Retro computing and monitor output

So I’m going through some kind of ‘retro computing’ urge at the moment. Sure I’ve used emulators, and I also have a Terasic DE-1 that can run as an Amiga or an old 8 bit Atari, but for whatever reasons its all ‘not quite the same as the real thing’. So I’ve collected my old Amiga 500 and Atari 800XL from my parents (after years of gathering dust)… and also bought an Acorn Electron on ebay (because I always wanted one). Somehow while testing the Amiga 500, it’s power supply blew up taking the keyboard controller with it, and I bought myself another 2nd hand Amiga 500 for $31 (mainly to get a working keyboard).

One of my biggest challenges with these old computers is finding a screen in 2015 that they will work with. So all these computers date back to the 80s. The early 80s ones like the Atari and Electron have RF modulator output and composite video. The Amiga had 15kHz RGB as well as black and white composite (you had to buy the A520 external TV adapter to get RF and composite output). So technically I could plug this into a modern LCD TV with a composite input, but rather than hog the family TV (which would actually be in the true spirit of the 80s home computer experience ūüėČ ) I want to plug these into LCD monitors in my lab. So I have long ago thrown out any CRT monitors (though I have a dead Commodore 1084S), I currently have two older LCD monitors, both of which take DVI and VGA.

My original bright idea was to get a SCART to HDMI adapter. I would use a HDMI to DVI cable to plug it into one of my LCD monitors. With the right cheap adapter a SCART input can also do composite so I thought ‘this should do the Amiga output and the composite output of the other machines’. Wishful thinking. It doesn’t work at all with my two older LCD monitors. If I plug it into our newish Sony TV I get a picture for a half a second, then it goes black for 2 or 3 seconds, then flickers the computers picture for half a second again. My thoughts are that the HDMI HDCP stuff is screwing everything up. It is effectively useless.

So I gave up on the SCART to HDMI and ordered a composite and svideo to VGA converter on aliexpress ($15 delivered, but looks like this one) and originally I just tried the composite input and it works significantly better (ie. I can actually see the output of these computers). Of course composite is not the greatest for text output (it never was), and getting something like 15kHz RGB working would be better. This seems to be the elusive dream of lots of enthusiasts that either have old computers or old arcade machines. The old CRT monitors always wear out over time, and the VGA standard technically has a minimum horizontal scan frequency of around 31kHz which is basically double what the PAL and NTSC standards use. There are some monitors that will do VGA down to 15kHz, but they are rare (and I don’t have one).

So I started reading about a lot of people doing SVideo from older computers and getting reasonable results. SVideo basically takes the chrominance information out of the composite signal and shoves it down a separate wire. So I set about working out how to get SVideo out of these old computers. SVideo wasn’t exactly common in the 80s (or may not have existed at all) so none of the computers I have actually have SVideo outputs … but generally their video circuitry often has a section that mixes the chrominance information with the raw black and white data … and if you can find those points on the circuit board you have ‘SVideo’.

So I made the SVideo change on the Electron and initially I had no colour at all … but a few turns of the trimmer near the 17MHz crystal and hey I have colour out of the Electron! It’s not 100% fantastic … but its not that awful. The next challenge was the Amiga. It also never had SVideo. It has RGB and a single RCA plug socket for black and white composite. Commodore opted to sell the A520 external modulator device for people who wanted to plug the Amiga 500 into a TV. The A520 also has composite (colour) output, and I did test this … but the consensus is that the A520 does a pretty crap job at composite. Fortunately, someone worked out some mods to the A520 to both improve the picture quality and do SVideo. Sadly the Amiga A520 SVideo mod is only available on archive.org¬†now but is a reasonably easy mod … and I now had two Amiga A520s to try this out on.

I am pretty impressed with the modded A520 SVideo output. I am running it straight into my composite/SVideo to VGA adapter and its very good. It is perhaps the most impressive SVideo output I have ever seen. The Amiga’s 80 column text output is very very readable.

So my next challenge is to do SVideo on the Atari 800XL. That doesn’t look that hard, but so far putting the¬†¬†Atari composite output ¬†into the composite/svideo to vga¬†adapter has been abysmal. It almost looks like the adapter is hating the sync signals as the picture is way offset from where it is supposed to be, and is certainly not in colour.

 

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