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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 10th Oct 2006, 8:31 pm   #1
jim_beacon
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Default Direct Synthesis of RF from VGA card?

I throw this open to people with far more knowledge than me. could this provide a method of direct RF synthesis for a band I or III tv?

http://fabrice.bellard.free.fr/dvbt/

Jim.
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Old 10th Oct 2006, 8:40 pm   #2
Kat Manton
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Default Re: Direct Synthesis of RF from VGA card?

Hi Jim,

Yes, and I've been thinking about it already...

..note that those are still images only, processed off-line first, the resulting synthesised waveform then played back in a loop.

...Synthesising a System A Ch. 1 signal, real-time, with moving pictures and sound with output straight from the DAC in a fast video card should be possible; I've no idea (yet) how much processing power it'll take. But in time I think we'll be connecting the aerial of TV sets straight to the graphics card... no extra hardware required.

Regards, Kat
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Old 10th Oct 2006, 8:46 pm   #3
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Default Re: Direct Synthesis of RF from VGA card?

Quote:
Originally Posted by Kat Manton View Post

Yes, and I've been thinking about it already...
Aaargh! the story of my life

Glad it's not a complete red herring. Even if you only use the DAC to drive an external modulator, making a simple(?) tunable modualtor.

Jim.
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Old 10th Oct 2006, 8:57 pm   #4
Kat Manton
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Default Re: Direct Synthesis of RF from VGA card?

Hi Jim,
Quote:
Originally Posted by jim_beacon View Post
Aaargh! the story of my life
Well, you didn't think I'd stop at what I've done so far, now..?

Quote:
Glad it's not a complete red herring. Even if you only use the DAC to drive an external modulator, making a simple(?) tunable modualtor.
Well, the point is you won't need a modulator.

All you need to do is compute the waveform of one carrier with AM video on it plus another carrier with AM audio on it, then bung the waveform out of the high-speed DAC conveniently provided by the video card manufacturer... it sounds easy when you put it like that...

Regards, Kat
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Old 10th Oct 2006, 9:09 pm   #5
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Default Re: Direct Synthesis of RF from VGA card?

Hmmmm, a promising procedure then, but well beyond my programming capabilities. I suspect this basically means writing a custom driver for the video card, and finding the correct way to pick the audio of the PCI bus at the same time - I think we could be looking at the current high-end range of machines to do this and still decode TV signals or playback a DVD at the same time.

Jim.
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Old 10th Oct 2006, 10:16 pm   #6
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Default Re: Direct Synthesis of RF from VGA card?

Kat told me about all this some time ago and I was sworn to secrecy. Jim, you've blown the cover

There was also a forum thread about direct digital synthesis plus a lot of emails between Darryl (tubesrule) and myself. We decided that a FPGA based solution would be feasible but not very easy. We certainly didn't think of using a VGA output.
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Old 10th Oct 2006, 10:25 pm   #7
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Default Re: Direct Synthesis of RF from VGA card?

Yes, I remember the DDS and FPGA discussion, but I hadn't seen the VGA approach before - a classic case of people in seperate places arriving at the same conclusion!

Jim.
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Old 11th Oct 2006, 10:08 am   #8
Kat Manton
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Default Re: Direct Synthesis of RF from VGA card?

Hi,

Quote:
Originally Posted by ppppenguin View Post
Kat told me about all this some time ago and I was sworn to secrecy. Jim, you've blown the cover
I'd discussed it with a few people outside the forum, we'd come to the conclusion that it was possible but I was keeping quiet until I'd done some "proof of concept" experiments (which I've not done yet.) But since Jim's raised it and found the same site I found after I'd started thinking about it, I reckon it's time to go public then we can bounce some ideas around.

Incidentally, related projects are GNU Radio and this project. Worth a look

The way some of these ideas come up seems to be by stepping back from the problem and looking at it in a different way. For example, my 'obsolete TV from a PC' experiments came about as I saw no difference between a vintage TV set and a fixed-frequency monitor. The X Window System (originated at MIT in 1984) has always been able to be programmed for fixed-rate monitors as these were common on Unix graphical workstations. Thus the X Window System predates both the "standard" rate graphical monitors (640x480, 800x600, etc) which appeared with the rise in use of PC-compatibles and the appearance of multisync monitors. Also, unlike other operating systems, Unix/Linux tend to evolve, new features may be added, but old features are seldom dropped. Thus current implementations of the X Window System still support fine-grained programming of the video waveform necessary for those ancient fixed-rate Unix workstation monitors.

