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New Coleco RGB board?

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2 hours ago, SearsRoebuck said:

... is there a chance this could help with a (true) s-video out for the CV?  With my setup, S-video is the best I can do, and even with a (very nice) composite modded cv, there is still some blurriness on some games with small text.

 

Yeah, going from composite (1 signal) to S-video (2 signals) will leave some of the image irretrievable.  RGBS (4 signals) or RGBHV (5 signals) are the best case where everything is completely split out already, which makes them a nice "mother" format for converting back down to fewer wires.

 

I'll answer your question with a strong "in theory it's possible".  You could add something like the AD725 near the end of the circuit, which would give nice, clean composite and S-video outputs in addition to RGB.  Judging from the datasheet, it looks like it'd be a relatively straightforward integration.  But the chip by itself (in single quantities) costs $11.  The small constellation of components that it needs (in single quantities) would add a few more dollars on top of that.  With a larger PCB and more output jacks, you'll be getting close to $20 more.  All to go "backwards" to lower quality outputs.

 

There is still a pretty big disparity between that and the $95 for the 2X-SCART from RetroTINK---which is the lowest-cost, "real" RGB-to-HDMI converter that I know about---but it's definitely worth considering.  HDMI isn't going anywhere for the next 10 years, but I haven't seen S-video on a new TV in about as long.

 

These sorts of "I know it makes sense in your situation but it doesn't for everyone else, so I'm not going to do it" answers are the worst!  So, I'd like to apologize that this initial reference board won't be able to solve the problem for you.  I suppose the good news is that the schematic and other files will be completely open so anyone else could come along and add their own new features.  As a hobby project, this has begun to consume most of my time (at the expense of my work, even :D ), so I'm excited to see it barreling toward its conclusion.  I would be happy to see others follow along afterwards to make different variations like one that had a proper S-video output.

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I have a large stock of AD725 from another S-video adapter design I did. It's neat. I was thinking of offering a header to the RGB socket that includes power, so this board could be connected and powered off the RGB adaptor.

Give it time.

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Posted (edited)

There is the soon to be retired, but widely available BH7236AF from Rohm. Less than 6$ by one from trusted sources like Mouser, 60 cents from Aliexpress.

It is used in the RGB board for the 2600 from Tim Worthington.

Higher pin count but near zero external component need (and no external clock). Correction, it need the color subcarrier too.

 

Add: It could serve as a proper 75ohm driver for all the outputs including RGB and CSYNC and clamp RGB out on sync. But does not have the LPF of the THS7374.

Could be tried in another revision to cut down the cost and adding composite and s-video.

 

Edited by emmanuelf
correction/addition

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I auditioned that chip, but rejected it for horrible dot crawl.

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Thanks guys, that would be great if that would be possible, in fact it was similar to what I had in mind (though I lack the tech know-how).  When it comes to classic gaming, I am a 'consumer grade CRT only' type, and S-video is the best I can do with my TV. 

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8 minutes ago, SearsRoebuck said:

Thanks guys, that would be great if that would be possible, in fact it was similar to what I had in mind (though I lack the tech know-how).  When it comes to classic gaming, I am a 'consumer grade CRT only' type, and S-video is the best I can do with my TV. 

Don't worry. I got you. The option board uses the AD725 to offer S-Video and clean composite (about the best I have measured in a sub-$150 device.)

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Small update for the past three evenings:

 

1. The "absolute maximum rating" output current of LM1881's pin 1 is 5mA... but the most you can usefully get out of it is apparently much lower.  I wanted the TTL-sync output (post Schmitt trigger) to be as clean as possible, so I tried tapping the little divider network (to get things down to SCART-level for the 75Ω sync) right off the LM1881.  I was seeing some weird droop right on pin 1, even when the total divider impedance was around 7kΩ.  It was only semi-passable once it was closer to 300kΩ, but at that point the distortion on the output of the divider makes it pretty useless.

 

Checking the "Electrical Characteristics" table in the datasheet revealed that pin 1 will already drop to 3.6V with 1.6mA on it, and will even reach as low as 4.5V with a paltry 40uA being sourced from it!  They really intended that to be a logic-only output that is buffered elsewhere!

 

So... divider after the Schmitt trigger it is!  Users will only realistically be using one kind of sync anyway, so if someone wants TTL, we could put a recommendation in the BOM to leave the divider out and simply ground Ch.4 of the THS7374 so it isn't left floating.

 

2. The edge speed on these Schmitt triggers is crazy fast.  They easily push the limits of trying to do all of this on a breadboard farther than any other step in this process.  With only a couple inches of wire, I'm seeing 2V of overshoot (on a 5V signal) on the scope!  This is one of those things I'm just going to have to cross my fingers in the hopes that a properly laid out PCB will fix.

