DOES BIAS BUY US ANYTHING ANYMORE?
Nov 1, 2002 12:00 PM, PETER H. PUTMAN, CTS
The timing couldn't be worse. I'm sitting in my powerless office, waiting for the electric company to turn on the juice again after the remnants of Tropical Storm Gustave created a gusty, blustery day and dropped more than a few tree limbs on power lines (including the one that feeds my office, apparently).
To make matters worse, nine brand-new LCD and DLP projectors are lined up out in the hall for my annual projector roundup, four of which were slated to go on the test bed today. About the only thing I can do is evaluate their owner's manuals.
So with some time in hand, I'll take a look at some of the issues in projector (and plasma) calibration practices, specifically focusing on image adjustments. Readers with experience in setting up and maintaining tube displays know the drill when it comes to calibrating color temperature and gray scale. Use a 20 percent video gray to set the bias and an 80 or a 100 percent pattern to set the drive.
WHAT IS BIAS?
Bias determines the idling current of the anode (plate). The more idling current, the more linear the tube will behave with smaller and smaller driving voltages. Drive is simply the gain of the tube, or the factor by which the driving voltage is amplified to the anode and thence on to the imaging surface.
With a purely linear circuit, setting bias (also known as offset or cut) and then tweaking drive is a pretty straightforward matter. But many electronic displays used nowadays do not use linear circuits. At least one of them is 100 percent digital. Are bias adjustments really needed anymore?
The truth is drive is the key control now for LCD, LCoS, DLP, and even plasma. In the case of DLP projectors and plasma, the imaging is accomplished by switching rapidly through on-off cycles. By varying the ratios of the “offs” to the “ons,” a technique called pulse-width modulation (PWM) achieves gray-scale images. The human eye can't keep up with these rapid cycles (in theory), so all you see are the changes in gray scale.
With such a circuit, the use of a bias control becomes superfluous. All levels of the gray scale from 0 to 100 percent white are dependent on the ratio of off and on PWM intervals. The switching function is nonlinear, because the gain or peak voltage level of the driven pixel is always 100 percent (full on to full off).
That is the reason why so few manufacturers of plasma display panels include a drive setting on their products and why adjusting those models that do have such a setting becomes a game of rock 'n' roll. You get the gray scale just right at a low luminance level, and the higher levels go askew. Or you tweak up high levels of gray, and the low end goes warm or cold.
The use of a bias circuit isn't quite as senseless with LCD imaging. LCDs are light-shuttering pixels filled with liquid crystal molecules whose job in life is to twist into a specific alignment in response to a particular voltage level. The twist and subsequent alignment determine the degree to which polarized light is passed or blocked through that particular pixel.
The tiny MOS transistors used to switch these pixels on and off are biased through their gate, but the bias can be set for pure cutoff (Class C, switched on and off) or into a linear region (Class B or even Class AB). For a switched-mode operation, simple on-off bias control is all that's needed.
PWM can also be used with LCDs, but the limiting factor there is the switching time of the LC molecules. A “slow” panel might have a response time of 25 to 30 ms, which is good only for static images. A “fast” panel, such as those used to view TV, would need to operate at twice that rate or even faster.
The key to understanding if bias is needed at all is to determine the operating mode of the display technology. Switched-mode operation or PWM operation will not require bias, whereas linear or quasi-linear operation always will.
A TWEAK HERE, A TWEAK THERE
I recently tested a new home-theater projector from Epson. The PowerLite TW100 uses three 1,366-by-768 (16:9) polysilicon LCD panels with the usual dichroics to produce RGB imaging. Like most other LCD projectors I've tested, it includes a menu for red, green, and blue drive settings, ostensibly to set the white point.
Unlike most other projectors, the TW100 also includes offset for red, green, and blue, as well as individual gamma settings for each color channel. That's an extraordinary amount of control for a $4,995 LCD projector, let alone a classic tube chassis.
What makes this projector interesting is that the offset (read: bias) circuit works pretty much the same way as that on a tube projector. Once you have selected the desired overall picture gamma and fine-tuned brightness and contrast, you can then use a color analyzer or tristimulus color meter (like my Minolta CL-200, calibrated to read short-arc lamps) and the bias and drive adjustments to slide right into (or darn close to) the magic 0.313x, 0.329y coordinates for D6500.
