Linearity in projectors and monitors
Nov 1, 1998 12:00 PM, Peter H. Putman
When the topic of large-screen displays comes up, we rarely hear anythingabout linearity, the ability of an electronic display to generate afaithful reproduction of the parameters of the input signal driving thatdisplay. Yet linearity is the most important property of any electronicamplification circuit, whether a high-performance stereo amp or a three-gunCRT video projector. Our world is linear, and our senses respond linearlyto aural and visual stimuli. How we respond to those stimuli depends upontheir linear nature; a horn blast that gets progressively louder tells us atrain is approaching. We can also judge relative intensities of stimuliover a wide range. This dynamic range lets us process a great deal ofinformation about the sounds we hear and the sights we see. Forlarge-screen displays to approximate visual realism, however, they mustexhibit a wide dynamic range-a tough job for many display technologiesbecause they try to define images with low or no voltage levels (black),high voltage levels (white) and everything in between (shades of gray orgrayscales).
Pumping ironIn any linear amp, there is a definable measure of performance-gain. Gainis the difference between the input signal level and the output signallevel, measured in decibels or volts. Gain is not particularly difficult tomeasure; simply connect the source signal, the measuring device and anappropriate load or termination, and you are ready.
Gain is not a useful figure without also measuring dynamic range, and anaudio amp would be useless if it worked at only certain frequencies or wasunable to deliver the rated power without creating distortion. For thatreason, dynamic range is usually defined as the range of input signals overwhich the rated gain figure is achieved up to the point at which the ampgoes into compression. That occurs when the output drops by 1 dB, alsoknown as the 1 dB compression point. Upon compression, the amp no longerbehaves linearly, and it may alter the input signal, yielding distortion.Distorted signals are no longer faithful reproductions of input sine waves;they are square waves, which can mean harmonics that mix with the inputsignal to create new signals (distortion products and picture artifacts).
Everything's fuzzyDynamic range in audio and RF amps is not difficult to calculate, but videodisplays are a different story. We must depend on test patterns and testequipment to tell us exactly what is happening in a given projector andmonitor. CRT-based displays (monitors and projectors) are truly linear, andit is not all that difficult to tell when they are operating incorrectly. Acathode-ray tube, however, is made up of three or four elements-thecathode, which emits electrons, one or two grids, which behave like faucetsto vary the flow of electrons, and the anode (plate), which is what theshaped electron beam strikes to form either a full-color image (picturetube) or single color (monochrome CRT phosphor).
The dynamic range of a cathode-ray tube can be quite high, particularlywhen a second grid is used (known as G2; the control grid is G1). If thetube is biased for Class A or Class AB operation, the ratio of the outputsignal level to input signal level (gain) can easily exceed 100:1. Still,there will be a point at which the CRT's gain will flatten out, sending thedevice into compression.
There is also a chance that the anode current will run too high,drastically shortening the life of the tube. For this reason, manymanufacturers incorporate a cathode current limiting circuit, such as aballast resistor, into the projector or monitor. Current limiting ensuresthe tube will not be pushed outside normal operating parameters but will,in effect, establish an upper limit on the tube's gain, thus setting thedynamic range.
Everyone's biasedThe control grid (G1) bias and screen bias (G2) determine a CRT's gain andlinearity. These controls can be adjusted to crank out the lumens at theexpense of tube life. Consequently, G1 and G2 circuits are heavilyregulated and may even be current limited. Use too little G1 bias, and thetube operates in Class B or possibly Class C mode, resulting in incorrectcolor levels, image distortion and uneven grayscale response at low-inputvoltages. With too much bias, anode current soars, possibly crushinghigh-input signals as the tube goes into current limiting. Too much powerdissipation on G2 may cause it to blow out.
Grayscale tracking (the color of gray at different input signal levels) isalso determined by the linearity of the individual picture tubes orprojection CRTs and will require additional bias adjustments. For thesereasons, CRT displays need to be calibrated to produce the widest possiblegrayscale, never the brightest image. The new eight-step ANSI test patternsand multistep grayscales available from Sonera Technologies and ExtronElectronics are useful for this calibration.
It is not hard to see why a matched set of CRTs would be a worthwhileinvestment in a critical projection application. Hopefully, themanufacturer of your particular projector has done his homework and matchedthe tubes beforehand so that any small differences in performance can betweaked with bias adjustments.
Bitstream biasDigitally modulated displays, such as LCDs and DMDs, are yet another story.These technologies use rapid changes between on and off states along withthe intervals of time between those states to create grayscale images. InLCD panels, small transistors work as switches to turn on and off voltagesto individual pixels. This creates a charge/discharge cycle that works fastenough to display video and many fast computer refresh rates.
DMDs are controlled by pulse-width modulation (PWM) circuits thataccomplish the same thing, but there are no analog voltages. Mirrors areeither on or off, and the ratio of on cycles to off cycles in a given timeinterval determines the grayscale value created. This on/off behavior iswhy digital light processing is often called the only true digital imagingtechnology.
Plasma displays are closer in behavior to LCDs. A voltage is applied to acell, ionizing the gas within to a plasma-like state and creatingultraviolet light, which, in turn, excites the red, green or blue phosphorinside the pixel. The duration of the charge/discharge cycle determines theintensity or grayscale value of that pixel.
In all these cases, the controlling electronics must digitally process thevideo/computer signal information by sampling it as three individualstreams of 8-bit, 256-step monochrome information. In a practical sense,256 levels of gray should be sufficient to convey realism, although somemanufacturers are tinkering with 10-bit sampling.
For LCD, DMD and plasma technologies, linearity has been a big obstacle toovercome. CRT-based displays still produce richer blacks and widergrayscales. Part of the problem lies with the off states of LCDs, plasmaand DMD displays-if they reflect or transmit any light at all wheninactive, that value becomes the lower limit of the display's dynamicrange. This effect is similar to turning up the brightness too high on aCRT display, resulting in a dark gray no-signal image instead of a richblack one. In plasma displays, what often happens is that black areas invideo images become solarized with oddball colors because the signalprocessing in a plasma display does not understand that something existsbelow NTSC black (4 IRE), and its signal processing becomes non-linear,resulting in a color artifact.
A properly adjusted CRT projector can reproduce a wider range of gray stepsthan we can perceive, resulting in contrast ratios far beyond 255:1(0=black). The best LCD displays are hitting 150:1 average with peaks to300:1. DMDs are still a little behind at about 100:1, while plasma displaysvary from 75:1 to more than 130:1 average. Improvements in pixeltransmissivity (aperture ratio) and mountings for pixel-based imagingsystems will surely improve performance at low levels of gray and even black,although it is doubtful that LCD and DMD black levels will approach thoseseen on CRT-engined displays anytime soon.
The notable exception is liquid-crystal light-valve technology, which usesa CRT to write the scan lines on a single-pixel liquid crystal. Contrastratios of 300:1 or more are routinely attained in this system, offered byboth Hughes-JVC and AmPro.