Replacing the CRT

With so many new display technologies available, which one will be the true cathode ray tube (CRT) replacement? CRTs have been the only practical display technology for video since the beginning; they are also the standard for visual confirmation of color accuracy. Even the standards for colorimetry and gray scale are all written for CRT displays and encoded into every recorded frame of existing video (e.g. gamma, color space, etc.).

Today, all this is about to change because several display technologies, most notably LCDs, have emerged, which has led to a rapid decline in the number of CRTs being manufactured. It’s imperative for broadcasters, as well as production facilities, to have display monitors that can correctly display NTSC colors and gray scale as well as today’s digital standards. There has even been some concern by engineers as to whether any of the new technologies are capable of matching the performance of the CRT. Later in this tutorial is a description of most current displays, either in production or soon to be, along with their capabilities and shortcomings. But before that, it would be helpful to go over current standards for broadcast monitors as well as to look at what organizations such as the Society of Motion Picture and Television Engineers (SMPTE) are doing to address the changing state of display technology.

Addressing new technologies

The SMPTE recently published the “Study Group Report on Display Technologies,” in which the group outlines topics that need to be addressed for an SMPTE Work Group to write a standard for current and future image displays. These are also issues that should be kept in mind when evaluating any new monitor today.

The SMPTE first discovered that it needed to define the capabilities of modern professional CRT monitors to create a comparison for newer flat-panel monitor (FPM) displays. Some of the issues that the report considered are: luminance ranges, contrast ratio, color gamut, screen pixel array sizes, deinterlacing, delay time, pixel defects, streaking and motion artifacts.

One of the points raised discusses square pixels typically used on FPM displays and how this relates to accurately displaying SD images. HD requires square pixels, while SD uses non-square pixels, which presents a problem for FPM displays. Another difficult question addressed is how forward and backward compatibility can be maintained while allowing technology to advance.

Current monitor evaluation standards

As the SMPTE discovered, when looking at new display technologies, it is best to become familiar with what these new devices are attempting to achieve. The European Broadcast Union (EBU) created a set of monitor requirements, under Tech 3320 “User Requirements for Video Monitors in Television Production.” This document groups broadcast monitors into varying “grades” (ranked one through three) depending on their intended use.

Grade 1 monitors are devices for high-grade, technical-quality evaluation of picture capturing, post production, transmission and storage. These monitors must possess, at least, the same quality properties of the equipment to be controlled. This means that artifacts should not be unduly masked, and additional artifacts shouldn’t be introduced. As a reference device, the settings of this type of monitor should be adjustable as well as lockable so only authorized access is possible. The Grade 1 monitor is a “measuring instrument” for visual evaluation of image quality; therefore, it’s necessary for the monitor to be able to reproduce the scanning mode of the signal in the native way (progressive or interlaced) or as it is intended to be viewed (e.g. 50Hz presentation of 25p material).

Typical applications for Grade 1 monitors include camera control, color grading and quality control as well as lighting control positions — that is, areas where video technical quality parameters are evaluated, controlled and corrected.

A Grade 2 monitor may have wider specification tolerances than a Grade 1 monitor for the benefit of a significantly lower price, or smaller size or weight. Grade 2 monitors are used in applications where tighter tolerances (for example, on accuracy of color reproduction and stability), as well as some equipment features, are not necessary. Areas of application for Grade 2 monitors are preview, control walls, edit suites and control rooms, if no picture quality manipulation is carried out. In many cases, it’s possible for Grade 2 and Grade 1 monitors to be used together — for example, in TV production control walls.

Grade 3 (observation or presence) monitors are devices equivalent in many respects to high-end domestic/consumer displays. For TV production applications, important considerations include the availability of professional interfaces and transportability as well as electromagnetic compatibility and acoustic noise.

Application areas for Grade 3 monitors include audio production, dialogue dubbing, signal presence monitoring, commentator positions and displays for the audience in a studio.

These requirements were originally written for CRT displays, but in the current version, updated May 2008, the EBU recognizes the advances of modern technologies and addresses some of them in the document.

Cathode ray tubes

CRT displays have come a long way since their beginnings, but they still suffer from a number of short comings: the brightness of the image is limited by the size of the tube, and tube brightness influences the resolution of the image (spot size). They also consume a great deal of power and they are large and bulky compared to their screen size.

On the other hand, CRT displays can adapt to different resolutions by changing scanning frequencies and, thus, display almost any video source in its native format without any transformation of the image. They display a wide range of linear gray scale and present accurate and precise color shading.

Any new display must be able to recreate the same gray scale and colors that were seen when the video was created. For professional monitors, SMPTE C phosphors where used to create the most realistic colors, while in consumer televisions, P22 phosphors gave a slightly different but brighter color. So consumers never saw exactly the same colors as those in the control room, and the same is true today. In those days, the brightness of a TV set helped to sell it. But today, it’s more a matter of cost, because reproducing the CRT color range requires much more technology — and money.

Only a single manufacture is still producing CRTs, and most of those are for consumer televisions. Gone is the day of high-end Grade 1 monitors that represented the height of CRT technology.

Next Time
The next "Transition to Digital" tutorial will continue with an overview of new monitor technology.

Editor’s Note: Some of the information for this tutorial came from Pete Putman of ROAM Consulting.