Professional Display Technology Overview – by Bram Desmet
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Professional Display Technology Overview – by Bram Desmet The following is a very high level overview of various display technologies currently in use for professional monitoring applications. It is meant to serve as a straightforward guide to the benefits, limitations, and realities of various technologies as they exist at this moment. Display technology is evolving rapidly so it is important to keep in mind that this overview reviews these technologies as they exist at this time and therefore this overview does not reflect the state of the industry as it existed 1 year ago nor will it likely reflect the state of the industry as it will exist 1 year from now. This paper is written with readability by both a non-technical and engineering audience in mind so some of the topics covered are admittedly oversimplified (on purpose). For those looking for more technical elaboration on any of the points below we welcome you to contact us at [email protected] . CRT Monitors CRT technology for many serves as the reference to which all other display technologies are compared. For this reason it is important to review the pros and cons of CRT monitors before discussing other display technologies. For decades CRT Monitors were essentially the only reference grade video monitoring devices available and as such the standards for video acquisition and post production were largely based around the characteristics of these display devices. Though CRT monitors have been the standard bearer of professional monitoring for many years they are not the perfect monitoring devices and do have many serious limitations. The primary performance benefit of CRTs, even to this day, is the achievable black level. CRTs have very high contrast ratios because they are emissive displays (light emission occurs at the screen level), so they have the ability to emit little or essentially no light when reproducing low luminance signal information. Another benefit of CRT monitors is response time. The rise and decay time of an excited phosphor is relatively fast, which leads to minimal motion blur. There is often a tendency to overemphasize these performance benefits when comparing other display technologies to CRT monitors and doing so often neglects to properly discuss the drawbacks and real world limitations of CRTs. Some of the most basic CRT drawbacks and limitations are: 1. Linearity: CRT monitors have difficultly achieving and maintaining linear horizontal and vertical speed of the electron beam’s position and this lack of linearity leads to onscreen irregularities. In plain English, if you put something like a crosshatch on screen it is difficult for the CRT monitor to maintain proper equidistant spacing between the lines of the crosshatch. On fixed pixel structure displays this is a not an issue (i.e. a crosshatch pattern will have perfect equidistant spacing between lines). 2. Geometry: Unlike fixed pixel structure displays CRT monitors have a difficult time maintaining proper image geometry. Simply put, an image as simple as a circle is not always shown as a perfect circle on a CRT monitor. Even a properly adjusted CRT monitor rarely exhibits perfect geometry and anyone that has spent an extended period of time working with a CRT monitor can likely attest to the time spent fine tuning geometry adjustments on the display over time. 3. Convergence Errors: The guns controlling the Red/Green/Blue components of a signal are physically at a distance from each other in a CRT monitor. This means that they are all influenced differently by the deflection coils. This can result in the beams landing in the wrong places and this leads to what is commonly called a convergence error. This error could easily be 1-2mm on many CRT monitors and as ©2011 Flanders Scientific, Inc. 3/15/2011 1 Professional Display Technology Overview – by Bram Desmet bad as 3-4mm in the corners of some displays. In plain English, this means that with something like a crosshatch pattern on screen operators would see individual Red/Green/Blue lines diverging from each other where technically a single (converged) white line should exist. Adjusting for convergence errors is a time consuming task and often requires both ancillary equipment such as a convergence gauge and a knowledgeable technician familiar with correcting for convergence errors. This problem does not occur on fixed pixel structure displays as the R/G/B sub pixels have a static location onscreen and cannot physically move. 4. Stability: CRT monitors are relatively unstable display devices. Drifts in luminance and color temperature tend to occur at a much faster rate on CRT monitors than many alternative display technologies. Regular recalibration of CRT monitors is necessary to stay within acceptable tolerances. Some facilities, especially those using aging CRT monitors, have adopted recalibration cycles of once, or even more regular than this, per month. 