Why is my picture so small? (the Postage-Stamp issue) - AVS Forum
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post #1 of 5 Old 05-30-2008, 04:33 PM - Thread Starter
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This question seems to be coming up quite a bit in several of the topics, so I thought it might be useful to give it its own thread.

Is your converter box giving you a small picture inside a big black frame? There is nothing wrong with your box. Here's an English explanation of what's happening, and why you have to use the "Zoom" button on your box's remote to fix it.

Digital TV stations are either 4x3 (standard screen) or 16x9 (widescreen). Only a very VERY few of them actually switch back and forth.

Obviously you've seen, on the analog stations, what happens when a 4x3 station shows a 16x9 source. Letterbox, right?

Well, the stations that are being transmitted with a 16x9 picture do the same thing with 4x3 material, only now the black bars are on the sides instead of top and bottom. We call that "pillarboxed."

OK ... now let's say your station is doing this. The converter box has no way of knowing this is happening. As far as it's concerned, the black bars on the sides are part of the widescreen picture. So what does it do to a widescreen picture? It letterboxes it!

Thus you have bars on the sides, which were put there by the station, and bars top and bottom, which were put there by the converter box. Voila - a fully framed picture, a/k/a postage stamp.

The solution is to tell the box that it's OK to chop off the sides, which you do by pushing the "zoom" button.

None of the actual image is getting cut off if all is functioning as it should. The "zoom" is preset to take that 4x3 postage-stamped picture and perfectly match it to the size of your 4x3 screen, cutting off only the black bars on the sides.

Now, all CRT TVs have a bit of overscan, meaning that about 5% of the picture is intended to be off the sides of the screen. That's part of the NTSC standard, and it's been happening ever since you got the TV. So if you don't see the very edges of the picture that you were seeing when it was postage-stamped, that's why. You won't miss anything of any importance. TV producers all know about overscan.

Now, there is a slight possibility that you have some dang fool local station with a CEO who hates black bars in all forms and has demanded that the engineering department reduce them as much as possible and he doesn't care how they do it. Such stations usually end up broadcasting their 4x3 programming purposely stretched to approx. 14x9. There is also a slight possibility that the station has inadvertently (caution - brief tech speak ahead) set their upscaler's digital NTSC input to square pixels instead of NTSC standard, which will produce a 3x2 picture (also stretched just a bit). In either of these cases, hitting your zoom button WILL shave off a bit on the side edges. This is not the box's fault, though. It's your nincompoop local station, and at this point, there's absolutely no excuse for it. Start making some irate phone calls.
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post #2 of 5 Old 05-30-2008, 07:42 PM
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I was all ready to refer you to some of the other threads on this issue until I actually read it.

If someone were to do a lot of research on all the new converter boxes, they would discover that the zoom feature is possibly the biggest difference between brands. Each one seems to do it a little differently and each one uses their own names for the different modes. (even when they use the same decoder chip internally)

Since the broadcasters can (and do) alter their programming on the fly, presets might not be of much use. The boxes seem to have been designed for live viewing and interaction where you can hit the remote to correct the size as needed. This will make recording problematic since you might not be able to compensate when playing back a recorded image.

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Sturgeons Revelation: "Ninety percent of everything is crud."
My Thoughts: "A reasoned argument must share some basic common points."
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post #3 of 5 Old 05-31-2008, 07:21 AM
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Dealing with two different picture aspect ratios is harder than dealing with SD and HD. This problem will start to go away when/if stations implement "Active Format Description" (AFD) and send information about the aspect ratio of the picture along with the program. Some Networks use it to control the upconverters at the Network. It could also be used to put "pan and scan" information into the signal so that the wide-screen picture wouldn't have to be modified to fit your (SD) screen by the Network, ruining the picture for everyone.

There is no "overscan" in the NTSC standard. Overscanned was caused by TV manufacturers building TV sets with poor high voltage power supply regulation. The set makers wanted to be sure that you wouldn't see the edges of the picture, so the just over-scanned the picture tube so that you wouldn't see the edges, even when the voltage dropped. None of this is necessary in a flat-panel display, but old ways die hard. Stupidly, flat-panel TV makers recreate overscan by zooming the picture electronically. This also has the "benefit" of making the people on screen seem bigger, making people think they have a bigger TV, I guess... At least my TV has a "computer" mode that removes the programmed "overscan" and gives me a 1:1 pixel match.

There are SMPTE documents that describe a "safe" area to guide producers in the creation of their programs (so important information doesn't get chopped off.) There are also standards for AFD, so this could get fixed in the future, if people complain enough about it. Maybe the sponsors would complain, too. I can't believe that they really want their commercial to air as a tiny postage stamp.
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post #4 of 5 Old 05-31-2008, 08:18 AM
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Quote:
Originally Posted by hphase View Post

There is no "overscan" in the NTSC standard. Overscanned was caused by TV manufacturers building TV sets with poor high voltage power supply regulation.

Don't forget that picture tubes used to be round and until the flat screen CRTs hit the market, even the flatest tube (Sony Trinitron) had some rouned edges. Since the lifespan of those TVs is in the 15+ year range, there are quite a few million TVs that will require a "safe" are for at least the next ten years. Some of the SDTVs (standard TV with digital tuner) being sold today don't use flat CRTs so overscan will be with us for a long time, as will letterboxed and window and pillar boxed images.

A quick scan through my local stations outside of primetime usually shows only one or two actual HD programs and I don't expect to see that change for many years either.

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post #5 of 5 Old 05-31-2008, 10:25 AM
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TVs have had pretty much rectangular bezels for over 50 years. Very early TVs even had round picture tubes with a rectangular bezel over the face. Overscan wasn't invented to accomodate slightly oval screens. It was introduced to mask an electronic design short-cut. The TV production industry had to grin and bear it.

Computer monitors have been CRT-based since the beginning, and computer displays don't use overscan. Menu bars couldn't be seen if they did. Computer monitors were built better than TVs were, and the price tag usually reflected that. Zenith was the first to make a ruler-flat CRT computer monitor.

Overscan is a legacy that has been perpetuated by the flat-panel TV makers, except that in this case it costs them money to do it, rather than saving money by allowing it. Todays TVs scale all incoming video formats to a native format for display. Adding overscan is "a few more lines of code" to them. Plus, it gets rid of those "annoying squiggles" at the top of the screen!

Just as it took a while for all prograns on TV to be in color, it will take a while for more programs to go HD. Make no mistake, the production companies that produce a lot of the daytime programs are planning for it and building for it. Local news might go HD, but maybe not soon in your market, unless someone takes the plunge early. Competition is a powerful motivator. Having the fanciest weather radar can be percieved as an edge for some stations.
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