Linearity And Displays
Although it rarely receives the attention it deserves, linearity is the most important property of any electronic amplification circuit.
GATHER A BUNCH of videophiles around a table, and sooner or later, a discussion on the merits of competing display technologies will start up. There will be plenty of acronyms tossed around (LCD, DLP, LCoS, PDP, etc.), and if you’re lucky, terms like “grayscale,” “uniformity,” and “native resolution” will come up. Depending on the technical expertise of the participants, phrases like “modulation transfer function” may surface.
But what probably won’t be mentioned is a discussion about “linearity,” or the ability of an electronic display to faithfully reproduce all the parameters of the input signal driving that display. Yet linearity is the most important property of any electronic amplification circuit.
Because our eyes have the ability to process a wide range of brightness levels and color shades, they’re said to have good dynamic range. In any amplifier, dynamic range is the ultimate determining factor for performance. If an amplifier can maintain constant output and gain at all input signal levels, the sounds it reproduces or pictures it creates will more closely be perceived as lifelike.
So it doesn’t matter if you’re partial to plasma, LCD, DLP, or LCoS technology. Any projection or direct-view display using these technologies must exhibit wide dynamic range. That’s a tough job for many display technologies as they try to define images with low or no voltage levels (black), high-voltage levels (white), and everything in between (shades of gray or grayscales).
Things get even trickier with pulse-width modulated (PWM) technologies like DLP and plasma. Both try to approximate a linear response curved with rapid cycles of on-off switching, and as a result can have difficulty with low-luminance signals. While that isn’t as much of a problem for large active matrix LCD (AMLCD) and small high-temperature polysilicon (HTPS) LCD, the higher black levels of LCD technologies cause that low-level information to disappear — almost as if it was vanishing into noise.
High brightness and/or contrast measurements do not a linear display make! A projector can crank out enough lumens to cause spontaneous ignition of projection screens while simultaneously crushing white and black levels. A plasma panel can dazzle you with bright, saturated colors, but creates artificial boundaries known as false contours through a smooth grayscale ramp.
Like any digital process, to reproduce linear image information, digital microdisplays and flat-panel technologies must first sample that information and convert it to bits. The higher the sampling rate, the more steps along that sine wave we can reproduce. The analog-to-digital converter (ADC) must process a video signal by sampling it as three individual channels of color information, using at least 8 bits per color pixel and preferably 10 to 12 bits for improved realism. For some time now, the standard for PC color display circuits has been 8-bit color, or 16.7 million total colors. Within each red, green, and blue color palette, 256 luminance levels should be sufficient to convey realism. But digital sampling results in square waves and associated artifacts, such as “aliased” curves. Increasing the number of samples is the only way to smooth out those curves.
Fortunately, RAM is cheaper and processors are faster than ever, making it possible to move to 10-bit processing — even in inexpensive home theater projectors. Along with that expanded capacity to sample and store luminance and color information have come expanded adjustment menus, which not only let you set color contrast and gain, but also tweak Gamma settings for individual color channels.
I’ve had a pair of front LCD home theater projectors in my studio lately from Sanyo and Panasonic, and both projectors have Gamma adjustments not previously available. On the Sanyo PLV-Z4, Gamma can be set over a wide range of steps for each color channel, while Panasonic’s PT-AE900U has three Gamma settings for all color channels. Both circuits make a dramatic improvement in color quality.
But neither of these circuits would be worth a dime if the projectors weren’t first set up for the widest possible dynamic range. I do that with a series of test patterns and try to hold a discernable step between 95 percent and 100 percent (white), and 5 percent and 10 percent (black). That means I ignore the factory settings and simply make my own adjustments, often reverting to a multi-step grayscale ramp to see if I can still see a difference between 100 percent white and the step just below that.
Think about it: The wider the dynamic range, the more color shades that can be shown. And as I mentioned at the start, wide grayscales and color shade palettes make for the most lifelike images. Using the “best grayscale” technique often means that measured brightness isn’t nearly as high as the manufacturer claims in its data sheets, but that’s not really as important as linearity.
The good news is that more display manufacturers are apparently figuring this out. Once projectors got down into the 6- to 8-pound range and broke the 2,000-lumens barrier, any further increases in brightness have been largely incremental. So, all of that lamp horsepower was tamed with better color filtering to produce video images with good dynamic range.
Tuning up a display for the best dynamic range doesn’t mean you sacrifice much in the way of brightness and contrast, either. The PT-AE900U delivered just less than 500 lumens with average contrast at 330:1 and peak contrast just a shade more than 500:1. Those numbers are way beyond what a CRT display can achieve, and in truth, the PT-AE900U can easily double the brightness reading. But then, it wouldn’t be linear anymore.
The PLV-Z4, while not quite as bright and contrasty, also put on a good show. My “preferred” settings resulted in brightness readings of 350+ lumens with average contrast at 275:1 and peak contrast close to 400:1. In a darkened room, your eye would be hard-pressed to see the difference between 400:1 and 500:1, both of which far exceed the eye’s instantaneous (non-irised) contrast ratio. For comparison, a properly adjusted CRT projector can reproduce contrast ratios as high as 300:1 (0 = black).
The black level issue is being addressed in some projection systems by using an automatic iris. In essence, this functions just like an audio automatic gain control (AGC) circuit, which dynamically changes the volume of an amplifier in step with changes in voltage levels of the incoming audio signal. The dynamic iris opens and closes in response to changes in overall picture luminance levels, and it works because the iris in our eyes opens up to view a dark scene as the projector’s iris closes down to produce an apparent reduction in black levels.
Up on the wall, manufacturers are mounting screens with less than 100 percent white surfaces in another attempt to improve black levels. Such screens will have 95 percent to 98 percent reflectivity and are often recommended for use with LCD and DLP projection systems. Under normal lighting, it’s hard to tell the difference between a gray screen and a white screen.
While both gray screens and auto-iris circuits work really well, it can’t hide the fact that some of the low-level luminance information in the original video signal is dropping below the projector’s black level, and won’t be seen at all. Do you really care that shadow detail is compressed, or even missing altogether?
Perhaps a more appropriate question would be: Can a display be developed that combines wide grayscales and dynamic range with the ability to show very low levels of luminance in a linear fashion, like the CRT? So far, none of the mainstream technologies have been able to pull it off.
One thing CRT technology has going for it is that unlike any of the technologies currently poised to replace it, a CRT is a voltage amplifier. With high voltages on the anode, producing an almost infinite number of signal levels is a snap with low-voltage input signals. That’s not the case with transistors and integrated circuits, which are current amplifiers. They handle square waves well; sine waves not quite as well.
Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at firstname.lastname@example.org.