If it weren't for the color uniformity problems characteristic of all transmissive LCD panels — amorphous or high-temperature polysilicon — you'd see a clean gray-scale ramp with no apparent color shift. As it is, the TW100 showed only a bit of yellow/red shift at the high end of the gray scale, making it one of the better-looking LCD projectors I have tested yet. What's more, the calibration is numeric and you can log the settings for RGB for future calibrations (see Fig. 1).
My experience in the past with LCDs is that I can usually find a value of gray somewhere between 40 and 60 percent that, with patience, can be calibrated near to D6500. However, the higher and lower values often wander into the red or blue region. That is often a consequence of manufacturers not wanting to apply additional dichroic filter correction for the short-arc lamps, which will produce better flesh tones but cuts down on light output rather drastically.
I have also tested plasma monitors with bias settings from NEC. In fact, NEC is one of two manufacturers (the other being Pioneer) that consistently provides a bias or “low” setting in its calibration menus. However, this type of bias is really more of a low-level drive setting. On the NEC panels, the bias and drive controls interact with each other substantially.
The result (combined with the overall coarse adjustment of the controls) is that you can't get as clean and neutral a gray scale as you'd wish from 0 to 100 percent. You also can't achieve the fine adjustments needed to get the white point smack-dab on D6500. In this case, I usually pick a gray value from 40 to 50 percent and try to get that bar as close to D6500, letting the rest fall where they may.
It sounds slipshod, but that's all the control you will get over these panels. The good news is that the NEC plasma tracks a given gray-scale setting quite well between 20 to 25 percent and 100 percent white. Only the very low values show any significant excursions.
In my tests using component video and selecting Standard mode, readings started at 7,500K at 10 percent white and quickly dropped to about 6,500K at 20 percent, hovering around that mark all the way to 80 percent white. The readings climbed back to 7,300K at 100 percent white (see Fig. 2).
Pioneer's circuitry was just as persnickety in terms of the bias and drive interacting. But the “pick a gray value and calibrate it” method resulted in another impressive gray scale, starting at around 7,500K at 10 percent white and quickly dropping close to 6,500K all the way to 100 percent white. If subfield switching of the lower gray levels is done correctly, the drive settings should be all you need across the board (see Fig. 3).
Of course, having bias and drive adjustments doesn't necessarily mean the resulting gray scales will track accurately. Zenith's P42W22, 42-inch plasma offers both controls in a hidden service menu, but the adjustments are way too coarse — one click of a number resulted in huge excursions across the white point. It felt like I was trying to shoot a bull's-eye at 100 yards from a ship on stormy seas! As expected, the resulting gray scale wandered all over the place (see Fig. 4).
HEADING FOR THE SCRAP HEAP?
The last bugaboo is in saving your calibration results. All NEC gives you is a graphic representation of a slider bar and no reference numbers for RGB drive and bias. Pioneer uses numbers for both, and the incremental adjustment is finer than the NEC. Zenith's coarse range of adjustments makes the numbers relatively useless.
As you migrate away from tube imaging and possibly toward more PWM or switched-mode displays, today's bias/cut/low/offset controls may disappear entirely. Certainly, an all-digital path from video sources such as set-top receivers or computers to projectors wouldn't need these controls — they'd be replaced by look-up tables generated by the projector and echoed by the video source. Instead, corrections to the color filters or dichroics would be made to offset the aging of projection lamps, adding blue to compensate for the increasing red shift. Because this correction would be applied across the board to all levels of luminance, it's once again a drive function and not a bias setting.
Keep your eye on the next wave of electroluminescent displays, such as OLEDs. Those devices emit light in response to a voltage differential between anode and cathode, making them, in effect, tubes without filaments. By changing the operating voltage (typically, from 0 to 5V in smaller LEDs and as high as 20V in some designs), the intensity of the red, green, or blue emitting layer is varied, just like a cathode-ray tube.
If manufacturers choose to operate these devices in a linear mode, then bias once again will play an important role in calibration. If, however, manufacturers of OLED TVs opt for a switched-mode operation such as PWM, then drive will be all that matters. I'll have more to say on the subject of OLEDs in 2003 and perhaps even some hands-on reports of how these unique products work and compare to AM-LCDs and CRTs.
Peter H. Putman owns PHP Communications, in Doylestown, Pennsylvania. The author of The Toastmasters Guide to Audio/Visual Presentations, Putman is also a regular columnist in S&VC.