Display Technology Notes

Many of our clients ask us for advice in selecting the right screen size, aspect ratio and displaying technology. The choice (and language) can bewildering to the initiated and we hope the following notes help to clarify your choices (or at least help you to ask the right questions from your retailer). We briefly cover the more contentious topics: Screen size, resolution, aspect ratios, line doublers and brightness.

Screen sizes

Our recommendation is always get the biggest you can afford and will fit in the room!

"Fitting in the room" does not mean that it fits on the wall, it means that distance from the screen to the closest viewer is greater than 1.3 to 1.5 times the diagonal size of the screen. So a 42in plasma screen should not be watched any closer than 4ft6in or 1.4m but best to be safe at 5ft3in or 1.6m. Look at it another way, if the closest person to the screen will be 6ft then you can go safely to a screen of up to 4ft (48in) diagonally.

Plasma screens sell in 42in, 50in, 60in and 70in sizes diagonally. Front projectors can project at very good quality into up to 120in-140in (using line doublers) and high gain screens may increase this by a further 50%. Two (or three) projectors can be mounted side-by-side and aligned to provide even larger screens (say up to 300in or 25ft). And of course, you can always go "industrial".

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Screen resolutions

Video material is presented by small dots of light that draws lines from left to right. Drawing these lines quickly enough forming frames that change gives the appearance of "moving pictures". In a standard analog TV broadcast, the image is created by drawing two half images (called field 1 and field 2) at about 50 (PAL, EU) or 60 (NTSC, US) times per second. This way of drawing an image is referred as "interlaced scanning".

More modern computer screens on the other hand, draw the complete frame at a rate (called the refresh rate) of anything between 60Hz and 85Hz. This is referred as "progressive scanning".

Now about resolution. First in the analog world. In these systems, vertical resolution is given by the number of lines that the system can draw. TV broadcast in the EU (PAL system) for instance is 625 lines and in the US (NTSC) is 525 lines. But these are interlaced, so the TV only draws half of these lines in each cycle.

The "screen resolution" is determined both by the resolution embedded in the signal as well as the the technical limits of displaying equipment.

The signal resolution is given by the maximum frequency of the carrier signal and it can vary from 4.2MHz to 100MHz. You can derive an "effective horizontal resolution" by resolving:

Scan frequency ~ vertical lines x horizontal resolution x refresh rate

Thus a standard PAL TV broadcast at 6MHz will give an equivalent horizontal resolution of 384 pixels (6Mhz = 312.5 x 384 x 50 ). The net effect is a resolution of 625x384 interlaced.

An analog computer monitor capable of accepting a signal with a scan frequency of 40Mhz and a refresh frequency of 50Hz will give a net resolution of 1024x768 progressive (40Mhz ~ 1024 x 768 x 50).

The final resolution can be limited by the displaying equipment. For instance, in the computer world you may be familiar with having to select a lower frequency to ensure your monitor works. More often special processing units referred as "line doublers" are interjected between the source of the signal and the displaying equipment to upgrade the signal to the maximum frequency (ie. maximum resolution) possible.

7in CRT projectors can provide over 600 lines of vertical resolution, 9in CRT projectors reach over 1000 lines and specially designed CRT projectors can reach circa 1,500 lines. Thus, theoretically at least, a graphics-grade CRT projector operating at 60MHz, 1,000 lines, can display an effective resolution of 1200x1000 progressive. This has been the limits of projector technology until recently. DLP projectors can now supply resolutions of up to 1024 lines (1280x1024) relatively easily.

In the digital world, resolutions are easy to find out. They are given in pixels (width x height) and given in the spec or user manual. 640x480 means exactly that, 640 pixels wide by 480 pixels high (ie. lines). 640x480 is commonly refereed as VGA resolution. High Definition TV format is 1920x1080. Other common resolutions are 800x600 (SVGA), 1024x768 (XGA) and 1280x1024.

