DTV and me

kw: observations, television, photography

A broken TV at the right time got us ready for the transition to Digital TV (DTV). About a year and a half ago, our 19" television began breaking down. Repairs would cost as much as we had spent to buy it, so we shopped for a new TV. We eventually found a model that would receive everything, both analog and digital. Apparently, it has three or four kinds of tuners inside, and can select the appropriate one automatically.

We already had a large antenna in the attic (I am too wary of lightning to put it above the house), and we had been receiving about ten analog channels with it. When we ran a scan, we found we could get nine digital signals, and most of them had subchannels. And we still had the same collection of analog channels. However, several of the digital channels were marginal, and would cut in and out, particularly on a rainy day.

I'd installed twin-lead between the antenna and TV originally, with a 300-to-75 ohm converter, but I read that twin-lead loses a lot of signal. I bought a length of coaxial cable to replace it, and the signals came in much better. No impedance matching needed, either. After a few months I added a preamplifier at the antenna to boost the signal by a factor of ten. Since that time, things have been pretty good.

The image above shows the signal meter that my TV can display. After the DTV transition last Friday, I went through all the digital channels and recorded their strength. I did this a few times. Strangely, some DTV channels that we received just fine before were not found at all when we re-scanned. At the moment we have seven channels, and most of them have sub-channels. This table shows them, with number of subchannels in parentheses, and the range of signal strength (on a logarithmic scale, I assume):

• 03 (1) - 85 to 92
• 06 (3) - 53 to 58
• 12 (3) - 100
• 17 (2) - 61 to 70
• 23 (3) - 45 to 48
• 48 (5) - 23 to 32
• 57 (1) - 40 to 50
  

According to infomation from TV Fool, I ought to be receiving channels 10, 29, and 35, but they don't get found on a scan. Strange. As it is, however, with subchannels, I am able to get eighteen signals, which is pretty good for a fringe area south of Philadelphia.

I also learned something while making screen photographs. You may have noticed the aliasing bands on the image above. The camera's grid of photosensors was close to being aligned with the TV's grid of pixels. In this situation, color banding is inevitable. So I tried changing the angle at which I was shooting. Rotating a screen by 30-35° is the technique used for color printing with halftone. If you don't rotate each screen relative to the others, terrible banding occurs.

This image was taken at an angle of 34°. I used an image editing program to rotate it back to horizontal, and cropped it. Since I've reduced both images to about 700x400 (you can click on them to see that size), I had pixels to waste. This image is much smoother, with hardly any aliasing visible.