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CRT Primer

Updated: April 2006

Index: 


Video vs Data vs Graphics Grade

 

All video projectors are usually called video projectors, however a higher end projector will also display data grade (higher resolution) and graphics grade (even higher resolution) signals than the regular video signal that is sent from a VCR or DVD player.

A standard North American video (NTSC) video signal consists of 525 horizontal lines that make up the TV image.

These 525 lines do not change whether you are watching a 14” TV or a 10’screen. This is why many larger pictures look grainy, as the scanning lines become very visible on even a 33” regular TV that is in good condition, and much more noticeably so on a projection TV.

Data and graphics grade video projectors will accept (usually) an HDTV signal, a line doubled signal, a computer signal via the high resolution RGB inputs, as well as a regular video signal. HDTV or line doublers insert an additional scanning line in between each of the 525 lines of a standard video signal, thus filling in the space between the scanning lines. Note that while true HDTV does actually send more video information to the projector, a line doubler is averaging the video information of adjacent scanning lines, and does not improve the quality of the basic video signal. Thus as with computers, garbage in, garbage out. Start with a good video (or S-video) signal, and a line doubler will be of great benefit to improve the basic video image.

Note that a video projector has to scan twice as fast for a line doubled signal as compared to a standard video signal. A standard video signal scans at a horizontal frequency of 15.75 Khz, so rounded up, a line doubled signal needs 32 Khz scan rate. The maximum scan rate of a CRT projector can be found in the model’s specifications.


Barco Cine 7

HDTV broadcasts 1080 scanning lines which are interlaced (alternate lines are projected, so 540 lines are being projected at during any one ‘field’ of video information, so HDTV requires a 36 Khz scan rate, or something just over line doubling. Most HD receivers can also be set for 720p, so that 720 lines are shown on the screen at one time. A data projector needs to scan to about 47 Khz to be able to show a 720p signal.

Line triplers and quadruplers are also available, and require signals of 48 Khz and 64 Khz respectively. In my opinion, a lot of people can get hung up on specifications, and for most home theatre applications, a scanning frequency higher than 64 Khz (ie: graphics grade) is not required. Many high end data grade projectors originally designed for CAD applications or flight simulation can take signals up to 135Khz, but I do not think you’ll ever use frequencies this high in a home theater application.  






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