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Jaguar VGA Cable Attempted

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32 minutes ago, Zerosquare said:

It looks like the LM1881 sync extractor could fit the bill: https://www.ti.com/lit/ds/symlink/lm1881.pdf

Indeed, and also recommended in the Retro Gaming Cables article I also linked in post #5, though for converting composite video -> composite sync.  I should have read my own research more closely the first time 🙂  AFAICT (which isn't very far), the vsync extraction circuit it has is just a much better/more general version of the integrator + gate in the image above with some stuff in front to filter out the actual video values from the composite video wave to get composite sync from composite video first.  My only concern with it was it appears the levels of the sync in composite video are much lower (~2v?) so it might get confused if you feed it 5v composite sync directly on the composite video input.  I'm hoping a quick and dirty test with the integrator into an XOR gate will yield results since I should have the parts on hand, but if it works, I'll probably order some of the L1881 chips to try those out too.


One thing I'm wondering is whether the Jaguar ever sends different video signals/timings out.  For example, when using the high-res interlaced image display code out there, does it send a real 480i interlaced signal, or is the code just faking it over 240p somehow?  I'm wondering if that would have any effect on how the sync extraction works.

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2 hours ago, cubanismo said:

One thing I'm wondering is whether the Jaguar ever sends different video signals/timings out.  For example, when using the high-res interlaced image display code out there, does it send a real 480i interlaced signal, or is the code just faking it over 240p somehow?

The interlace code generates proper 480i signals. Apart from it (and the homebrews that use it), I'm not aware of any software that uses anything else than 240p.


I also did manage to get VGA-compatible 480p (at 31 kHz) out of the Jaguar a few years ago. But it needed a bit of extra electronics, because I couldn't figure out any way to disable the equalization/serration pulses completely from software, and the VGA monitor I used for testing wouldn't display anything if there were any extra pulse. Also, it required software changes and twice as much bus bandwidth as regular 240p, so it was more like a proof-of-concept than anything else.

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Finally some progress here.  I used this circuit to extract VSync from Composite Sync:




Based on the HD Retrovision sample circuit and the parts I happened to have on hand (XOR Gates).  Here's what it looks like on the scope:




Channel 1 is HSync straight off the edge connector, Channel 2 is the recovered VSync signal, and Channel 3 is the integrated composite sync signal before passing it through the XOR gate.  Vsync is offset a bit, which was expected I suppose, but the two monitors I tested (NEC MultiSync EA190M and 1980SX) don't seem to mind; once I plugged HSync into HSync and the extracted VSync into VSync on the VGA cable rather than the other way around (It's been too long since I worked on this project), the screen lit right up and worked just as well as when I pass composite sync to HSync:




Nevermind the missing blue bits.  The blue wire appears to have been ripped off the edge connector yet again while it was migrating around my desk for the past several weeks.  The next thing I'm going to do is solder all the VGA wires and edge connector pins to some breadboard leads and wrap the joint in a bunch of superglue and/or electrical tape in hopes I can continue to prototype without constantly ripping the tiny wires off.


Unfortunately, the 1980SX still didn't really handle the mode properly, displaying the same heavily-offset image as before, and wouldn't let me adjust anything, simply telling me the values are out of range.  This was more or less expected, but I was still holding out hope it would do something better once I got a proper signal into it.


I tried to test some 640x480i images using the Win10-compatible QuakeGen from this thread:

But the ROM I got (a one-image slideshow) doesn't appear to do anything but display a black screen (on a TV using S-Video and on VGA).  Anyone have a working 640x480i demo/test available?  I noticed Rebooteroids has a menu option to display hi-res logos.  Is that using the 480i mode?

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qUaKe is what I tried.  QuakeGen was the name used in the thread I referenced, the name of the executable in the package, but same thing.  Interestingly, the ROM it generated works fine on virtualjaguar, but does not work on hardware when I send it to my skunk.  I just get a black screen, or sometimes a deep blue or purple screen.


I was aware Zerosquare worked up the timings/video register values used in qUaKe and probably elsewhere for interlaced mode, but I hadn't seen anywhere his raw code was released, so I did a little googling.  Is this comment on the jagware thread:




The final version of your hires demos @Zerosquare?  Interestingly, I see that a note in the Atariage mirror of that thread that Tempest 2000 used an interlaced mode as well (Or not).  Can anyone confirm this?  If so, is it for normal gameplay, or just in some particular screens?

