The CRT display, for many decades the only choice for viewing video content, is now history. And while there are several display technologies jockeying to replace it in the professional arena, it's not yet clear which will be the winner.
There has been a sea change in the world of electronic imaging over the past 10 years. Manufacturers have moved away from small assembly-line models to large-scale semiconductor “fabbing” of new display technologies, combining brighter displays with high resolution. The increased importance of the consumer TV marketplace led to dramatic drops in retail prices as flat-panel TV screens grew even larger.
Today, it is possible to buy a 50in consumer HDTV with full HD 1920 × 1080 resolution for less than $1000. That's great news for the home viewer but problematic for the video and film professional who is faced with the prospect of replacing older professional CRT monitors with … what? Plasma monitors? LCD monitors? Organic LED (OLED) monitors?
Defining the task
What, exactly, is a reference monitor? In the simplest terms, a reference monitor is one that stays within close tolerances for performance, can be calibrated to specific brightness levels and color temperatures, has a color gamut that matches one or more standard color spaces, and exhibits consistent gamma performance from 0 IRE to 100 IRE — and higher.
The CRT was the standard for years because it behaved just as a well-designed tube amplifier ought to. Small changes in input voltages resulted in corresponding steps in luminance values, a linear response that correlated to the signals being captured by tube video cameras, and later, solid-state cameras using CCDs.
The range from video “black” to full white on CRT monitors (i.e. dynamic range) was not extreme, but wide enough to evaluate electronic images for broadcast and transfer to recorded media. A well-designed reference monitor with black levels measuring .2 nits (.06ft-L) and peak whites at 100 nits (29ft-L) exhibited a contrast ratio of 500:1 — more than adequate for everyday work.
This reference monitor could also accurately reproduce a specific gamma curve to match standard gammas used with live video content and filmed content transferred to video. The monitor might not be blazingly bright, but its light output was sufficient to be viewed under controlled ambient lighting.
Equally important, a reference CRT monitor would be expected to track a specific color temperature value from black to white, and all steps in between. This track could vary only by a small amount — say, 250K — to be considered for critical color and exposure correction, where larger changes in white point values create noticeable color shifts in subtle, pastel color shades and flesh tones.
As far as color reproduction was concerned, professional CRTs conformed to the SMPTE-C standard gamut, which closely matches the NTSC standard and slightly exceeds the EBU (PAL/SECAM) standard. Any limitations on the reproduction of certain color shades were due to the maximum saturated levels of red, green and blue phosphor compounds used in these monitors.
It can be said that HD video basically wrote the obituary for CRT technology. At the heyday of CRT reference monitors, most video production was done at SD resolutions like 720 × 480 (NTSC) and 720 × 576 (PAL). Because of the link between spot size and resolution, the highest resolution achieved by the best CRT monitors capped out at 1100 lines — about half of what was needed to accurately display 1920 × 1080 HD content.
Unconstrained color gamuts for a reference plasma monitor
reference LED LCD monitor
reference OLED monitor
(The black outline represents the ITU BT.709 color space.)
There are several candidates to replace CRT technology for evaluation-grade monitors. They fall into either of two categories: transmissive (light shuttering) or emissive (burst).
The leading contender here is the LCD. LCDs use liquid crystal compounds that align with changes in voltage to pass or block white light from a CCFL or LEDs. (See Figure 1.) Red, green and blue microfilters are applied to the front surface of each pixel to obtain full RGB color.
If a modulated light source is used, the light source can also generate color (red, green and blue LEDs). Modulated light sources are unique in that they can be used for local area dimming to enhance intrascene contrast for higher dynamic range.
Plasma display panels are the leaders in this category. Plasma pixels, which contain a mixture of neon and xenon gases, emit ultraviolet light when electricity is discharged through them. The burst of UV light in turn stimulates red, green and blue phosphors to glow. Plasma pixels switch at very high speeds and can handle faster frames rates than LCD displays. (See Figure 2.)
Because plasma monitors function as high-speed switching displays and provide only two operating states (on and off), they create grayscale images by using a pulse-width modulation (PWM) technique. The luminance level of the image is determined by the ratio of “on” cycles to “off” cycles within a specific time interval.
OLEDs are also classified as emissive displays and function just like other semiconductor devices, with a flow of electrons from anode to cathode at low voltages with high current. As electrons collide with “holes” in the organic film structure, photons are emitted with different colors. The color is dependent on the materials used to create the organic film layer. (See Figure 3.)
How they compare
CRT-like gamma curves as emulated on plasma
When discussing contrast performance, the limiting factor is typically the level of black. A reference plasma monitor will easily achieve black levels at or below .1 nits (.03ft-L) after calibration, whereas professional LCD monitors can range as high as .4 nits (.12ft-L) with using conventional CCFL backlights. (LED-equipped LCD monitors with local area dimming can achieve the .1 nits threshold.) OLEDs essentially have no lower black level threshold, as they can be modulated at extremely low levels.
Plasma, LCD displays and OLEDs are all capable of equaling CRT color gamuts. All three can exceed SMPTE-C and BT.709 standard color spaces and cover all or most of “extended” color spaces, such as xvYCC and P3. (See Figures 4a-c.)
How about gamma performance? The standard gamma for a CRT is typically in the range of 2.3 to 2.4, and while the new generation of LCD, plasma and OLED displays can reproduce everything from flat (1.5 or less) to s-curve (film) gammas, they are also able to emulate CRT performance, provided they are not operated at extremely high levels of brightness. (See Figures 5a-c.)
Consistent grayscale (color temperature) tracking for LCD, plasma and OLED is not difficult, but is dependent on consistent gamma performance. Sheer brightness is not the goal of a reference display; accuracy is. The latest generation of reference plasma monitors and LED-backlit LCD monitors can produce accurate grayscale tracks at any standard color temperature. (See Figures 6a-c.)
Grayscale (color temperature) tracks for a reference LCD monitor
reference plasma monitor
reference OLED monitor
(The solid center line represents 6500K.)
Reference monitor products that use all three of these technologies are now available commercially. Panasonic is now shipping its TH-42BT300U (42in) and TH-50BT300 (50in) reference plasmas, while Dolby has ramped up production of its SRM-4200 42in LED LCD reference monitor that uses local area dimming for higher dynamic range.
Sony and TV Logic are both exhibiting and taking orders for OLED monitors. Sony's Trimaster EL-series OLEDs are available in 17in and 25in sizes, with additional screen sizes coming to market in 2012. TV Logic sells a 15in OLED monitor for studio, remote control and master control use.
It's still too early to say which of these technologies will eventually rule the roost as CRTs once did, but in all likelihood it will use emissive technology. Emissive technologies offer the widest viewing angles and best color saturation and combine them with high dynamic range and low black levels. (Wide viewing angles and low black levels remain a challenge for LCD displays.)
For now, plasma has that level of performance locked up in larger screen sizes, but bigger OLED displays are definitely in the works. Will we see any make their debut in 2012? Check back after NAB.
Peter Putman is president of ROAM Consulting.
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