Several days ago we looked at my pile of converter boxes and compared the quality of their pictures... today we'll look at how well they perform on the receiving end, and how well they handle a poor signal.

We're used to how a bad signal looks on an analog TV:  the ghosting and snow, sometimes tearing and rolling if the signal gets really bad.  Still, there's a recognizable picture to be seen.

DTV is a different animal: weaken the signal a bit or add some distortion, and the picture looks just as good as if we had a perfect signal.  Knock the signal back some more, and the picture still looks perfect.  Keep dropping the signal enough, and you suddenly go from wonderful to nothing at all... or at least, to a blocky mess.  What does it take to have a consistently great picture?  First, a receiver that is both reasonably sensitive and yet tolerant of distortion; second, an antenna that delivers a good signal to the receiver; and third, a receiver that gives you a useful indication of how good the signal is.  Right now we're looking at the receiver issues... antennas will come later.

When we talk about signal quality, most people think of stregnth first.  A typical cure for a poor signal has traditionally been to insert an amplifier in the line to boost the signal... but that doesn't necessarily work for DTV.  Quality also depends on keeping out the stuff that doesn't belong.  Let's look at the most common probem:

Ideally, you will have an unbroken path between the transmitting tower and your antenna... that's the gold arrow.  You want as few obstructions as you can get -- trees, buildings, hills, and so on.  In the real world, you also have things that will reflect the signal like buildings, water tanks, and trucks driving past your house... as in the dotted salmon-colored line.  One might think that picking up the reflections can be a good thing: it adds to the signal, right?  Well, yes... but what it adds is delayed, because it took longer for the signal to hit the other object, bounce off it, and then travel to your antenna than it takes for the "real" signal to come directly from the transmitter.  That delayed signal winds up adding stuff that doesn't belong, and cancelling out the stuff you need.  Sticking an amplifier in the line won't fix the problem -- it just kicks up the reflections right along with the signal you want.  If you amplify the signal enough, it actually makes things worse rather than better:  the stronger main signal gets chopped off at the amplifier's limit while the reflections still get boosted, making it even harder for your receiver to tell the difference between what is real and what isn't.

How do you fix this problem?  The best way is to choose an antenna that is most sensitive to signals where you point it (toward the transmitting tower, we hope!) and least sensitive to signals coming in from other directions.  The next thing is to pick a receiver that's especially good at locking to the true signal, and ignoring the other crud that shows up.

All right, already! How well did the receivers work?

I've tested the bunch at my house (buried in the valley, obstructions between the transmitter and my antenna but lots of things for the signal to bounce around on... a digital version of bumper pool).  I also tried them at the station, which isn't much of a challenge since we can walk out the door and see the tower in Pompey.

The LG-made receivers (the Zenith and Insignia) worked the best.  They're the quickest to lock to a weak signal, and slowest to break up or black out when the signal gets bad.  You can call up the on-screen signal display with a single button on the remote, and a beeping sound gives you a way to aim your antenna without having to keep your eyes glued to the screen.

The Magnavox receiver was nearly as good at keeping a locked picture as the LG twins, but getting to the signal quality display is an annoying procedure that requires going through several menus.  There is no audible indicator.  The display also blocks most of the picture, making it harder to see what's going on with the station's picture.

Third on the performance scale was the Digital Stream... not by a huge margin, but enough that you might want to look first at the LGs if you live in an area with difficult reception.  It was the first to break up and the last to restore good video.  This is unfortunate because the main quality display is rather nice looking... even though you have to go through menus to get there.  It also does not give an audible quality indication.

On the plus side, the Digital Stream is the only converter that gives you a signal quality reading with the station information -- and that is a single button push.

Next week I'll try to get back to our homebrew Gray-Hoverman antenna project... by coincidence, one of the trade magazines that showed up on my desk this morning has an article about it.  Apparently we're ahead of the curve!  Have a good weekend --

-- Jeff