Teletext support

I've had this idea for quite some time now. How about trying to get the video output circuts to embed some teletext pages in the signal?

Those who know about teletext should skip this paragraph. Teletext is a set of pages that is transmitted along a normal tv broadcast. The pages are usually some news, program schedules and once in a while, closed captioning. Tvs need special decoders to receive the pages. When used, you press a button on the tv remote labeled text or texttv and then you see the front page, usually page 100. You can then enter a three digit page number to view that page. Pages can be set to overlay on top of the visible picture, usually for closed captioning. (You know, subtitles.)

The idea is to get the final framebuffer to extend into the VBI section of the signal where the teletext data usually is sent. Then it's just a matter of formating the pixels right. That should not be an issue since the teletext format is quite well documented, at least for the core parts. It's not that well documented for things like formating and advanced features, but that's a minor issue at best.

Couldnt you just use teletext on your TV? Or go on the teletext website on Opera Channel? I'm assuming you want to display Teletext through the Wii, but I'm not 100% sure, your post is quite confusing (to me). And anyway, the Wii doesnt recieve a signal so can't get Teletext TO output to the TV.

He doesn't want to display or relay Teletext, he wants to generate it.
I also thought about that. Would be a cool, but propably pretty useless.

Showing it can be done would be all the fun

Ah, now I understand. My bad, sorry. But what would you display in the generated teletext? Could you have a page for System Info, a page for memory observation, etc?

For example, yes. The idea is mostly to do it to show that it is possible.

If we can do that, we might even be able to create a change-aspect-ratio signal

This would be cool, but a lot of modern TVs can only do teletext over RF, not scart or composite.

Realy?
Never expirenced such kind of TV

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The idea is to get the final framebuffer to extend into the VBI section of the signal where the teletext data usually is sent. Then it's just a matter of formating the pixels right.

framebuffer pixel format is fixed and cannot be changed
the maximal height is 480 lines for NTSC and 574 for PAL
teletext data are sent in the VBI aka vertical blanking "unused" lines (there are a total of 525 lines in NTSC and 625 in PAL)
there is no way you can access those lines via software, they are output by the Video Encoder chip and the only things you can configure are the ones which were initially available (interlace/progressive, PAL/NTSC , YUV/RGB, etc). Some VI registers also let you modify the frame/line timings but it won't help in that case.

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ekeeke
Some VI registers also let you modify the frame/line timings but it won't help in that case.

Any idea, how they might be abused to achieve teletext or any other other effect?

Is there something that indicates Widescreen signaling support?

My tv does do teletext trough SCART, also trough composite, all TV's in this house actually. And all TV's here are old so you're TV doesn't have to be that new.

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Any idea, how they might be abused to achieve teletext or any other other effect?

[hitmen.c02.at]

actually, it seems you can configure the number of lines (half lines) for each part of the video display (odd/even active fields, blanking, etc...) but It's very likely that modifying the default values, if possible, could produce an incompatible video signal (PAL and NTSC are standards and TV are more or less tolerant to deviations from the standard)

In all cases, you can only write the active area (the framebuffer), so you can not input data in the blanking lines, which teletext protocol requires. I think this can only be done with dedicated video signal generation hardware.

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Is there something that indicates Widescreen signaling support?

It does not seems so but not evrything is controlled by registers, I know from libogc that there are a lot of stuff that can be controlled by sending I2C commands to the Video Chip but it's not very documented.
I don't know how widescreen works but I guess it's just an output pin from the Video Chip that tells the TV the signal is widescreen (16:9) and will be displayed ("stretched") so it fills the whole TV screen. I don't think think the display is "magically" adapted to widescreen by changing a bit in the configuration, more likely the developper decides to "downscale" the framebuffer horizontally to take the further "upscale" that is done by the TV in account. This might be completely wrong though but it seems the way my TV is working (there is no difference is the ouput image if I only change the "widescreen" configuration of the console)



Edited 1 time(s). Last edit at 07/17/2009 06:12PM by ekeeke.

I am not completely sure that teletext requires that the data is written in blanking lines, just that's where everyone does it. Some, if not most, of the decoders shouldn't even care about what line the data is on, they just chose to use the blanking lines since they where not displayed.

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SifJar
Ah, now I understand. My bad, sorry. But what would you display in the generated teletext? Could you have a page for System Info, a page for memory observation, etc?

It'd be great for debug info. I know it's capable of placing the text anywhere on the screen, and (though I've only ever seen it when a crappy signal was messing up the text) displaying in multiple colours. So you'd have some nice debug output that can be toggled from the TV remote.

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I am not completely sure that teletext requires that the data is written in blanking lines, just that's where everyone does it. Some, if not most, of the decoders shouldn't even care about what line the data is on, they just chose to use the blanking lines since they where not displayed.

Sure it does... and everybody do it like that because it has been specified like that (so as decoders). That's precisely why they choose to use the VBI: because those unused line can hold data for a short period of time.
On the contrary, the vsiible lines of the video signal contain pixel data in analogic form and, even if compressed, Teletext data cannot be embedded in those lines.
Also, the video encoder obviously does not switch between pixel & teletext data in the active lines when you hit the teletext button on your TV, it's done the teletext decoder in the TV receiver which switch between normal and teletext mode.

Unfortunately, you sometime have to admit that not everything can be done with software ;-)

Can we control the blank lines in interleaced mode?

just to point out, most tvs wont support teletext in a few years anyway since teletext is being dropped by tv networks and tvs themselves

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SteelSLasher
just to point out, most tvs wont support teletext in a few years anyway since teletext is being dropped by tv networks and tvs themselves

Source? Almost all my TV-channel's have Teletext, and all new TV's in store have teletext support and I think they won't drop it for a long time.

Why would they drop it? I think that would make a lot of hearing-impaired people very angry, unless they're replacing it with something better. I've heard of no such replacement.

Steel, Darkness, Hyperhacker? Where do you live?

The only channels in Germany, that dropped Teletext are CNN Europpe and BBC world news (those are of course not German).
All others send teletext over all broadcasts (analogue, DVB-x)

Support for hearing impaired people is minimal over here.


The major digital TV system in Europe (DVB) offers DVD-like subtitle support - no teletext is needed.



EDIT: Why do I hit the SPAM filter that often?