5. Environmental Variations: CRT monitors are greatly affected by magnetic fields. Environmental variables such as the CRT monitor’s location in the world, physical positioning within a suite, and the equipment surrounding the monitor can all significantly impact the performance of the CRT display. All of these variables need to be accounted for, often manually, on CRT monitors and are particularly troublesome if the monitor needs to be moved regularly as is the case in field use applications. 6. Poor Multi-Format Support: All of the concerns listed above are amplified when the CRT is used as a Multi-Format monitoring device. CRTs in general do not switch well between SD and HD formats. Different components are physically used inside the CRT monitor when dealing with different formats. This means that individual monitor adjustments are required for these different formats and on many CRT displays this means complex manual adjustments per format to account for the variations between these differing relays and circuits. In addition to these difficulties CRT monitors are physically incapable of natively supporting certain formats (such as 24p material). 7. Fixed Color Space and Gamma: CRT monitors have a fixed color space determined by the phosphor set used (EBU, P22, SMPTE C). CRT monitors cannot accurately reproduce many defined color spaces in common use such as Rec 709 (HD) and DCI P-3. So while CRT monitors may be still be in use at many facilities as reference displays for work on HD material they are demonstrably incapable of correctly operating within the Rec 709 color space. Furthermore, CRT monitors do not have programmable or adjustable gamma making them unsuited for many workflows requiring a specific display gamma response. 8. Limited Peak White Luminance: CRT Monitors in general cannot operate well at high luminance levels required for operation in brighter environments. Moreover, many CRTs are actually incapable of maintaining industry recommended peak white luminance levels as they age. In many facilities that have traditionally used CRT monitors this has led to the in-house adoption of lower peak white luminance levels that were often a function of manufacturer’s recommendations or the facility’s own experience with expected longevity/stability of the monitor at a given luminance level, but these in-house standards did not necessarily have any relation to the recommended practices set forth by the industry’s various technical standards organizations. 9. Curved Screen: CRT monitors do not have flat screens. The lack of a flat screen makes it inherently more difficult to display a straight image. ©2011 Flanders Scientific, Inc. 3/15/2011 2 Professional Display Technology Overview – by Bram Desmet 10. Limited Form Factor: CRT monitors come in very restricted size options. Large format CRT monitors are difficult to produce and at any size are generally much bulkier and heavier than alternative display technologies with the same screen size. 11. Environmental Health Concerns: CRT monitors have traditionally relied on large amounts of lead to shield operators from the otherwise harmful effects of the electron beam. In larger CRT monitors the amount of lead used could easily be as much as 5lbs. The concern over disposal of CRTs and their contribution to high levels of lead in landfills led many governments to adopt Lead Free Initiatives that essentially banned the sale of such CRT monitors. 12. Poor Availability: Even for facilities wanting to use CRT monitors despite some of the technical limitations listed above the general lack of availability of high quality CRT monitors has become a real world problem. Lack of availability is attributable to a combination of environmental health concerns, limited form factor, and technical limitations all of which have led to a drastic reduction in demand. The production of glass used in CRT monitors requires 24/7 operation of large glass furnaces that cannot be easily switched on/off as needed. Without sufficient consumer demand for products using such glass it has become economically unviable for the glass manufacturers to keep these facilities running on such a required non-stop basis. Despite the limitations listed above the tendency to review other display technologies as they compare to CRT monitors is a rather natural inclination as CRT monitors are a known quantity with which a lot of operators have decades of experience. LCD Monitors LCD Monitor technology has undergone a rapid evolution in recent years. Early generations of LCD monitors exhibited very poor performance and were typically not suited to any critical monitoring environments. Limitations in viewing angle, contrast ratio, pixel response time, achievable color gamut, and backlight lifespan meant that early generation LCD monitors were typically reserved for less critical viewing environments where the benefits in power consumption, form factor, and cost outweighed the performance limitations. However, this began to change in the early part of the 21st Century as LCD monitor technology rapidly evolved to become suitable for more and more critical monitoring applications.