using Composite to VGA converters

[PeBo]

When I set up my shiny new 4A through composite connectors to a CRT television the picture was perfect (attached photo is deceptive - have never been able to take pictures of CRTs with a moire pattern - but only the camera sees it!)

 

Anyway, I had also purchased a CompositeRCA/Svideo to VGA converter (a cheap one) hoping to use an old 4X3 LCD I have lying around.

 

When I hooked it up, I got a black & white picture with severe vertical banding (the attached photo is NOT deceptive this time).

 

I've now tried 2 different TI composite cords, 3 different VGA cords, and 2 different LCD monitors...all with the same results

 

so... my questions are:

 

1. Is my problem just a cheap converter box (would I likely have better results with one of the $150+ boxes, or would my results probably be the same??

2. Is my problem just a lack of understanding of the intricacies of interfacing a 30 year old VDP with a modern VGA monitor?

3. Since I've seen LCD's hooked up to TI-99/4A's, I know it can be done,

 

Sorry if I've missed a thread somewhere that I should have read before posting. I did read a thread about a guy who couldn't get colour, but I get colour just fine through composite....just not able to convert the signal to VGA.

 

Troubleshooting code is usually a no-brainer, but when it comes to troubleshooting hardware I suck the big one!

[+Lee Stewart]

Here are some links:

• http://atariage.com/forums/topic/172937-getting-s-video-from-the-ti-console/?p=2819013
• http://www.unige.ch/medecine/nouspikel/ti99/tms9918a.htm [EDIT: Previous post had Thierry's old site—sorry!]

 

...lee

Edited December 23, 2014 by Lee Stewart

[+OLD CS1]

BTW, the TI Tech Pages have moved.

 

You will have a helluva time finding a composite/S-Video - to - VGA adapter these days. Your best and easiest best is probably to pick up an F18A from Code|Hack|Create. It gives you SVGA output and a whole lot more!

[+Ksarul]

It is cheaper than the high-end converters and adds some additional capability to your machine too--I can definitely recommend the F18A.

Edited December 23, 2014 by Ksarul

[PeBo]

BTW, the TI Tech Pages have moved.

 

You will have a helluva time finding a composite/S-Video - to - VGA adapter these days. Your best and easiest best is probably to pick up an F18A from Code|Hack|Create. It gives you SVGA output and a whole lot more!

 

Actually, I've found several... from cheap ones like the one I bought (about $20 US) to units that cost several hundred. One of the major Canadian computer chains carries no less than 6 different models, and every store I've checked has at least two. (lots of folks (us for example) still have older equipment they want to run through their monitors and HD TV's)

 

I just made the mistake of taking the cheapest route (having already spent through the nose for the 4A and peripherals)

 

As far as an F18A... I bought a TI again after 30 years, for the authentic vintage computing experience. While I may be cheating trying to hook up a modern monitor (unless someone has a working PHA4100 they want to give me for Xmas), I really want to keep this particular console (bought as "New-Old" - I'm its first user) as original as possible. To upgrade the hardware would be a sacrilege to my goal of recreating a bit of my past. With the exception of my nanoPEB (which was soooo much cheaper than a PEB right now), if I couldn't buy it in '83, I dont want it now.

 

I'll consider an F18A if and when I buy a second console as a "project" computer, but for this one I want a Fantomworks restoration, not a Fast 'n Loud rebuild.

[PeBo]

Here are some links:

• http://atariage.com/forums/topic/172937-getting-s-video-from-the-ti-console/?p=2819013
• http://www.unige.ch/medecine/nouspikel/ti99/tms9918a.htm [EDIT: Previous post had Thierry's old site—sorry!]

 

...lee

 

 

Appreciate the links...(trying to take shortcuts as holiday-hectic rolls into full gear...otherwise I would have done a proper search myself)

 

Looks like the first link describes my experience exactly. Didn't help since he never found a solution either, but misery does love company, so it was nice to see I'm not alone!

 

The second link looks like a good read for when the wife and kids are finally down (I keep shooting them, but they keep getting up damnit!)

[+OLD CS1]

Whoops, I actually meant you will have a hard time finding ones which will work with classic systems.

