Deal with the Devil

Posted on December 5, 2005 by psu

My friend Dave used to say that the exponential increase in hard disk capacity over time was sure proof that not only was the storage industry in league with the darker forces, but also that every disk platter was clearly populated with the souls of the poor industry employees who had signed the contracts needed to rev the latest generation of the hardware.

I have been shopping for a large screen television lately, and after becoming familiar with the prevailing technologies, I have to say that Dave is wrong. It’s not the storage industry, but the TV people who have the true pact with Satan.

Even the oldest type of television has an air of magic about it. Consider: an electron gun at the end of a large vacuum tube shoots a beam of charged particles towards a large piece of glass. This glass is covered with light sensitive phosphors which light up where the beam hits them. By modulating the beam faster than you can see, the television scans the entire screen and generates an image line by line. At any point in time, the electron gun is only scanning one point on the screen. The only reason you ever see a whole image is that the phosphors stick on for a bit while the rest of the screen is scanned. That, and the fact that the screen is re-scanned a few dozen times per second. To generate color, you use three guns, one for each of the three primary colors. Broken down into its component details, it seems preposterous that displays like this should work at all. After 50 years of development, you can only marvel at the fact that along several axes the CRT still works better than all of the new screen technologies.

One of these new screen types is the LCD, seen in laptop computers, Palm organizers and Blackberries everywhere. These screens started out as those 12 segment displays on calculators that could display 0,1,2,4,5,6,7,8, and 9, but couldn’t really make an “A” correctly. LCDs work by sandwiching a special liquid crystal between panes of polarizing glass. Normally, the crystal is oriented such that light passes through it and the polarizers, and looks clear. When you apply an electric field, the crystal twists and causes the polarizer to block the light so that the liquid appears dark. Shrink these cells, and the switches that run them small enough, and add color filters, and you get the modern color LCD display. Of course, it is completely preposterous that you can take some glass panels and squeeze special crystals and color filters and switching circuits all in between the panes and make something like this work.

Similarly, plasma displays are obviously impossible. Here again you have two panes of glass and some phosphor. Between them are millions of microscopic tubes that contain ionized gas. Run a control current behind the cells and the gas lights up the phosphor in front of the cell and you get a picture on the front of the screen. It’s like each one of these cells is a tiny little CRT tube (although really, they are completely different), built in miniature, and they have collected them all together glued between two panes of glass to make a panel about 4 inches thick that displays images. It’s really too bad that they have to trap the soul of an electrical engineer in each one of those plasma cells.

Having covered direct view, we can move on to rear projection sets. Confusingly, LCDs make another appearance here. Rather than making a huge panel of LCD material and switches and so on so you can look directly at it, you could shrink the panel down to a really small size. You then combine three of these panels, one each for red, green, and blue, and project a color image to the front of the TV using a powerful backlight. The TV takes a normal TV signal at its inputs, digitizes it, scales the picture from the normal NTSC resolution up to the resolution of the panel and projects the resulting image to the front of the screen for you to see. Magic.

Of course, this process can work in the opposite direction too, giving you a front projector. But I’m not going to get into that right now, because now I have to cover DLP.

The heart of a DLP television is an integrated circuit whose surface is covered with microscopic mirrors that are hinged. The control system can tell the mirror which way to tilt. Tilt one way, and the pixel is on. Tilt the other way, and the pixel is off. Make the mirror dance back and forth really fast, and you can get dithered gray scale values between 0 and 1024. To generate color, the image processing system spins a wheel with color filters on it past the chip so that you get a red, green, then blue image flashing past your face fast enough to look like a color picture.

So, just to be clear. The TV uses microscopic mirrors, that flash back and forth faster than you can notice. Yeah right. Those poor souls.

After all this shopping, the TV that I settled on actually uses a combination of LCD and DLP type technologies to get its job done. The Sony SXRD televisions uses a form of “LCoS”, or Liquid Crystal on Silicon chips to generate a picture. Here, instead of a hinged mirror, you have a reflective substrate behind an LCD layer. Turn the pixel on, and the LCD is clear, and light reflects off of the substrate. Turn of the pixel off, and the LCD goes black. No light. LCoS combines many of the advantages of the LCD projection machines (3 panels, no color wheel) with the DLP sets (less space between pixels, because the switching hardware can be behind the glass, and they are reflective rather than transmissive, and thus have higher contrast).

Again, this is completely unbelievable. What is really going on is that the the souls of millions of poor electrical engineers are packed into the cabinet of that 50 inch diagonal back projection TV that you just ordered, and they are all shining tiny little flashlights at you to allow you to watch TV.

Or at least that’s how it may as well work.