsung's screen with nary a trace of green!

That's chapter one. Chapter two: the same show was broadcast by my local PBS station during a pledge drive a few weeks ago. It was in standard definition this time. I watched it on my basement plasma, not my DLP, and it looked distressingly, disappointingly green.

Fortunately — and this is chapter three — it was even more recently shown yet again on INHD (or was it INHD2?) in hi-def, and when I watched that transmission on my plasma, no green tinge was apparent. The TV's grayscale appeared to be spot on, with no tint in sight.

Thus when a B&W show comes into my digital-cable DVR box over a standard-def channel, such as that of my local PBS affiliate, it can betray a green tincture. When the same show comes into that same DVR box over a hi-def channel such as INHD or INHD2, there can be no tincture. (In both cases my DVR box sends its output signal to my plasma TV over a digital HDMI/DVI connection, by the way, so the difference has nothing to do with the signal pathway.)

Oddly enough, I think I can explain this anomaly.

The explanation has to do with how TV signals are packaged. With the advent of color TV way back in the 1950s, there had to be tricks by which the three color primaries of red, green, and blue could be mixed to make a suitable black and white picture, if only for the benefit of the many existing non-color TV sets. The resulting signal was called "luminance," or Y. After it had certain other necessary processing steps applied to it, it was subsequently called "luma" or Y' ("Y-prime").

Luma or Y' is the sum of fixed proportions of the three chroma signals R', G', and B'. They in turn are derived from R, G, and B — shorthand for red, green, and blue.

The trouble today is, the fixed proportions of R', G', and B' that are used to compute Y' are different for HDTV than for standard-def TV.

Other things being equal, an HDTV set that expects any signal it receives to conform to the new HDTV luma-encoding standard will do strange things when the signal was actually encoded for SDTV-style luma.

In order to drive its tri-color screen, the HDTV will take apart the luma component — with the help of two "color difference" components it also sees, R' - Y' and B' - Y' — to get R', G', and B'. But if the luma was encoded with SDTV's version of the numerical coefficients, the HDTV will forward too much of the received luma component to the green sub-image of the overall picture, and too little to red and blue. Or so my reasoning goes.

That can turn Peter Lorre's frightened face in Casablanca a tad greenish. It can make Roy Orbison's famously tinted glasses on "Roy Orbison and Friends: A Black and White Night" really tinted.

(For techies, the difference in the two forms of luma encoding, one for SDTV and one for HDTV, can be researched by looking up "Rec. 601," a nickname for ITU-R Recommendation BT.601, the international standard for television studios' non-HDTV digital signals, and "Rec. 709," a nickname for ITU-R Recommendation BT.709, the international standard for television studios' HDTV digital signals. These two standards specify other things besides techniques for luma encoding, but luma encoding is one of the biggies. You can begin your research with this Wikipedia article. Note that the relevant SDTV standard is sometimes called by an earlier name, "CCIR 601.")

It is apparently easy, given equipment that can accomplish the task, to convert between the two luma encodings. That's how the same program could wind up with two different encodings, one on the high-definition channel and one on the standard-def channel. Engineers at the TV studio might apply the requisite conversion matrix — mathematics embodied in electronics — and voilĂ .

My plasma TV seems to be capable of reversing the process — but only when the signal reaches it in analog form via its component video or "YPbPr" input. In that case alone, the TV offers a user menu item which purports to let me switch between luma encodings manually.

Unfortunately, the same is not true of the digital connection I'm actually using between the cable DVR box and the TV. There, the signal comes out of the cable box's HDMI (High Definition Multimedia Interface) port, passing through an HDMI-to-DVI adapter into the realm of Digital Video Interface, or DVI, the digital video format that my two-year-old TV model is actually capable of receiving. DVI uses digital video signals exactly like those used for HDMI.

Such all-digital signal pathways into the TV simply assume the signal needs no conversion — or if it does, it takes place in the source device, in this case the cable box, prior to HDMI or DVI transmission. That's apparently why the TV, in its user menu, offers no color-encoding selection option for DVI.

* * *

Since filing the above I have done a little more experimenting with my Hitachi plasma TV. I now find that there undoubtedly exist more than one reason why black and white material can appear greenish.