[PeBo]

Whoops, I actually meant you will have a hard time finding ones which will work with classic systems.

no biggie...hope I didn't come off sounding arrogant (wasn't my intention)

 

Just wish I would have found the story Lee linked me to BEFORE buying another unusable gadget...I've bought a few things like that recently. Oh well, future ebay store I guess.

[Gazoo]

New item!

 

So will this work with our AVPC, EVPC, Geneve, other 80 column devices?

 

http://www.ebay.com/itm/Brand-NEW-MDA-RGB-CGA-EGA-to-VGA-industrial-Converter-/250851103017?pt=US_Video_Cables_Adapters&hash=item3a67e41129

 

The Ambery converter and the Amiga RGB to Component converters have been failures. The only device that works so far is the real expensive XRGB unit.

If there's not going to be an F38A, then we need SOMETHING!

 

Gazoo

[+Lee Stewart]

This one says it handles progressive scanning, which is what I think @Tursi said the 9918A does—it does not do interlaced scanning. I don't know about the 9938, though.

 

...lee

[Imperious]

See my reply in Your other post You just resurrected.

[+InsaneMultitasker]

New item!

 

So will this work with our AVPC, EVPC, Geneve, other 80 column devices?

 

http://www.ebay.com/itm/Brand-NEW-MDA-RGB-CGA-EGA-to-VGA-industrial-Converter-/250851103017?pt=US_Video_Cables_Adapters&hash=item3a67e41129

 

The Ambery converter and the Amiga RGB to Component converters have been failures. The only device that works so far is the real expensive XRGB unit.

If there's not going to be an F38A, then we need SOMETHING!

 

Gazoo

This looks promising. Would certainly be nice to know if it is compatible!

[kl99]

I used the Hama 2-3 video converter with an European TI-99/4a console with success on a VGA beamer.

The European TI-99/4a has a slightly different VDP chip which doesnt feature Composite but a Y-U-V kind signal.

Use this as input for the Hama converter, and use the VGA ouput.

 

I didn't try a VGA monitor yet, Fred Kaal described he had problems with it.

http://ti99-geek.nl/Projects/hama_converter/hama_converter.html

 

The Hama 2-3 video converter is no longer produced but they show up on ebay, i got mine for 10 bucks I think.

Edited December 24, 2014 by kl99

[+OLD CS1]

If that unit will do component then it should handle a sufficiently modified console with a 9928 (if I read the specs correctly, that is: I expect the 9928 can output Y-Pb-Pr.) That would give you the "almost-stock" quality as you would only be replacing a chip and making minor changes to the motherboard. Any VDP experts confirm this?

[gregallenwarner]

What I have discovered from experience is that those little cheap composite to VGA converters that PeBo mentioned: Some work and some do not.

 

I had purchased one of those exact same converters, and it suffered from the vertical banding. Then I searched eBay for another one, and although it was an identical form factor, it apparently had different electronics inside and it worked perfectly with the TI.

 

Unfortunately, I can't look up which one it was from eBay because it was such a long time ago, and I've since ceased using it in favor of the F18A. However, my theory is that it indeed has something to do with the "interlaced" vs "progressive" nature of the color signal. From what I can tell from my research, true NTSC color spec requires the color carrier be 180 degrees out of phase with itself on each successive scan line. (This allows for the color signal to effectively "cancel itself out" on older black-and-white TV's without color filters, a throwback to the introduction of color era.) However, based on the appearance of the image coming from the TI, it doesn't appear to be alternating the color phase every scan line. Each scan line appears to be in phase with every other scan line.

 

There are many methods to decode NTSC color, however, one of them: the "comb filter" presents a problem here. The comb filter works by storing each scan line in a buffer, and "subtracting" the next line from it, in order to extract the color information. If the color phase doesn't switch between scan lines, the comb filter subtracts all the color information right out of the image, and the result gets interpreted as luminance (brightness) information, resulting in the vertical banding.

 

So my recommendation is to try purchasing another composite to VGA converter, preferably of a different brand name. They're not all the same, and what you're hoping for is to get lucky with one that doesn't use a comb filter, but rather some simpler form of color decoder. Like I said, I was able to successfully locate one, and they're cheap enough to try a couple of brands.

