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MillimeterWhat to Know about the Move to LCD Monitors
Sep 28, 2005 4:31 PM, Gary Mandle Senior Product Manager, Display Systems, Sony Electronics
With the need to upgrade on the agendas of many organizations, video production professionals are continually making plans to implement the next iteration of technology. In the professional display world, this means a transition from cathode ray tube (CRT) monitors to liquid crystal displays, commonly referred to as LCDs.
But there is a range of issues associated with this migration, and while LCD offers several distinct advantages over CRT and is clearly the way of the future, CRT is still maintaining its lead in certain applications, specifically critical evaluation.
There is still no better way than CRT to exactly replicate an electronic image. Professional LCD monitors certainly have their place, and they’re growing in popularity because of form-factor, weight, power savings and how they reject ambient light. But for the most critical evaluation, there is not yet a display technology that can replace CRTs for grey scale reproduction, high contrast range, standardized color gamma, and near-perfect reproduction of blacks. CRTs have also always held the highest level of resolution, uniformity, bandwidth, and color accuracy over other display technologies.
Technical video evaluation entails the judging of image quality, color consistency, noise level, and making quality assurance decisions. Operators often need to create action plans to resolve system or signal problems, verify signal continuity, and evaluate a program’s technical quality. This is where use of a CRT is required.
On the other hand, judging video content, which commonly involves decisions about camera blocking, set lighting and confidence monitoring, is a perfect fit for LCD technology. In this case, LCD performance is on par with the CRT monitors currently in use and LCD video monitors even add flexibility in terms of input options and integration simplicity.
And significant strides are continually being made in the development of LCD monitors that promise to put them on equal or even higher footing than CRT in the very near future.
The advantages of LCD over CRT are many. First on the list is the fact that an LCD display, such as Sony’s LUMA™ LCD monitors, is a purely digital monitor from front to back. If a facility is inputting SDI or HD-SDI streams, then LCD offers the advantage of never having to convert images to analog.
The only part of an LCD monitor that can wear is the backlight, but even this should become less of an issue with new backlight technologies planned to hit the market in the near future. The result may be a near maintenance-free display that could eliminate concerns over convergence alignments, white balance, and other errors. This, coupled with LCD’s high level of reliability, can add up to a very impressive return on technology investment.
Other advantages of LCD are reductions in power and weight without sacrificing performance, important considerations for facilities on a budget that need to closely watch such costs as power and air conditioning. Low power and weight take on even greater importance in mobile applications such as large production trucks.
LCD monitor walls can now be designed less than 10 inches deep allowing more room for staff or other equipment. Power consumption is lowered by a factor of 3, and heat by a factor of 7, times. This is all money put back in the pockets of the customer.
A third big advantage is integration and operation. As facilities become more software-dependent, multi-monitor systems—combinations of video and computer monitors—are becoming commonplace. With video LCD monitors it’s possible to integrate these two sources into one display, saving the cost of a second display while simplifying the system design. Current LCD video monitors on the market have computer, composite and SMPTE digital inputs; in other words everything connects into one box.
The rapid technological evolution of LCD monitors for video applications is also noteworthy as a significant trend, one that many video professionals were hesitant to follow upon their first experiences with LCD, but have since begun to fully embrace.
There are some issues associated with making a change in monitor type, and several factors need to be taken into consideration by a facility that’s deciding whether or not to stay with CRT or move to LCD. There are also several misconceptions about LCD versus CRT.
To begin with, high contrast levels are often used to tout LCD’s capabilities to reproduce blacks, but it’s rare to see any statistics that state the actual black level of the display. That’s why a high contrast spec, which sounds good on paper, really doesn’t mean anything. The same holds true for bit rate and response time. Although there are established industry standards for measuring contrast and response time on LCD, they are not always followed. Therefore, it’s not always safe to use a “better the number the better the panel” approach when making purchasing decisions. You need more detail to make that judgment on your own.
One misconception is that current CRT high definition monitors don’t display full resolution HD. This confusion is partly the result of a division between computer manufacturers, who specify resolution in number of pixels, and video manufacturers who specify resolution in TV lines (TVL), which is basically measuring line pairs.
These are very different measurements and it is often unclear as to which is the more accurate statistic, but the real truth is that there are issues with both. CRT monitors, such as Sony’s BVM-D series, are specified at 1000 TVL. To try and look at it as pixels, you could count the number of wires in the aperture grill. At that point you’d see that there are more than 2050 of these within the image area. If you stay within the physics of HD, a 1080i signal can produce no more than 873 TVL. That’s the upside.
To really look at this correctly, you must also consider the mean transfer function of the imager. At 1000 TVL, Sony monitors for example, show a 30MHz multi-burst at about 50 percent depth of modulation. To be more specific, the peak white versus the peak black of the signal is 50 percent of what it would be compared to the 500 KHz pulse.
This argument can also be applied by pixels, but then dynamics change radically. An LCD monitor is a fixed matrix, which means that for whatever input is fed into it, the scaling of the monitor must reformat the image data to fit the LCD matrix. This will add errors or artifacts since no conversion can ever be fully true to the input signal. This is one primary reason why LCD should not be used for critical monitoring.
And while the LCD matrix may offer an HD pixel count, it will not display full HD. All the panels currently on the market are 8-bit , while a normal HD-SDI signal input is 10 bit. As a result, the best grey scale and color scale mapping can only be 256 levels when compared to a CRT monitor’s 1024 levels.
Another issue is frame-to-frame response or temporal resolution. Most manufacturers specify switching speed or the time between when the pixel opens and when it closes as one number. While it’s a great marketing hook it really doesn’t have any significance when less than 35ms. What is critical is the light level at which the pixel returns to black.
The effect of this is similar to a CRT monitor’s modulation transfer function (MTF) capabilities. As the input frequency moves higher, the black to white difference diminishes. Many of the LCD technologies lose black levels as faster pixel switching speeds are required regardless of this specification due to residual static charges. This is why a 10ms panel may look worse with video than a 35ms panel.
However, the operational benefits of switching to LCD include lower total cost of ownership and more flexibility and higher performance, as well as cost savings with little or no sacrifice in performance for many viewing applications. The bottom line is that LCD provides reliable operation at a much reduced cost when compared to CRT. And, unless you are operating in an extremely critical environment where you must have the added performance of CRT, there’s really no better way to go other than LCD.
For more information about Sony displays, visit www.sony.com/professional
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