I've recently purchased a four-DVD collection of the old British mystery movies starring Margaret Rutherford as Agatha Christie's amateur sleuth, Miss Marple. From the early 1960s, they're all in B&W. Played into the Hitachi via YPbPr (i.e., component video) from my Bose Lifestyle system's control unit, or via S-video from my Samsung DVD player, the first movie in the series, Murder She Said, exhibits an interesting anomaly. In the middle of one particular scene, at a transition to a new camera shot, the B&W picture changes from not greenish at all to just faintly greenish!

I'm not sure I can confirm this effect on my other HDTV, a Samsung DLP rear projector, using a different DVD player. It is, after all, quite a subtle effect. But it does show up big as life, in my iMac's DVD Player software, so I don't think I'm imagining it.

My best explanantion: I expect there is information recorded on the DVD which triggers a change in (I'm guessing) the so-called color space (Rec. 601 vs. Rec. 709) the DVD player is supposed to use when it decodes the DVD. The digital picture information on a DVD is, I'm aware, accompanied by a raft of on- or off-bits which tell how the video was recorded, how it should be played back, etc. Maybe during the authoring of this DVD a crucial flag was changed at the transition in question. Maybe some DVD players take the change into account and some don't.

As I said earlier, my Hitachi plasma's user menu allows me to change color spaces only for its YPbPr input, not for DVI or S-video input. In general I find that forcing its YPbPr color-space decoding method to that for Rec. 601, or SDTV, does indeed make a very slight difference in a B&W DVD being sent to the TV from the Bose player. It adds quite marginally to the greenishness ... but not enough to account for all the greenishness I see!

I also find that, strangely enough, adjusting the Hitachi user menu's color control affects the greenishness of a B&W picture! When the color setting is lowered, the greenishness of B&W increases. When the color setting is raised, the greenishness (almost) goes away. Furthermore — and this is really strange — this color-setting dependency applies to YPbPr when the Pb and Pr input cables are both disconnected(!), such that the only input the TV receives is the supposedly colorless Y, or luma, signal.

Which suggests that the TV's internal color decoding algorithms do some odd things. You would think — obviously erroneously — that the luma signal would be treated the same, no matter what setting the color control has, since color (chroma; Pb and Pr) and black-and-white (luma; Y') are nominally independent. So if the B&W picture were on the greenish side with one color setting, it would be equally on the greenish side with another. But no! There's a clear-cut difference in the greenishness at different color settings.

That suggests that maybe I've been too hasty in dismissing the possibility (see above) that I ought to have my Hitachi professionally calibrated.

Yet, if my Hitachi is capable of, in effect, changing its calibration settings when the user color setting is altered, I wonder how much good professional calibration would do. I envision the calibrator being totally stumped by the TV's complexity — "They sure didn't tell me it could do that in calibrating school" — and telling me that compromises must inevitably be made. Live with it.

By the way, I think I can conclude from the foregoing that the Murder She Said DVD, at least prior to the camera-shot transition mentioned above, causes signal information to be put out on the Pb and Pr (i.e., chroma) channels such that the Y or luma signal is modified to offset what would otherwise be a greenish tinge. After the transition, perhaps the compensating Pb/Pr signals disappear, and there is greenishness. As I say, this change happens also in my computer software DVD player.

So there may be several possible sources of greenishness in a B&W picture:

  • the DVD player or other source device using the wrong color-space decoding parameters, i.e., Rec. 601 rather than Rec. 709
  • inaccurate TV grayscale calibration
  • other oddities in the TV's color-decoding methodolgy
  • oddities in how the DVD was authored or the source program was broadcast
  • and perhaps many others

If that's so, I can only conclude that TV in the digital age is so complex that it's almost impossible to get a "perfect" picture, if by "perfect" you mean a B&W rendition that is totally free of tint.


Anonymous said...

Hi, A lot of great info on the green problem. I have a Samsung 46-inch DLP HDTV that is almost two years old. I never had a problem with the greenish tint until I bought one of those up-convert DVD players and I switched to an HDMI connection. Now, most often (but not always) I have some green tint in my B/W, and I have some green tint in some color situations: indoor lighted scenes(incandenscent), fire, skin tones of African Americans but never fair skin people. If a scene goes from one camera to another the greenish can decrease or increase. It is worse if I watch with the picture in CINEMA mode. It gets better in STANDARD mode. I have tried several HDMI cables and 3 different and new DVD players with only a small improvement.

eric said...

Dear Anonymous,

There should definitely be no false green tinge in properly done HDTV material, such as a live football game, which stays in the Rec. 709 colorspace from camera to screen.

