NEW YORK (AP) – Did my TV screen just shrink?

That's the question a lot of people will be asking after installing one of the converter boxes that will keep their older TV sets tuned into over-the-air broadcasts after February 17, when most stations will switch from analog to digital transmission.

The National Association of Broadcasters estimates that 70 million sets are in danger of losing their picture.

We tested two boxes and found them to produce a picture quality far better than any analog channel we could find, giving a whole new lease on life for a tube TV that lacks cable or satellite service (which will still work with older TVs after February 17).

The boxes also gave us access to more channels than analog reception.

But the boxes also have a peculiar problem: Unless users manually change settings from show to show, the picture from many stations either won't fill the screen, or it will be so big that parts the picture are cut off by the edges of the screen.

We tried the Digital Stream DTX9900, sold by RadioShack Corp., and Insignia NS-DXA1, sold by Best Buy Co. Each costs $59.99, which can be partly defrayed by a $40 coupon available from the government at www.dtv2009.gov.

Both boxes, with some programmes, produced "windowboxing" or "the postage-stamp effect": The TV picture occupies the centre of the screen, leaving black bars above, below and on either side of the picture.

This occurs because the digital broadcasts of network stations are in most cases formatted for widescreen HDTVs. When shown on a non-widescreen TV, the image will be "letterboxed'', showing black bars above and below.

But it doesn't end there. A lot of these digital broadcasts are actually of non-widescreen content. Practically no daytime fare, like syndicated shows and reruns of old sitcoms, is widescreen.

The stations deal with this by inserting black bars to the left and right of the image to pad out the widescreen frame. On a non-wide TV, the result is windowboxing. That nice 32-inch TV you got five years ago is turned into a 24-incher.

The solution is to press the "Zoom" button on the remote that comes with the converter box.

That will expand the picture so it fills the screen. Everything good now? No.

When the next show comes on, it might have been shot in widescreen. The "zoom" mode will still be on, which means only the center of the image will be visible. The left and right sides will fall outside the screen, but you won't know that, unless you start mashing the Zoom button again to cycle through a few options until you get to the letterboxed mode.

Now, a lot of people might accept that they're going to be missing part of the frame, especially since many stations use a "centre cut safe" principle, which means they try not to put important information on the left and right edges when shooting widescreen content.

But when we watched "Everybody Loves Raymond'', having the zoom engaged cut Raymond's brother out of the frame. I don't think Robert appreciated that.

The converter boxes have another issue that may affect some people. It's been widely reported that all analog broadcasts will go away next year.

That's not quite true. More than 2,900 low-power stations and about 4,400 signal-relay stations known as "translators" that extend broadcasts to rural areas will not be required to go digital by the deadline, and may lack the resources to do so.

The converter boxes we tested don't let analog broadcasts through to the set, so these low-power signals will be blocked. None of the other converter boxes carried by the big US electronics retailers have analog passthrough either.

One model sold by online stores, the Philco TB100HH9, has passthrough, but it was sold out at the sites that listed it.

Lack of analog passthrough may not be a fatal flaw. You may not miss the low-power stations. If you have a TV that takes a "composite" input, usually via a yellow socket, you can plug the converter into that, leaving the antenna socket free for a second antenna, which you can use for remaining analog broadcasts. (Even if you don't plan to use a second antenna, plugging the converter box into the composite jack is a good idea, because you'll get better image quality.)

The two models we tried are quite similar, and opening them up reveals that their major components are in fact identical.

We preferred the RadioShack's Digital Stream model for its user interface, which contains a programme guide that allows you to look ahead at a channel's programming for up to 24 hours.

It's bare-bones compared to the program guides that come with cable or satellite subscriptions, but hey, it's free. The Insignia also has a programme guide, but it only tells you what the current and following shows are.

Another minor difference is that the Insignia remembers the zoom setting for each channel, while the Digital Stream applies the same setting to all widescreen channels. Either way, you'll be using the zoom button a lot if you want to watch TV the right way.