HDTV image encoding question

[Mathetes]

I experienced an odd thing with my relatively new HDTV hookup over the last couple of days. It's been resolved, so I'm not looking for advice; but I am looking to confirm a technical assumption that this all led me to make.

The situation was this: the Sony Bravia LCD screen started displaying HD programming with the right and left sides clipped off. Only the 4:3 aspect ratio portion in the center of the screen would display; the rest was black. And it was clearly clipped, not compressed.

After an hour with Sony's on-line tech support, taking me through all the trouble-shooting I'd already done, we concluded that the most likely cause was the cable box somehow not sending the full picture ... I had discounted this hypothesis only because nothing had happened (to my knowledge) to have caused the cable box to malfunction. But I had just installed a DVD recorder (separate unit) and hooked it to the TV, and switched it through different levels of "upconverting" the image, going by HDMI cable to the Sony set. And it seemed to me as if that 720, 1080i, 1080p cycle might have "confused" the Sony ...

So all of those smoke screens and diversions have been resolved. The problem WAS the cable box.

My question (assumption I'm wanting to confirm): it turns out that when the HD signal was coming through that should have been in 16:9 aspect ratio, what was actually displaying was a perfect 4:3 aspect ratio portion from the center (I measured and calculated; perfect 4:3). I couldn't get an answer from the tech support people at Cablevision (or at Sony, for that matter), but it appears to me that HD pictures are sent consisting of three components, the central image is a 4:3 aspect ratio primary image; that portion can also be used by the network to satisfy the non-HD viewers. The sides then are separately processed. And to get to the common denominator of 9, here, we convert the 4:3 central portion to 12:9, leaving us with two 2:9 segments, one on the right and one on the left. They're added together to give the 16:9 full image.

That would explain how it IS possible to have a minor malfunction such as I appeared to have, where the central part of the image was perfectly acceptable HD (sharp, good color, etc.) but absolutely nothing on the sides. (Presumably, and more disconcertingly, it could be reversed, but that's another story).

Is my reasoning sound?

 

[Biggiesized]

You've made some great strides in deduction, but that's simply not how it works. I can explain, but it'll have to be later since I'm at work (thank God this site ain't blocked).

As for your problem, the first thing that came to mind, if you picture does appear to be in HD, is to check your cable box's menu and see if HD output it set to 4:3. The cable box will chop off the sides of your picture and give you a 1440x1080i output. It's not your TV's fault if that is the case.

There are modern CRT 1080i HDTVs that have a 4:3 aspect ratio that'll take that 4:3 signal.

 

[Mathetes]

I'll look forward to your explanation. The picture (the portion that was displaying) was definitely HD.

I just checked the cable box settings menu: couldn't find anything for altering the video output, setting it to 4:3 aspect ratio ... and even if I had, why would it (which was displaying the full 16:9 whenever available) suddenly switch to only 4:3 on all channels? I definitely hadn't done it, and the only other person in the house never adjusts anything like that on any gadget!

Now, an additional piece of information that I've learned since first posting: the cable company (coming for a service call tomorrow morning) tells me that the signal strength coming into the house is weaker than it should be, and inconsistent (over time). Since the single coaxial cable supports me on the internet and the TV systems (two cable boxes, three TV sets, with usually no more than one computer and one TV on at any given moment) .... the tech support person on the phone simply said that it's the high bandwidth stuff (like HDTV) that is going to suffer.

Still doesn't explain those two bands specifically .... so, as I said, I look forward to your explanation. And, thanks in advance.

 

[Hi Ho]

My guess is that the box was defective and was, for whatever reason, outputing a 4:3 image. Options for output resolution are not found in the standard onscreen menus. I don't know who your cable provider is or what kind of box they use but to access those settings in the Motorola boxes used by Comcast one must turn the box off and press the menu button. This brings up the "service" menu for the box that the consumer is not really supposed to see. It is there that options for aspect ratio, resolution, and several other parameters are present.

HDTV signals are not brodcast in seperate blocks. They are broadcast in either 1280x720p or 1920x1080i. The standard definition signals are broadcast on an entirely different channel. For example, you probably have two sets of local channels, one HD and one SD.

 

[MDS]

Scientific Atlanta cable boxes are similar. To access those settings you have to turn the box off and press guide and info at the same or up and down arrow at the same time then info (varies between models).

I bet the issue is that the box is set to pass thru the resolution without scaling it and the broadcaster is sending a SD 480i 4:3 aspect ratio image in a 16:9 carrier and the box doesn't touch it. In other words the black bars are 'built in' to the image and the TV sees a 16:9 aspect ratio and just displays it as is.

 

[Mathetes]

continuing the mystery ...

If you all don't mind, that is

The box is Scientific Atlanta, Explorer 8300 HD (a DVR box); the service provider is Cablevision.

And this clipping of the image that I'm talking about was definitely with HD source programming that was being broadcast in full 16:9; What was being displayed was purely the central portion, and it was a perfect 4:3 aspect ratio. Not the 16:9 image compressed; letters would be missing, for instance, from "CNN HD" on the left, all one would see would be "NN HD" and what was displayed had none of the aspect ratio that might have been there.

Another interesting anomaly when the set was malfunctioning: normally, the TV senses correctly what kind of image is coming in and sets itself to "Normal" mode, meaning a 4:3 image is displayed as 4:3, a 16:9 as 16:9 (filling the screen). What happened when it was clipping the image was that it would take those images that were supposed to be 16:9 but were instead only the 4:3 central portion, and switch itself (the TV did) to wide, distorting the picture in the process. I would have to manually switch it back to "Normal" in order to at least see what was there properly.