Edited by cubanismo
Saw that temptest does not in fact do interlaced mode

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Yes, these are the final releases for the hires demos. But if I remember correctly, the hires code in qUaKe is a later version with a few enhancements. So if the hires demos don't work properly, try a qUaKe slideshow.


Regarding Tempest 2000, there's an "interlace" option you can enable in the settings, but it doesn't actually use interlacing. It just emulates some screen flickering.

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Cool, those demos work fine on my TV, and with the VGA cable:




The camera doesn't capture it that well, but the image looks very crisp in person, much better than on my CRT TV with S-Video where it looks slightly flickery.  The LCD controller seems to be deinterlacing it rather nicely.


Still need to resolder the connector, then I'm going to get one of those LM1881 chips ordered and compare that on the scope to the simple integrator I used to see which seems to have the best signal.  Then it's PCB prototyping time!


I can't decide whether it's better to use the real hsync for the hsync line, or use composite sync, which works fine even with an invalid vsync on this monitor, and also seems to be what the Apple II circuit referenced above used.  I might just put a switch on there that allows selecting between the two as I've done on my current breadboard prototype.

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On 9/9/2020 at 8:30 AM, Zerosquare said:

Fix that blue signal, that tree looks sick :D

How's this:




So crisp you might feel like you're breathing cool mountain air!




Courtesy of the LM1881 and a quick prototype board intended to test out my Skunkboard silkscreen tweaks:




The silkscreen test part didn't work out (The back is just blank. I may not be going with that PCB manufacturer for production skunk PCBs after all. Still working on resolving that), but the VGA connector part works just fine. Of course the layout is not ideal, and I didn't realize how much so until I went to plug it in. That VGA connector ends up about 2cm from the Jag power connector and lined up perfectly vertically with it, so you have to plug the VGA adapter in, unplug your power connector, plug the VGA cable in, then very gingerly pry it all up, flexing the PCB and connector until you can get your power connector plugged in again. Oh well. The form factor and the layout were entirely designed around fitting the skunk logo on the back and not putting any through-hole parts or vias in the way of detailed regions on it. The VGA adapter prototype functionality was just a bonus.


Note there's an 1/8" audio line out jack as well, and some headphones are plugged in to my monitor in that Val d'Isere shot. That part works great too.


There's a switch on the board that let's you select between feeding composite sync (as output by the LM1881, so a filtered and probably lower voltage version of the jaguar's raw composite sync output) or the Jaguar's raw horizontal sync signal the the VGA connector's HSYNC pin. In these pictures I'm testing the Jaguar's HSYNC signal, and the picture is rock steady with that. Even 50Hz mode works, which wasn't the case on this monitor with my prior experiments. Really good stuff. With the composite sync mode, it works but the picture flickers in and out. I might cut the trace on one of these boards and try wiring it straight to the Jaguar's raw composite sync signal instead to see if that works better. The idea here is to offer both in an attempt to improve monitor compatibility: Apparently some prefer composite sync on the hsync line rather than just hsync for these low-clock modes. However, the tiny dip switch is somehow the second most expensive component on the board and has a pretty big footprint. I'll have to look for a cheaper and ideally smaller-footprint switch if I end up keeping this feature in the next iteration.


I'm not planning on doing more with these immediately, but eventually I'll redo this board in a more sane form factor (Make it much smaller and mount the VGA connector on the top, right next to where the audio one is here) so it can be used practically, then I'll probably just build a few for myself and post the PCB design and gerber files along with a sample BOM for anyone that wants to order their own. These are super trivial to solder up. If anyone really wants one built for them, let me know, but if you haven't been reading along, recall these only work on a select few monitors that you'll probably have to track down on eBay. I'll have parts for around 10 just because some of the things here come in quantities of 10 at a minimum, so I can sell a few at cost+shipping (Probably around $20 plus shipping. Have to see how much the boards end up costing). I might try to 3d-print a little case for mine too, but my 3D printing skills are pretty meh so far. If I get something that works there, I'll share those CAD files as well.

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