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CRT Primer

Updated: April 2006

Index: 


Projector Resolution 'Sweet Spot'

 

Each CRT projector model has what is known as a resolution ‘sweet spot’, depending on the tube size and construction, as well as the quality of the internal circuitry. Speaking in very general terms, it is possible to feed too much video information to a projector, so much so that the circuitry and the tubes cannot accurately process the large amount of  video information fed to it. A less detailed picture results, and some loss of detail may be noticed if the ‘sweet spot’ is exceeded.

The general idea is to increase resolution until the individual scanlines are close enough together such that you do not see scanlines from your seating position.  If the resolution is too low, scanlines will be visible (and distracting).  If the resolution is too high, scan lines will overlap creating a softer or smeared picture.

For example, a 7” CRT system with ES focusing is best suited for line doubling (480p) or tripling (720p) if the tubes are in good shape. Feeding a line quadrupled signal (960p) into one of these projectors will show less detail and a bit of smearing will occur on occasion.

The below settings are approximately what I have found are the best signal to be fed to the projectors. Naturally these findings are subject to interpretation, and your own experimentation is encouraged to find the picture that is best for you.

ES focus:

7” tubes: Line doubling (480p) or tripling (720p)
8” tubes: Line tripling (720p)
9” tubes: Line quadrupling (960p)

EM focus:

7” tubes: Line tripling (720p)
8” tubes: Line tripling (720p) or quadrupling (960p)
9” tubes: Line quadrupling (960p) and above

The screen aspect ratio will also determine how well the projector will be able to resolve certain resolutions. For example: An image projected onto a 4x3 ratio screen uses more tube surface area vertically than an image projected onto a 16x9 screen. This means that the 4x3 image has more space to display the scanlines so more scanlines may be displayed before they touch.  A real world example of this would be that 8" EM tubes are probably limited to 720 lines (720p) on a 16x9 screen but can resolve 960 lines (960p) on a 4x3 screen.

HDTV

Where does HDTV fit into this?   The two most common resolutions for HDTV are 720p and 1080i. 

720p: As you can see from the list above, even an Entry-Level 7" ES focusing CRT projector should be able to display a 720p HDTV signal in most cases.

1080i: This resolution is actually easier for a projector to display as the image is interlaced (ie: it takes 2 passes to build a picture) and is equivalent to 540p in terms of bandwidth/scanrate requirements.  Every CRT projector listed on this site (other than a couple of the extremely lowest-end models) is fully capable of resolving 1080i HDTV.

1080p: This is a new and upcoming resolution found on some HD-DVD and Blu-Ray discs that (IMHO) is completely over-hyped and does not look much different than 1080i in real world tests.  While many CRT projectors are capable of displaying a 1080p signal, only High Performance 9" units will do so adequately (though some will argue that very high end 8" machines do have the resolving capabilities to do 1080p justice).  If you decide that you do require 1080p resolving capabilities then you need to take this into consideration.  For projectors with 8" and 7" tubes, it's best to use 1080i or 720p resolutions for HDTV.

Below are simulated examples of what may be seen as you feed a projector different resolutions or levels of scaling:

Projector is fed too high of a resolution (image is smeared as the scanlines are overlapping):


Projector is fed too low of a resolution (space between scanlines is too obvious):


Projector is fed the best resolution suited for  this particular unit (image is perfect as scanlines are just touching!):







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