Any deviation from that scenario may well introduce a green tinge that is especially evident on black and white images (which, of course, are rare in HDTV programming).

Any other scenario may involve hopping once, or many times, between Rec. 709 and the old Rec. 601 colorspace for standard-def TV.

On any given hop, the proper colorspace compensation may fail to happen.

The fault may lie in the signal that your equipment first "sees" — say, in a DVD itself.

Or, your DVD player, cable box, satellite receiver, etc. may be culpable.

Or, the TV itself may be to blame.

I've also noticed that some of the B&W movies on TCM look greenish while others do not. Go figure.


DavetheF said...

Interesting post. I too have the dreaded green tinge. It is just visible on the television satellite picture, in very dark areas, where it is sometimes accompanied by writhing purple in the background. Not noticeable in bright TV picture. But DVDs seem to excite the phenomenon to varying degrees. It is visible in dark and sometimes even grayish areas or mild shadows. The Pang Brothers movie The Eye 2 was unwatchable, with the green tinge turning even the characters bilious. On several other DVDs it is almost tolerable (The Bourne Ultimatum isn't bad) but it never goes away completely.

I know a very clued-up plasma engineer, and without seeing it, he promptly diagnosed it as a voltage problem with the sustain board, which would need replacing.

The comments here have given me food for thought. My plasma is a Samsung 42-inch, about four months old.

eric said...


What you said about writhing purple in the background makes me think your plasma does indeed have an idiosyncratic anomaly such as quite possibly the voltage problem with the sustain board that your engineer friend diagnosed.

Not that I know what a sustain board is, mind you.

But for what it's worth, my problem with green tinges in black & white material on my Hitachi plasma does not involve any similar problem with false background colors that writhe on the screen.

However, many, many years ago I did have a VCR (remember those?) that made writhing false colors appear on the screen. I never knew exactly what caused it, but I was able to figure out that it was some sort of signal interference. When undesirable analog signals leak inappropriately into the actual video signal, they can "beat with" the real signal and superimpose themselves on it in a dynamic, "writhing" way. The result can be faint patterns snaking around on the screen, in addition to what is supposed to be there.

I don't know whether this kind of thing explains what you are seeing or not, since in theory we are now dealing with digital signals and circuits, not analog. But I thought I'd pass it along for your consideration anyway.

Also, I'd like to add this to my original post: I now have a Sony 1080p LCD in addition to the Hitachi plasma. The Sony does not seem prone to green tinges in high-definition B&W material, but such tinges do show up in standard-def B&W material.

I attribute that to the program originator's use of the old "color matrix" appropriate to SD television, rather than the revised color matrix for HDTV.

As I said in the original post, it is possible to convert the signal from one color matrix to the other, so the result will look right on a high-def TV. But this does not always seem to happen as it ought. I'm not sure who or what to blame for this, as there are many places along the route from the original video signal to the final TV display where a slip-up can occur.

Still, the fact that most B&W looks right on the Sony and has a green tinge on the Hitachi shows that the TV itself can impose unwanted tinges of color, even on an input signal that uses the "right" color matrix.

Anonymous said...

Thanks very much for the detailed info. My plasma guy is very busy because of inferior work by Samsung agents here (Cape Town) so I don't know when I'll get rid of the problem (hopefully). The sustain board will have to be replaced by Samsung techies anyway.

I might try getting a mains purifier to prevent interference

This is a very good blog. I'll be back!

Not updated since October 2007 ...

Anonymous said...

I have a hitachi 55" Plasma with the same green problem.
My solution, I have a preset with the color set at zero and the tint set at more red than green, this eliminates 99% of the green in the B&W shows.

eric said...


You said:

I have a hitachi 55" Plasma with the same green problem.
My solution, I have a preset with the color set at zero and the tint set at more red than green, this eliminates 99% of the green in the B&W shows.
I, too, have found the green problem on my Hitachi plasma disappears when color is turned all the way down to zero, but I don't have the ability to set a preset for it on mine. It is interesting that the tint setting would have any effect at all with color set to zero!

I am of the opinion that the "color space" used by the phosphors of a plasma TV don't very well match those expected by either the new HDTV standard or the standard for old standard-def TV. The input signal has to be compensated, inside the TV, for that discrepancy. The compensation is only partly successful. There is a certain amount of compensation error that can show up most prominently in supposedly B&W material, when in standard definition and when the TV receives it over HDMI or DVI. I have also noticed that deep reds sometimes veer over into orange, rather than being true red. I attribute that to the nonstandard plasma color space as well.

