Projector Technology Explained Part II

In part I we discussed resolution and aspect ratios. In future posts we will cover additional areas such as the different technologies used to project those images, and other advances.

The combinations of resolution and aspect ratio are summed up in specifications called the format. Designations like XGA, WXGA, WUXGA refer to these combinations. HDTV formats do this as well but include one other designation such as 1080p, 720p, 1080i and 720i references. The pixels on a screen are not changed simultaneously, but scanning across the lines and changing one pixel at a time. This happens so rapidly that the human eye does not perceive the change in this way. In the HD formats the “p” refers to a “progressive” scan that follows line by line from top to bottom. The “i” stands for “interlaced”, meaning that odd numbered lines are scanned top to bottom and then the scan starts over at the top and scans the even numbered lines. A complete cycle, regardless of the format is called a “frame” (with “interlaced” technology it is referred to in “fields”). Frames can be changed in a number of different rates (also referred to as refresh rate or expressed in Hz). This gives the illusion of motion to the human eye.

With computer images a 60Hz rate is common but some gaming systems and higher end equipment may have higher rates. Devices adjust these rates to match, using buffers either in the computer or other playback device. Different content will have different rates that may come from the same device, but the buffer will make the adjustment as needed. For example, a movie might reproduce at 24 frames per second, but a spreadsheet may produce this change only when there is a change in the content of a cell, sometimes once per second or less. A gaming system might have faster action and refresh at 120 frames per second, so a display that refreshes at 60 frames per second will discard half of that data. As long as the signal being sent is below what the display can handle then the projector will be able to handle the display rate. When the rate goes faster than the projector’s refresh rate, then the projector will have to slow it down to meet its own capabilities (if it is able to do so) which will mean the loss of this information and a reduction in the signal. A gaming system that can produce 120 frames per second may still project, but not at its full quality.

The chart below gives a breakdown of some of the more common aspect ratios and their resolutions.

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Always remember that you want to find the “Native Resolution” which refers to the actual image produced by the technology in the projector. Any images sent to the projector that are not the same as the native resolution will be adjusted in some way to be able to be projected by that technology. A higher native resolution in a projector will produce a lower resolution being sent to it, but you will not be getting the full capability of the projector. A native resolution lower than what you are sending the projector may also project, but will have a degraded image from the original content. Keep in mind that when you project an image you are blowing up the size of the pixels, so every change in the image is going to show far more on a large screen than it does on your laptop monitor, or even a home TV. Also, just because a projector is advertised as being HD compatible does not mean that the native resolution of the projector is capable of HD formats. What it means is that the projector is capable of altering that signal in order to display it, but not necessarily the way it was sent to the projector.

In the next few posts we will look into the technology used by projectors to display the images.

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Read Part I         Read Part III          Read Part IV            Read Part V

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