Projectors are pretty simple. The zoomies go in, the zoomies go out, voila, pretty pictures on the wall—not!
While on the surface it may seem that simple, it can be a rather complex evolution to get from input to output on a video projector. Every manufacturer has unique methodology for accomplishing this. We will look at a basic overview of signal flow through a projector and some of the other key components that make projectors work.
An oversimplification would be input stage, pixel processing and mapping, and output stage. It is what happens in that pixel processing and mapping stage that makes a difference in the signal chain. However, a projector can live or die by some of its other functions and of course its power supply, but more about that later.
Most mid- to upper-range projectors now use modular input cards allowing us to configure our projector. We can install the inputs we need, or more importantly, leave out the ones we don’t. This also allows us to have a small inventory of those “other” formats that we typically don’t use without having to outfit every projector with extra cards.
The entry- to mid-level projectors that don’t use modular input cards will have an onboard selection from which to work. These units will configure the inputs in a handful of ways. One way is to offer a variety of discreet inputs with each using a different format. Alternatively, the projector may use some generic universal input and require us to configure it for our application.
Another way might be to offer inputs based on types of signal. For example, the S-Video and composite signal are Input One and there would be an RCA and S-video connector assigned to the same input. Input Two could be component and VGA and use an HD15 connector. A third input might be DVI or some other digital format.
In each of these cases, some amount of signal processing will be required before the projector can start drawing pictures. In a modular card system, this processing will typically take place on the individual card. In a smaller machine, the signal still must be processed but the signal format has to be determined first so the machine knows what to do with it.
The pixel processing and mapping stage is where most of the fun is. For example, if a projector allows you to dissolve from one input to another, then those two inputs will have to be buffered and synched to allow the dissolve to happen. This will cause one or both inputs to be delayed. If this delay is more than a couple of frames, it will be noticeable to the eye. Remember, this is just the delay in the projector. If you have delay occurring elsewhere in the system (e.g., a switcher that synchs internally) then the delay to the screen could be several frames and quite annoying to the audience.
Another thing that occurs in this stage is the scaling of the image to fit the output panel. Here is where image quality comes into play. If a 1080p image is being sent to an XGA panel, then the processor has to decide to cut off the side of the image or black out the top and bottom of the panel. It also must decide how much of the image to actually use. Now keystone correction or other image manipulation (edge-blending, etc) can do serious harm to our image (both personally and to the signal!).
So we got the signal into the machine, processed and made it usable; what happens next? That depends. Is the machine LCD or DLP? Does it have one chip or three? We will go through how the chips work another time. For now, let’s look at three-chip DLP.
In an earlier article we talked about how DLPs reflect the light while LCDs filter it. In the three chip projectors, white light from the lamp is sent through a prism to split it into its red, green, and blue components. These colors are reflected off the chips that have the corresponding picture information. These three separate reflections are combined back into a single image and passed through the projectors lens. Voila! Pretty pictures on the wall.
Very quickly, a single chip DLP uses a spinning color wheel in front of the light to create its red, green, and blue images. The wheel and reflectors move faster than the eye can see and so the brain processes it as a single color. However, in high speed or high motion video (e.g., sports television), we will sometimes get a rainbow effect as the image changes faster that the color wheel can keep up.
While the chips reproduce the image, the lamps are how we see the image. There are several key things to remember about your lamps. Think of them like a temperamental sweetheart. Or like a mental sweetheart with a bad temper. Either way, if we don’t take very good care of them, we will regret it.
Most large format video projectors use xenon lamps. The nice thing about these lamps is that they produce a nice, even light. That means that the wide the image is, the less the light falls off as you move to the edge.
As these lamps age, the light output will drop off. After a while, you will begin to notice the corners getting dim or seem slightly grey compared to the rest of the image. Eventually it will drop so far that you will notice a ring or bright spot in the middle. Lastly you will hear a loud pop and an arcing sound and you might smell smoke as your projector’s power supply destroys itself. This is a bad thing.
Most xenon lamps last about 1200 to 1500 hours, according to the manufacturers. I recommend replacing the lamps at no more than 1000 hours. As the lamps age, they place a larger load on the power supply and if you think lamps are expensive, try replacing a power supply.
So there you have it, the digestive tract of a video projector. Now let’s do this backwards and talk about cameras!!! See you next month.
When Paul J. Duryee is not watching pretty pictures he is the Systems Design Lead at Maxx Technology. He can be contacted at pduryee@plsn.com.