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Screen aspect ratios

The "normal" TV screen has a 4:3 ratio (or 1.33 ratio). This means that if the screen width is 4 then its height is 3 (or width is 1.33 times height). A widescreen TV (and plasma TV) has a 16:9 ratio (or 1.78 ratio) thus width is 1.78 times height. If you use projectors then you can choose your screen aspect ratio to match the format of your choice. But watch out:

• Film formats vary. There are 1.33, 1.66, 1.78, 1.85, 2.20, and 2.35 ratios.
  The 2.60 ratio correspond to Cinerama formats ("How the West Was Won"). The large curved cinema screen, if you remember, but it is no longer used.
  
• 2.35 ratio is achieved by a number of technologies going by names such as Cinemascope (The original "Star Wars" series), Super 35 ("Terminator 1 & 2"), Ultra Panavision 70 ("Ben Hur"), Panavision, Todd-AO or Technirama. It is quite common.
  
• 2.20 ratio is also referred as Super Panavision 70 ("2001 A Space Oddysey").
  
• 1.85 ratio is achieved with a technology called Vistavision ("Vertigo").
  
• 1.78 is the "new" widescreen / high definition TV format.
  
• 1.66 is a format favoured by some movies-for-TV producers which fits well in both standard and widescreen TVs.
  
• 1.33 ratio is also referred as Academy ("Casablanca", "Citizen Cane") - as it was the original ratio for films (until "The Robe", 1954, the first widescreen film).

To further complicate matters, when films are transferred to DVD format the actual ratio may be altered (sometimes even adding stuff you don't get to see in the cinema!) and the final product labeled as "full screen", "anamorphic", "letterbox" or "anamorphic letterbox". The good news is that, in practice, all movies will play reasonably well on a 1.78 ratio screen ("widescreen"):

• "Full screen" means a transfer ratio of 1.33. The original film may have been modified to fit in this frame size. The transfer will play "as is" on a standard TV. If you have a widescreen set, then you will see vertical black bars at each side of the screen (or you can switch to a zoom mode but you will lose the top and bottom of the film).
  
• "Anamorphic" means a transfer ratio of 1.78. So the film will play "as is" on a widescreen TV (you may have to select the widescreen mode on your TV) or to view on a standard TV, the DVD player will have to select the "pan and scan" option (effectively chopping off the right & left end of the film).
  
• "Letterbox" means a transfer ratio of 1.33 but the film (originally in a higher ratio) has not been reformated (significantly) resulting in black (empty) space on top and bottom. The film will play on a standard TV (with black bars at top and bottom) and on a widescreen by selecting the "zoom" mode (albeit the quality of the image in large screens will be poor).
  
• "Anamorphic letterbox" (sometimes just grouped as simply anamorphic) is and anamorphic transfer for original films with aspect ratios over 1.78 (ie. the popular 2.35 ratio described above). The net result is that even in widescreen screens you see a black bar on top and bottom. Sometimes the subtitles are placed on the bottom bar.

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Source and projector resolutions

The source material (say a DVD) is normally produced at a standard resolution of 640x480 pixels (approx) interlaced. Projecting this material in anything bigger than a 42in screen will result in visible black lines between the lines of light drawn by the display. To reduce (or eliminate) this, technology is employed to first convert interlaced frames into progressive frames (essentially doubling the number of lines) and further by interpolating in real time to produce resolutions of up to 1920x1080 progressive frames (1280x720 and 1700x960 are also possible). For further information see for instance Faroudja or Snell-Wilcox.

Obviously, your screen/plasma/projector will only be able to process these enhanced signals if it has been designed to do so.

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Brightness

Today, CRT type projectors provide the best resolution, brightness and blacks. But this may change quickly as digital light processing (DLPs) get better.

Brightness is measured in ANSI lumens. CRT projectors however, have the ability to concentrate their light output in specific areas of the screen (up to 10-20% of the screen over short periods of time) - given up to five times the brightness of a similarly rated LCD/DLP system. So if you compare a 250 ANSI lumens CRT system with a 1500 ANSI lumens LCD, it is most likely that the CRT will look better!

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