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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 29th Aug 2006, 5:08 pm   #1
Sean Williams
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Default RGB to composite combiner

Hi All,

Just a quick question really - I have found a unit that was used as part of a wireless link system to connect a PC to a television screen.

As far as I can tell the unit is combining RGB and sync pulses, and sending them out as composite video - The picture on the 'scope looks promising.

So, the obvious query would be - Is there any reason that I couldn't use it for the FOTH TV converter?

Cheers
Sean
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Old 29th Aug 2006, 5:48 pm   #2
Kat Manton
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Default Re: RGB to composite combiner

Hi Sean,

As long as all this is doing is combining RGB+sync while leaving the video timings from the PC unchanged it should work.

I'm aware of units which would take VGA/SVGA from the PC, then scale and interpolate and produce 525i/60 NTSC or 625i/50 PAL video - as these change the timings from the PC they're not going to work as "The FotH TV System" relies on a little X configuration hackery to get correct video timings directly out of the VGA card. One of these type of PC to TV interfaces would get confused if fed with 405-line interlaced RGBHV...

What's in it? If there's a TV encoder chip in there it might be a "dumb" one which leaves the video timings alone; or it could be a "clever" one with scaler/interpolator...

If it's simple enough then you're in with a chance (and for "simple" have you seen the "KISS" one I tested which uses 2 resistors, a cap and a transistor?)

Regards, Kat
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Old 29th Aug 2006, 6:08 pm   #3
Sean Williams
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Default Re: RGB to composite combiner

Hi Kat,

Well, loads of trickery inside the box, along with an unhealthy quantity of multilegged fuses!

Here is a link to the spec

http://www.techbuy.com.au/products/2..._Converter.asp

Perhaps I should investigate the mode line mod and see what happens?

Cheers
Sean
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Old 29th Aug 2006, 6:18 pm   #4
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Default Re: RGB to composite combiner

Definitely not suitable.
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Old 29th Aug 2006, 6:42 pm   #5
Sean Williams
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Default Re: RGB to composite combiner

Sounds fair to me!

Well, looks like it re enters the junk box......

Cheers
Sean
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Old 30th Aug 2006, 7:42 am   #6
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Default Re: RGB to composite combiner

Hi, this thing is a complete standart converter. If you are able to change its soft, you can use it.

Kind regards,
Darius
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Old 30th Aug 2006, 4:48 pm   #7
Kat Manton
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Default Re: RGB to composite combiner

Hi,
Quote:
Originally Posted by oldeurope View Post
Hi, this thing is a complete standart converter. If you are able to change its soft, you can use it.
Quite right, but that's a big "if". The unit could contain a processor of some sort running software, or it might contain an FPGA. I'd suspect if you ask the maker for source code they'd refuse. There's also a possibility that datasheets for devices used in the unit could only be available if you sign a non-disclosure agreement and commit to buy several thousand devices.

Reverse-engineering could be possible; but that depends on where the software's stored and whether there's any protection on it.

There's also the question, "why bother?"

Consider what would happen when playing a PAL DVD on the PC through a VGA/TV converter assuming it has been possible to modify it for 405-line.
  • PAL DVDs are 768 x 576, 25fps and may or may not be interlaced depending on the source.
  • SVGA is 800 x 600, 60fps, progressive (non-interlaced.)
  • "Digital 405" (square pixel) would be 504 x 378, 25fps, interlaced.

Here's roughly what happens:
  • Extract digital frames from the DVD.
  • De-interlace (if required.)
  • Scale 768 x 576 to 800 x 600 in the digital domain.
  • Duplicate a few frames to match SVGA frame rate.
  • Output through the D/A converter on graphics card.
  • Convert back to digital with A/D converter on VGA/TV converter.
  • Drop frames (or interpolate if it's 'clever').
  • Scale 800 x 600 to 504 x 378 in the digital domain.
  • Output through D/A converter on VGA/TV converter with 405-line timings.

Compare with "The FotH TV System."
  • Extract digital frames from the DVD.
  • Scale 768 x 576 to 504 x 378 in the digital domain. [1]
  • Output through D/A converter on the graphics card with 405-line timings.

With a VGA/TV converter, expect motion artifacts resulting from two changes in frame rate along with a loss of quality as the images have been scaled twice. I think these things are more intended for presentations than TV/movies, so these problems don't matter much.

With FotH TV, using digital sources originating inside the PC (DVD, digital terrestrial tuner card, etc.) there's not a great deal of processing going on. FotH TV isn't a standards converter; though if you have a video capture card you could use it as one. But the system works best with digital sources originating inside the PC as this reduces the amount of processing going on.

Wouldn't it be great if some company made a combined DVD player, digital TV set-top box and hard disk recorder which output 405-line video directly so you don't need a standards converter? I doubt anyone would, though. But that is what FotH TV is, except you make it yourself with an ordinary PC, consumer PC peripherals, free open-source software and a bit of electronics.

Regards, Kat
[1] Actually, we're running at 968 x 378 as I need to get deep into the inner workings of the graphics card and figure out what the PLL is doing. Then I'll figure out some slightly more sane resolutions. The point still stands. Scaling happens once, from DVD resolution to something with 378 lines.
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