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Know It All: DLP -- Reflected Glory

Published in Geare magazine, Issue #24, 2004

Competition is stiff amongst those who make projectors for the home theatre, and those who generate acronyms for them. There are LCD projectors, LCOS projectors, D-ILA projectors, CRT projectors and -- the subject of this issue's erudite discussion -- DLP projectors.

So what is a DLP projector? DLP stands for Digital Light Processor, which sheds no, er, light on the subject at all. All DLP projectors are built around DMDs, or Digital Micro-mirror Devices. What's a DMD?

A projector takes a video signal -- say, from a DVD player -- and turns it into a bright light which can be blasted onto a screen. All the digital projectors (LCD, LCOS, D-ILA and DLP) use one or more small panels to control the light from a bright light globe (these are usually 200 or more watts). The front-ends of these projectors take the analogue signal and digitise it -- convert the continuously fluctuating signal into a series of numbers. The electronics in the panel adjusts the brightness of each pixel according to the matching number.

LCD, LCOS and D-ILA panels have no moving parts. But here's the tricky thing: DMD panels, which form the heart of a DLP projector, are not solid state.

Even though most DMD panels measure just 18 to 23mm in size, each has hundreds of thousands of tiny mirrors on its surface, all nicely lined up into a grid. The grid on the current top-of-the-line version (aka the 'Mustang') measures 1,280 mirrors across and 720 down. Multiply those two numbers and you find that there are 921,600 mirrors!

Now those mirrors don't just sit there. If they did they couldn't control (or 'process') the light from the lamp. No indeed. Each of those mirrors is hinged at one end and can flap through an angle of either ten or twelve degrees (the latter ones cost more). If the video signal requires a black dot, the mirror swings and the light falling on it is reflected away from the lens. If it requires a white dot, the mirror stays put.

But what about grey? Actually, the mirrors aren't limited to the slow speed of the incoming video signal (which only requires the pixel to change its brightness 50 times per second). They can actually flap furiously up to 1,000 times per second. If the dot needs to be grey rather than black or white, the micro-mirror flaps between the two positions; the relative proportion of the time spent in the two positions determines the brightness of the dot.

That's one panel. Most panel-based projectors use three panels, one for each of the three primary colours. Most DLP projectors only use one (I guess DMD panels are expensive). So how do they produce the red, green and blue primary colours? Simple! Just have a wheel in front of them, punched through with holes, filled with red, green and blue bits of plastic. The flapping of the mirrors and the spinning of this wheel are carefully synchronised to make sure that the right proportions of colours are produced.

And therein lies the weakness of DLP projectors. Their main advantage over LCD projectors is that they are better at blocking off light in the black dots, so their blacks are blacker. Another advantage is that there is less need for clearance between the individual dots on the panel, so the displayed image is less obviously marred by a grid pattern -- usually called the 'screen door effect' -- and so the picture is smoother.

But these spinning wheels and flapping mirrors produce an optical illusion called the 'rainbow effect'. This is the occasional flicker of short multi-coloured stripes on the screen, usually just off the viewer's centre of focus, usually when the picture is more black-and-white than coloured.

The solution is simple. Instead of using one DMD, use three. With one for each colour there is no spinning wheel. Unfortunately, this is expensive and consumer level DLP projectors (ie. less than $25,000) are still based on just one. But the way prices are dropping, expect three-panel DLP projectors within the next year or two.

2009 thoughts

Obviously things have moved on since then. There are excellent full high definition DMDs available now (ie. 1,920 by 1,080 pixels) in projectors selling for just a few thousand dollars.

But, looking back from 2009, that last sentence was a real clanger. So why was I wrong?

There were two reasons. One to do with the innovatatory environment, and one technological.

First, Texas Instruments holds the patents for DLP technology. While it does so, it seems likely to remain, more or less, a monopoly provider of this technology. That will keep the prices of DMDs high. High DMD prices give a strong incentive to use fewer DMDs.

Second: technical issues. Using a single chip has at least one advantage over three chip systems: colour alignment. Since all the colours are bouncing from the the one chip, they can't be sent in different directions (assuming the optics are otherwise okay). The very best three chip projectors, even though they don't produce noticable 'boundaries' for colour areas, still tend to assume different colour casts in different parts of the screen with certain high resolution test patterns. DLP projectors don't.

DLPs also permit more manipulation of colour due to the ability to put whatever number and hue and ratio of colour filters into the colour wheel that the maker might like.

Finally, SIM2 has even been experimenting with a variable output lamp. Since this apparently switches level extremely quickly, it can work not only at the frame level, but be used to dampen or intensify particular colours.

© 2002-2009, Stephen Dawson