Originally published 10/15/2004, HDTV Magazine, editor Dale Cripps
Updated 07/18/2005
By Richard Fisher
Let us begin with technology that is over 100 years old but remains the bench mark for video performance, the CRT or cathode ray tube. CRT remains king because of one very special attribute, the ability to morph itself into different native scan rates. This means no scaling or signal processing is required to present the image to the screen. CRT displays are available in three different forms, DV-direct view, RPTV- rear projection TV and FPTV-front projection TV.
Direct view is the most common for the last five decades. The new HDTV versions come in both 4:3 and 16:9 aspect ratios with sizes up to 38" 16:9 and most offer two native scan rates, 480P and 1080I. All suffer from screen glare so a controlled viewing environment is required to reduce or eliminate this problem. These can have very high light output and the smaller the screen size the brighter the image. Recommended for bright to medium rooms. Due to the small screen sizes and the viewing distances commonly used bias lighting may be necessary. They will not meet HD CVD of 3 screen heights but again I doubt many will watch a 38" from 5 feet so this is optimal for the casual viewer and it should be noted that these sizes are too small for home theater applications.
Edge focus is poor. Perfect convergence of the red, green and blue is not possible. Near perfect geometry is unlikely. Overscan is commonly 5-9%. Direct view CRT uses a shadow mask that creates moire artifacts in the image. If you are a videophile and intend to use one of these at 3-4 screen heights then definitely look at the Sony Fine Pitch series...there were only two but that has been expanded to some 4:3 models as well (as a videophile 16:9 is preferred). Even with these, direct view CRT hides problems and does not have the resolving power of other displays. Those that do are professional broadcast monitors running $25k plus and require controlled lighting in a dark room. Burn in is of little concern but is still possible. Medium uniformity. Very wide viewing angle horizontally and vertically for numerous casual viewers. Low maintenance. Color temperature and tracking alignment required every 1-2 years.
RPTV has become quite common in the last 8 years and screen sizes range from 40-73". For videophiles wanting a large screen on a budget this has been the best technology for under $5000 until recently. Many offer two native scan rates, 480P and 1080I. Most suffer from screen glare so a controlled viewing environment is required to reduce or eliminate this problem. In many cases the screen protector can be removed by a technician but beware as this modification could cause a problem if the unit should need to be swapped out by the manufacturer while under warranty. Mitsubishi still offers their models with optional screen protector that you can remove yourself. As a projector there are no pixels and the only picture structure concern is scan line visibility. Most will meet HD CVD of 3.0 with an HD source.
Scaling SD and DVD to 540P/1080I will also get you the same results. Many offer near perfect convergence of red, blue and green and geometry if you choose to have the display ISF calibrated. Uniformity varies from poor-to-medium and with an ISF calibration a modification can be performed to make poor become medium. An RPTV is nothing but an FPTV with the light path shortened and folded with a mirror. This creates an artifact caused by light scatter inside the cabinet where bright content in an image washes out dark content in an image and is directly related to picture size which determines cabinet size. The best screen size to overcome this is 60" or larger, is the recommended size for videophiles and requires a dark room. 55" and less is recommended for casual viewers and can work in medium rooms with 50-55" screens and possibly bright rooms at 40-50" screens. Another function of this design are short throw lenses which are notorious for poor imaging accuracy in any field making it difficult to get good center to edge focus. Also, with these lenses comes an artifact called lens flare which means light is being reflected off surfaces within the lens assembly creating bright shadows elsewhere in the image. Most of these lenses are hybrid designs using plastic and glass. Burn in is possible but if you turn down the contrast and mix your images you can significantly reduce this potential. Due to the nature of this product there is a significant drop off in light output at extreme viewing angles beyond 30 degrees and also viewing height requires the viewer is vertically in the middle of the screen. For videophiles using a CVD of 3-4 screen heights, the best viewing angle limits you to three people per row with the center being optimal using a 60" or larger screen size. For casual viewers increasing the viewing distance to 6 or greater reduces this problem.
FPTV is what made home theater possible and is still considered the industry reference for accurate imaging. Unlike consumer grade RPTV these products are typically data grade projectors for graphics and support numerous native scan rates such as 480I, 480P, 720P, 1080I, 1080P and others related to PC video. Due to being front projection many of the problems with RPTV are overcome. Dark room is required. Long throw high quality glass lenses are used producing flare free optics. Screen sizes vary due to light output which is directly related to CRT tube size being 7, 8 or 9 inches. Per the ISF a reference FPTV requires 9" CRT