Likewise using a video card for DDS seems obvious to me - step back from it a little and a video card looks like three high-speed DACs on a board with some other hardware sitting in the way. So if we can work around this other hardware (the graphics controller), we can use the DACs for whatever we like.

Now, back to bouncing those ideas around. Getting access to raw video and audio data isn't difficult, Linux and other operating systems provide library routines for all this sort of thing, we don't have to either hit the hardware directly or reinvent the wheel. But, that's getting ahead of ourselves.

My own thoughts are to start by playing around with accessing the video hardware through X in the first instance along with some very 'funky' X modelines. Then synthesise, off-line, a 41.5MHz carrier amplitude modulated by 440Hz tone as a file which can be thrown at the video card and looped - essentially a "still image" as far as the video card is concerned. Next, a little more complicated, synthesise, off-line again, a 45MHz carrier amplitude-modulated by a 405-line video waveform consisting of Test Card C. Finally combine the two waveforms. In all cases you have a file containing the synthesised waveform which, as far as the video card is concerned, is a still image - so we can just use X and readily-available software to "display" it; and if all's well a 405-line TV set tuned to Channel 1 with its aerial socket connected directly to the video card will reproduce Test Card C and tone

That's my thoughts on the "proof of concept" side; once that's been shown to work we can work on real-time synthesis of images and sound retrieved from a TV capture card or decoded MPEG from DVD etc. I think it's going to take a fair bit of processing power, but processing power is always on the increase. If it can't be done with current hardware; it'll be possible eventually.

Regards, Kat

Last edited by Kat Manton; 11th Oct 2006 at 10:11 am. Reason: Typo
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Old 12th Oct 2006, 4:31 pm   #9
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Default Re: Direct Synthesis of RF from VGA card?

Kat & Jim

Quote:
Originally Posted by Kat Manton View Post
Likewise using a video card for DDS seems obvious to me - step back from it a little and a video card looks like three high-speed DACs on a board with some other hardware sitting in the way. So if we can work around this other hardware (the graphics controller), we can use the DACs for whatever we like.
It should be feasible to use the RGB DACs to output whatever you like, however a possible obstacle is that some graphics controllers clamp the DAC outputs to 0 Volts during H & V blanking; needless to say this is not helpful in your application.


Quote:
Originally Posted by Kat Manton View Post
My own thoughts are to start by playing around with accessing the video hardware through X in the first instance along with some very 'funky' X modelines. Then synthesise, off-line, a 41.5MHz carrier amplitude modulated by 440Hz tone as a file which can be thrown at the video card and looped - essentially a "still image" as far as the video card is concerned. Next, a little more complicated, synthesise, off-line again, a 45MHz carrier amplitude-modulated by a 405-line video waveform consisting of Test Card C. Finally combine the two waveforms. In all cases you have a file containing the synthesised waveform which, as far as the video card is concerned, is a still image - so we can just use X and readily-available software to "display" it; and if all's well a 405-line TV set tuned to Channel 1 with its aerial socket connected directly to the video card will reproduce Test Card C and tone

That's my thoughts on the "proof of concept" side; once that's been shown to work we can work on real-time synthesis of images and sound retrieved from a TV capture card or decoded MPEG from DVD etc. I think it's going to take a fair bit of processing power, but processing power is always on the increase. If it can't be done with current hardware; it'll be possible eventually.
Your proof of concept approach seems sound to me.

When you move on to real-time synthesis, may I suggest that a useful intermediate step would be to synthesize the composite video waveform without performing the AM modulation step.

I can state that current PC processors are capable of performing the required calculations in real-time at least for synthesizing composite video; I've worked on design and implementation of a couple of PC based video processor / standards convertor projects where vertical / temporal interpolation was performed in real-time by software running on the PC CPU.
Should you require, I'd be happy to offer assistance in any software aspects of video processing.

John
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