 

3. Speaking of edge speeds, I had thought squaring the output of the LM1881 burst pin was just sort of a "nice to have" because the tiny Schmitt trigger happened to have two circuits and I only needed one at the time.  But, looking at the timing diagram in the mux datasheet, the burst pin's transition speed was just out of spec.  Now it isn't. :D

 

4. The LM1251 uses more power than I would have expected.  I happened to brush it with my finger during testing and noticed it was... warm.  Just for fun (and because I have no legitimate reason to own one of these things, so I try to make an excuse to use it any chance I get) here is a thermal shot of the breadboard (from before I switched back to the THS7374):

 

499833142_TMS-RGBthermal.thumb.jpg.451f661f70382319ceaf9bf961618598.jpg

 

Checking with my meter, measuring through the 5V pin coming from the CV to the breadboard, I saw 140mA with the color bars test cart and 131mA at the Popeye level select screen.  The LMH1251 datasheet says it's responsible for 70mA of that and judging from the picture, I'd believe it! hehe.

 

In a back-of-the-VDP installation, I wonder if it'd be a good idea to add a tiny chunk of heatsink to the LMH1251, just tall enough to make it contact the rear RF shield?  That would give it a ton of surface area to dissipate heat.

 

5. Of course I ordered an NO switch from Digikey instead of the NC needed for the sample-and-hold (during logic-low h-sync).  New parts on the way... again. :(

 

That said, I measured the actual propagation delay through the mux and it was only 3-4ns, so this extra switch might be a little superfluous.  I mean, every little bit helps, but is that 4ns worth the extra $0.25 and board space?  I'm not sure.  It does make me wonder where the other 10ns is coming from though.

 

I've had my doubts about L9 on the CV board.  I might try bridging it to see if there is any effect on the Y-only overshoot I've been seeing this whole time or if it has any impact on this slight phase offset.

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Be careful with overshoot: Most of the time it is measuring artifacts, you must take the ground as close as possible as the measuring point.

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If you meant the overshoot I mentioned in section 5, you can actually see it on the TV screen (just barely).  And it only shows up on the Y line (and not B-Y or R-Y) on the scope, so I'm pretty sure it's really there.  The real question is whether it's always present on all CVs and where it comes from (e.g., something on the CV board or is it inherent to the VDP)?

 

I'm not too worried about it because it's nearly invisible, but I figure if we're already going to this much effort, what's a little more?

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L9 on the CV motherboard is responsible for the overshoot on Y!

 

274640459_YwithL9.thumb.png.99c72059cfccbd9c1c238283bddcd217.png

Typical Y signal (showing color bars) on a Rev.H2 Colecovision

 

7829499_YwithoutL9.thumb.png.1772dd662a38c8fbdee5449fd8af3f1c.png

Y with L9 bypassed (using my tweezers) :D

 

There is a little more noise (which is presumably why L9 was there in the first place), but it's nothing the LPF on the THS7374 can't take care of.

 

The results are immediately visible when you zoom way in:

 

243749709_L9results.thumb.jpg.265bb3f5a967e6035ec794cb39291f9b.jpg

 

Bypassing L9 might be a new, optional step during the install of something like this.  I know I'm going to do it on my system when the time comes, hehe.

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When you bypass L9, it seems the clock goes to pieces causing per line left/right shifts of entire rows.

Most apparent in the white square and the left edge of the R.

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Posted (edited)
2 hours ago, MrPix said:

... it seems the clock goes to pieces...

Sorry!  Those slight line shifts are just an artifact from the capture device that are always present in random parts of the image.  They don't show up on the TV and if you capture a frame ten times, they'll always be in ten different places.  I should have spent more time trying to get the "stable" part to appear in the place I was zooming into.  That shift is not representative of bypassing the inductor.  If anything, it's more likely to be caused by building all this on breadboards, but those artifacts have been present for the whole two month process now across all five prototypes.

 

I'm going to try a different method (frame grabs from a video instead of the screenshot feature from the capture device's app, which is frustrating because it outputs relatively low-quality jpeg without any knobs to adjust it) and average several frames together.  I'm planning a fairly comprehensive installation instructions page to go along with the open source files, and having a nice, animated GIF-style comparison of several games showing the results of this optional step will be a nice addition.  I'll try to control for that unrelated effect a little better.

 

Something like this.  Here's a two frame animated PNG (or APNG), which is supported in most browsers.

 

The slight shift in the pixels occurs because the inductor also caused a tiny phase shift on Y, which you can see if you alternate between that pair of scope shots in my previous post.  Without it, the channels are better synchronized.  (This will be improved again when Y also goes through the mux.)

 

Overall it's subtle, but it's a definite improvement!

 

3 hours ago, emmanuelf said:

Or replace it by a ferrite bead.

That would probably work, too.  Although, if inductance was the cause of the problem, I'd be hesitant to add even trace amounts with something else.  I read the schematic as "we need a filter but can only afford to add a single passive; let's go with 32mH".  Now that it'll be replaced with the fancy, active, "6th order", etc. filter in the THS7374, a plain jumper is probably as good as anything.

