PDA

View Full Version : EGA RGB to pure composite signal; no no no, not VGA! Pure composite!



Kazblox
November 28th, 2017, 07:20 PM
EDIT: Impossible passively. I lied about my NTSC composite monitor being able to support a plethora of EGA-type refresh rates, I am as stupid as the average Pentium II kiddie, and I need to do my research. But I guess it can be a probable source of discussion for what it is worth with the video modes that have a lower amount of lines. I know my electrical signals, but not television, dammit!

Off to find a Multisync!

KC9UDX
November 28th, 2017, 07:30 PM
I think you answered your own question. I cobbled something together with a few transistor amplifiers once. Don't actually recall why those were needed.

But your feature connector option is by far the best solution.

kgober
November 29th, 2017, 06:47 AM
By "low refresh rates" I assume you mean the 200-line resolutions? Because the 350-line modes aren't really compatible with composite afaik.

If all you want is the 200-line modes (but with the full set of 64 EGA colors) then I don't know of any off-the-shelf converters but I suppose you could try building something with some open-collector 7400-series logic and some resistors.

Retro Canada
November 29th, 2017, 08:21 AM
A composite monitor is compatible with NTSC video which is 262.5 lines (525 interlaced). It won't support 350 lines because the vertical refresh is different. You can't passively convert it. You would need a FPGA for that and you'd lose a lot of resolution. The only thing that is easily doable is converting CGA/(EGA 200 lines) into SCART RGB video.

But composite needs color encoding, which is much more complex. In this case you'd need a NTSC/PAL encoder IC like the AD722.

With a lot of logic components you could convert the RGBHV to a monochrome composite video (again 200 lines only) but why would you want it ?

KC9UDX
November 29th, 2017, 09:19 AM
I sure seem to recall that it worked. But I would have been using an older CRT monitor which would be pretty tolerant of that.

kgober
November 29th, 2017, 10:16 AM
If all you want is the 200-line modes (but with the full set of 64 EGA colors) then I don't know of any off-the-shelf converters but I suppose you could try building something with some open-collector 7400-series logic and some resistors.

In case it wasn't clear, the TTL logic and resistors are just to convert EGA 6-bit RrGgBb to analog RGB, which is easier to work with. You would then need to feed that analog signal into an NTSC encoder to get the final composite signal.

reenigne
November 29th, 2017, 10:27 AM
I have occasionally thought it would be interesting to duplicate the composite encoding logic from a CGA card as an EGA feature connector module. This would allow most composite CGA games to display correctly. More interestingly, in the mode that would be 640x200x16 on an RGBI display, it would allow for a very large number of colours. I haven't figured out the exact number, but it may be something on the order of 65,000. Not quite 65536 because there are a few duplicates but still a very colourful mode. If I ever find myself in possession of a suitable EGA card, I may have a go at doing this.

Kazblox
November 29th, 2017, 11:48 AM
Aaaaand I just started off the thread with a very big misleading assumption that 350 lines works on a regular composite monitor. Hypothesis really works out when you're naive!

2icebitn
November 29th, 2017, 04:31 PM
There are/were any number of lcds that accepted signal in the ega region. I learned this several years ago when perusing posts regarding a strange Tektronix unit. Provided you could fenagle an adapter that would convert ega to ntsc, you probably wouldn't want to look at it. You'd lose loads of detail, composite monitors aren't that sharp, and any perceived benefits of using ega would be lost.

Why don't you convert your ntsc monitor to accept rgbi signals :)

Kazblox
November 29th, 2017, 05:40 PM
No way I want to do that to my precious Apple monitor. It's too good for any modification... Best that it's off to getting a Multisync then!