I doubt I will ever buy a plasma TV again. I find that LCD HDTVs are much better in terms of how true the colors are. LCD flat panels are now plenty big, have black levels that nearly match plasmas', have fast enough response times to prevent motion blur, etc. They do have problems with off-center viewing (but not as much as they once did) and they can have uneven screen lighting ("clouds" or "mura"). But for the most part I think they are much better values than plasmas.

Anonymous said...

Hi. I have a plasma screen that also suffers from a greenish cast to the screen. In black and white pictures, as well as in dark scenes in a color movie or show. My plasma has 3 presets for color temp, cool, normal and warm. As you move from cool to warm, the green issue becomes worse.

I have found out it is 100% a grayscale issue. I recommend everyone who thinks they are having this problem to invest in the Digital Video Essentials Blu Ray disc, and take a good look at a grayscale pattern from the test disc, on your tv. It seems that in an effort to increase contrast levels, and enhance the green pop in the picture you see on tv, they RGB grayscale balance really pushes green over red and blue. It is possible for a tv to have a perfect tracking grayscale, with little variation from black to white, WITH the green drive higher then blue and red, since green is pretty neutral, and doesnt effect color temp too much. Also, if you have an SD card slot on your tv, download a grayscale pattern and have a look. cycle through your color temp presets and see if matters get better or worse.

99% of the time, this is the problem. The green can seem worse in Standard def because of the Rec. changes, but the true problem almost always lies in too much green in the grayscale. Sometimes you can't really see the green in a test pattern that well either.

eric said...


Thank you for a very valuable comment.

Too much green (or any other color) in a TV's grayscale is a problem that, technically speaking, is closely related to the TV's color temperature.

Ideally, adjusting the color temperature sets the "color of white," i.e., cool, warm, or in between. If the color temperature is uniform at different levels of light output, then when using a grayscale test pattern from the Digital Video Essentials Blu-ray disc, the user should see all the different shades of gray from white to black, and none of them should betray a color tint.

Even more ideally, one of the TV's color temperature presets (often, "Warm") should give a measured color temperature of 6500 degrees Kelvin ... though one needs special equipment to measure that.

Many plasmas and other HDTVs don't have a very good grayscale, as you said in your comment. Of the three primary colors (red, green, blue) they sometimes "push" green so that the grass on a football field, for instance, will look "good."

They also sometimes push red, because that makes for more "pleasing" skin tones.

Red and/or green "push" also allows the "brightness" (controlled by cranking up the contrast setting all the way, actually, or using the TV's "Dynamic" preset) to be more "convincing" in video stores with their bright fluorescent lighting. Usually, the blue phosphors in a plasma display have the highest available light output, so pushing red and green lets them balance out the hyper-blue when maximum contrast is being used.

With a color image, it's almost impossible to tell that the grayscale is out of whack. With a B&W image, any grayscale problem that the TV may have shows up more readily.

Getting the TV calibrated may eliminate part or all of the problem with incorrect or non-uniform grayscale, and also with incorrect or non-uniform color temperature.

Calibration by a technician certified by the Imaging Science Foundation involves setting up light measuring devices in front of the TV screen (or attaching them to the screen) and then using their readings to tweak the adjustments hidden in the TV set's service menu.

These adjustments include "bias" and "gain" for each of the three primary colors, red, green, and blue. Bias sets the level of each of the primary colors for black parts of the image. Gain sets the level for each of the primary colors for white. If bias and gain are adjusted properly for all three primary colors, then in theory both color temperature and grayscale will be right for all shades of gray.

That's the theory. In reality, getting a perfect grayscale is very hard to do. If the three nonlinear gain functions happen to line up properly going in, but bias adjustments are needed to get the proper color temperature, that alone can throw the grayscale out! At that point it can be hard to achieve ideal grayscale.

This is why the enthusiast magazines and online reviews often tell you that, after calibration, a TV's grayscale and/or color temperature were not absolutely perfect!

Add to that the problems I mentioned in my original post that arise due to the two forms of luma encoding, one for SDTV and one for HDTV: Rec. 601 for SDTV and Rec. 709 for HDTV. When an HDTV receives a program that uses SDTV encoding, a B&W picture can have a tint to it. There's not a lot an ISF calibrator can do about that!