 

I also suspect that "short piece of wire" will be better than "pair of tweezers in parallel with L9"! :D

Edited by Falonn
Attaching an animated PNG didn't work; linked to it instead

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Fairy knuff. Artifact ;)

I just wanted to drop here that this is an impressive amount of work and level of detail. I'm so relieved I didn't have to do this myself on my own workbench - I would have done a lesser job compared to you.

I did get a new 'scope out of it, though, so that's good!

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Alright, that's that.  The little SPST switch arrived.  After some bumbling, I was able to get Y routed through the same mux for propagation matching.  The result is very slightly sharper now that all three channels are ~8ns closer to perfectly synchronized.  Really, it's challenging to see the effect---even zoomed way in using an image editor.  In a pinch, we could probably leave that IC out and just use the 3rd mux channel for the sample and hold.  It's only $0.20 in reasonable quantities and not much more board space than an 0805, so I'll leave it in my version for completeness.  But it's worth mentioning that there is a cost/size-reduced version of this circuit that could omit it without much quality loss.

 

Here's the final breadboard prototype screenshot (with L9 on the CV board bypassed):

 

1394732339_afternoL9(2).thumb.png.4a950585855788a6aa86ed5d4165ccc6.png

 

Now it's time to draw up and release a proper schematic!  I got a head-start on it (really, I'm almost finished) while waiting for the SPST switch to get here.  That's next with the board layout following shortly afterward. :)

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6 hours ago, Falonn said:

Now it's time to draw up and release a proper schematic!  I got a head-start on it (really, I'm almost finished) while waiting for the SPST switch to get here.  That's next with the board layout following shortly afterward.

Then the next challenge is to fit it all on a Citrus3000-style underneath board :D

 

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8 hours ago, Falonn said:

It's a lot of stuff!  I think I can do it without going down to 0402. 😅

 

Go super small!  I have an oven and a scope haha. :)

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This won't be a kit project - I think about 90% of hobbyists would have severe problems assembling it.

I'll make it fit by using some very careful design.

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Posted (edited)

I think this is everything (see attached PDF for a zoomable/printable/etc. copy):

 

tms-rgb-v1-schematic.thumb.png.215d9e7fb100a237f4b25f56520afb9e.png

 

At least, I hope it's everything... because here is a board layout that I've already ordered from OSHPark & OSH Stencils for testing. 😅

 

tms-rgb-v1-board.thumb.png.37df338de67c7c8e1d6161506dc2ea5b.png

 

The density there is a little crazy for hand-assembly.  The labels on the passives just to the right of U3 & U4 suffered a little, but otherwise it should be legible everywhere else.

 

Once they arrive, we'll see how impossible it is to drop all these things in place with SMD tweezers.  Assuming everything goes smoothly and the board actually works, I'll open/share the OSHPark project as a public link.  The GitHub repo has the Eagle files so far.  Otherwise the information page just a stub until I get the boards in hand for installation photos and whatnot.

 

In other news, my broken PAL Colecovision should be delivered in the next few days, so (with any luck) we'll be able to get a PAL-compatibility data point pretty soon.

tms-rgb-v1.pdf

Edited by Falonn
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I've replied in PM as part of an ongoing conversation with Falonn with feedback of a couple of changes I'd recommend. They're only small 'perfectionism' things, mainly involving the debounce caps organization and a small issue with signal routing near U6:15. There will likely be a small revision to the board in a couple of days, so if you're planning to order the board from OSHpark you might want to wait just a couple of days.

Meanwhile, I'm setting up for a production run of 250. There will be plenty for everyone who wants one. We'll also have a variety of RGB socket options for various machines. They will be economically priced.

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Well I’m def gonna want 2 of those. One for my colecovision, and one for my Ti99/4a I’m gonna have to swap some chips on to get it to work

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8 hours ago, MrPix said:

I've replied in PM as part of an ongoing conversation with Falonn with feedback of a couple of changes I'd recommend. They're only small 'perfectionism' things, mainly involving the debounce caps organization and a small issue with signal routing near U6:15. There will likely be a small revision to the board in a couple of days, so if you're planning to order the board from OSHpark you might want to wait just a couple of days.

Meanwhile, I'm setting up for a production run of 250. There will be plenty for everyone who wants one. We'll also have a variety of RGB socket options for various machines. They will be economically priced.

Awesome - I am for sure in for 2 as well!

 

Matt

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13 hours ago, MrPix said:

I've replied in PM as part of an ongoing conversation with Falonn with feedback of a couple of changes I'd recommend. They're only small 'perfectionism' things, mainly involving the debounce caps organization and a small issue with signal routing near U6:15. There will likely be a small revision to the board in a couple of days, so if you're planning to order the board from OSHpark you might want to wait just a couple of days.

Meanwhile, I'm setting up for a production run of 250. There will be plenty for everyone who wants one. We'll also have a variety of RGB socket options for various machines. They will be economically priced.

I am definetly down for at least 2! Thank you!

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