exxos's Atari, Amiga & retro forum

And for my next trick: Elite


Though I doubt it very playable with the framerate as is...

And Galaxians:


Which runs at a very nice pace.

I'm also wondering why your screen is B&W.

HigashiJun wrote: 01 Apr 2026 11:54 I'm also wondering why your screen is B&W.

It's odd for sure. As I mentioned, the signal at my display is showing as NTSC even on different displays. The crystals are both as specced in the BoM and on the schematic I have.

I'm beginning to wonder if my 6847 is NTSC rather than PAL so I need to check the part number.

I did try an NTSC to PAL converter but it's B&W regardless.

EDIT: It seems the 6847 is NTSC only. So there fore there are additional tricks done to convert the video from NTSC to PAL.

There is apparently a 74LS393 dividing the system clock and generating the extra blank lines to make the frames suitable for 50Hz.

Theres a 74LS74 gating the 6847, a 74LS00/74LS04 combo combining the timing signals and an MC1372 PAL encoder.

I don't remember the MC1372 during the build so I need to go back to the schematic.

rubber_jonnie wrote: 01 Apr 2026 12:00 I did try an NTSC to PAL converter but it's B&W regardless.

Correct me if I'm wrong, but isn't this CRTC programmable, so you can set it to PAL or NTSC ?

rubber_jonnie wrote: 01 Apr 2026 12:00 I'm beginning to wonder if my 6847 is NTSC rather than PAL so I need to check the part number.

Correct me if I'm wrong, but isn't this CRTC programmable, so you can set it to PAL or NTSC ?

(Sorry, quoted the wrong sentence...)

Yay!

That's odd. I did a post and clicked submit but it obviously never appeared.

My Atom would indicate a grey scale NTSC signal via composite video

For colour you need to fit a colour board or build the rgb2hdmi board I've sent you :D

rubber_jonnie wrote: 01 Apr 2026 12:00 EDIT: It seems the 6847 is NTSC only. So there fore there are additional tricks done to convert the video from NTSC to PAL.

Yep. There's a whole section of the Dragon PCB just to try to convert NTSC to PAL (badly)!

I think Acorn decided well to use it as monochrome out as otherwise it'd be all green, like the CoCo/Dragon. Motorola, in their infinite wisdom, never thought that people may want "white" as a colour.

PhilC wrote: 01 Apr 2026 12:21 That's odd. I did a post and clicked submit but it obviously never appeared.

My Atom would indicate a grey scale NTSC signal via composite video

For colour you need to fit a colour board or build the rgb2hdmi board I've sent you :D

Interesting, I wonder if the green is only from the modulator then? So A bit like the Beeb & Electron that need mods to get colour out of composite. That would make sense.

@HigashiJun From what I can tell there is only one flavour of 6847 and it's NTSC.

rubber_jonnie wrote: 01 Apr 2026 12:25 Interesting, I wonder if the green is only from the modulator then? So A bit like the Beeb & Electron that need mods to get colour out of composite. That would make sense.

Sense has nothing to do with it... The Motorola engineers baked it into the chip design.