So ... to me the mystery still is what in the image encoding or in the circuitry and options of the box would cause the image to be clipped as it was.

 

[Biggiesized]

Did you check your cable box's settings like we told you? If it is set to output HD resolution in a 4:3 aspect ratio, it will chop off the sides.

Otherwise, as MDS said, the network was broadcasting SD material in a 16:9 carrier. Although that sounds the most likely, I give pause to the fact that you mentioned letters were being cut off in the horizontal axis. It definitely sounds to me like your box is set to 4:3 output.

 

[Mathetes]

Well, it's far too late to check the box's settings. That one was exchanged for a new one three days ago. And the new one is not displaying the problem.

To challenge the hypothesis that it was the box's settings, however, I present (A) the evidence that the information displayed on the screen was "clipped" -- that's my word, but by it I mean that you could readily tell that information was missing as in "NN HD" rather than "CNN HD" and similar off-the-screen but it should have been there.

And (B) that it HAD been working just fine and nothing had (consciously) been done to change the setting --- and, as you all have pointed out --- that setting isn't even available to most users (and wasn't, knowingly, to me).

(Of course, if it HAD been working just fine, why would it be the box at all? That's why I concentrated initially on the set, spent time with SONY on-line, etc.)

 

[Biggiesized]

I understand what you are saying and I do believe the box was set to output a 4:3 HD image by cropping (more connotative term for "clipped") the 16:9 image. Now if your box outputted the correct 16:9 picture but your HDTV was set for 4:3 then you'd see the picture be anamorphically squeezed.

I found out how to change the box's settings by reading the manual Comcast supplied with me.

I'll check if my box (Motorola DCT6200) will emulate the problem you described. If I can't reproduce it, then maybe your box was really junked to begin with because you cannot send a 16:9 HD signal cropped like that.

 

[Midcow2]

Better solution ...

You will be much. much happier if you go to satellite (DirectTV or DishNetwork) or fiber optics (AT&T Uverse). The difference is phenomenal!

 

[Mathetes]

Thanks to all. This has been a good learning experience, and in the process I've gotten my cable service improved. They came yesterday and upgraded a few connections to improve signal strength.

In the process, the technician showed me those various settings, many of which are hidden from the typical user (for good reason, I might add). And he also demonstrated how a very specific setting in the cable box could produce the cropped/clipped image. So.....maybe that setting had been inadvertently, coincidentally, selected on the former (wrongfully accused) cable box. I'm still left with a simple question: why in heaven's name would anybody ever want to crop the 16:9 image down to 4:3, and even if there's an occasionally valid purpose, why make it something that can be selected without wilful intention?!

Ah well....

I'd been wondering about one of the satellite services. Thanks for the strong recommendation.

 

[allargon]

I'm still left with a simple question: why in heaven's name would anybody ever want to crop the 16:9 image down to 4:3, and even if there's an occasionally valid purpose, why make it something that can be selected without wilful intention?!

There's actually a reason for that. Some cable companies are switching to SDV (switched digital video) across the board. This includes standard definition channels (and therefore displasy) as well. Cox gave my 75 yr. old father a HD cable box for his 25" SD CRT. (The thing doesn't even have component inputs.) It is set to output 480i 4:3 letterbox (thanks to me--no pan and scan crap in my family) over composite.

They told him he had to use that box if he wanted the NFL Network.

 

[Mathetes]

Cox gave my 75 yr. old father a HD cable box for his 25" SD CRT. It is set to output 480i 4:3 letterbox (thanks to me--no pan and scan crap in my family) over composite.

I'm not sure that's the same thing: If mine had been outputting letterbox, albeit in the 4:3 framework, it still would have been complete. This was cropping (or, back to my original word, clipping) the image, taking what could have been letterbox, and making it "standard" ---

I guess the point, whether or not my problem was a setting that you've employed with your father's set (I'm within a decade of him in age, by the way, just that I spent a LOT of time with computers and other technical stuff in my career) ... whether or not you are using what I was mildly complaining about ... there is somebody out there who is looking for every kind of odd setting.

I mean, I have a problem with setting a 16:9 set so that it takes a 4:3 original image and stretches it horizontally (distorts it) just to fill the frame. The Cablevision technician did that, unilaterally, with mine, thinking he was doing me a service. He quickly restored it when I said I wanted the images to be accurate in both sizes, not cropped, not anamorphically stretched or shrunk, and, please, don't have it do any of these things on its own: I'll do any adjustments manually, thank you very much.

That's also my preference when it comes to photography; I have a digital SLR, but still am very happy with my old manual lenses, which, since it's a Pentax, fit and operate just fine. Not that I'm phobic about automation and technology, but I like to be the one directing it ...

Thanks again to all for your help in understanding this issue.

 

[Biggiesized]

When I got my HD set top box from Comcast, the box was set to output the HD channels in 16:9 480p. When I switched it to 1080i, it made a huge difference. So they do make mistakes with respect to the box settings.

 

[Hi Ho]

So they do make mistakes with respect to the box settings.

Oh yes they do. Part of why I have a job is to fix their mistakes. They may know how to get the cable system to work properly and get the cable signal into your home but when it comes to HD sets they don't seem to have a clue in general.