 

All the theory above is purely a thought-experiment of mine, based on lots and lots of reading and research. I don't own an oscilloscope to test my theories, but I believe my reasoning is logical. Hope that helps somebody out.

[Fredrik Öhrström]

You are right in the conclusion, some work, some don't. :-) Here is however a bit more info on the reasons for this. (Which has already been partially explained by Tursi earlier in this thread.)

 

The 9918/9928 and all the other home computers could have created an interlaced signal for display, but they choose not to. The reason is that, even if the same image is displayed during both fields, the computer graphics horizontal lines and text with high contrast will jiggle up and down very visibly 30 (ntsc)/25 (pal) times per second. Not very pleasant to look at.

 

In the video signal there are two, slightly different vertical retrace signals, one that triggers an odd field, and the other triggers an even field.

 

And since the tv-circuitry is forced by the video signal and the tv:s are generally forgiving, it is possible to send the same vertical retrace trigger all the time and not alternate. Thus the computer graphics will be drawn on the same phosphor lines 60/50 times per second, no jiggeling! Will there be black space between the (now) progressive lines of phosphor? Nope, because on a TV-set the spot size of the electron beam is wide enough to cover the gap between the lines anyway. I connected my PAL TI994a through the RGB scart converter sold by Texas Instruments to a decent RGB monitor of the day (198ish) and then the black space between the scan lines were quite visible, because the monitor supported higher resolutions and had a smaller spot size.

 

Here is a good text: http://martin.hinner.info/vga/pal.html

in particular check the last paragraph.

 

The NTSC does not alternate the phase between lines, it is PAL that does that (hence its name Phase Alternating Lines).

 

The cause for the problem is probably simple, the developers of the software in one Composite->VGA converter probably has never experienced and therefore not tested a slightly borked input signal as ours, and therefore the software fails to recognize this as a composite signal with color and therefore falls back to treating the signal as a black and white signal. The bandings perhaps arise because now the color signal is no longer filtered away digitally and is allowed through aliasing during sampling to influence the video picture. Another converter that works has tested this or just happens to deal with it by chance, just like the good old TVs. :-)

 

 

 

 

What I have discovered from experience is that those little cheap composite to VGA converters that PeBo mentioned: Some work and some do not.

 

[gregallenwarner]

Good info Fredrik. I've long scratched my head over this issue, and since given it up in favor of the F18A, and my previous post was as far as I had gotten with my debugging. Your info closes this case for me.

[+OLD CS1]

Good points. Even if you were to use Y-Pb-Pr the interlace/progressive issue would still stand.

[Omega-TI]

Here is however a bit more info on the reasons for this....

 

 

Very well written and easy to understand explanation, even for newbies. This would make a nice addition to a sticky or Wiki somewhere about why some things on the TI don't work properly. Possibly even an addition of known devices that do or do not work correctly.

[+Ksarul]

You could always add the data to Ninerpedia. . .

[gregallenwarner]

Soon as I get a moment later today, I'll do the writeup on Ninerpedia for this.

[PeBo]

Thanks to all of you....as I've looked around and read several articles, I have a much clearer (no pun intended) understanding of the issues at hand.

 

it's frustrating that PAL consoles seem to fair better with these low cost converters than NTSC systems do. I've found countless pages of European TI users using the cheapest converter and having it work first time (exactly the opposite of what I read from North American TI users)...But there is no arguing that a $79 F18A ($92 CAD - Thank you OPEC) is cheaper than a $468 xRGB (a whopping $543 CAD). Especially considering the console cost me $100 including shipping and import fees!

 

It was encouraging to hear that I "might" luck into a cheap unit that works, but if I have to go through 2 or 3 to find one, then the F18A is still cheaper (and as someone else mentioned, provides other enhancements as well),

 

As always the AtariAge TI crew demonstrates a more finely tuned synapse-firing mechanism than I posess in such matters. I guess I've got to get myself a second console, keeping one stock, and making the other one a 1981 sci-fi hybrid by adding an F18A. In the interim, I found a sweet little 12" (13"?) Flat Screen (CRT) Trinitron for $10 on craigslist that I pick up next week, which will be far more practical than this honkin' 20" mofo I'm using now.

 

If I do break down and try another one of these converters, and find one that works, I will be sure to provide the sku in this thread.

[+Ksarul]

Of course, you could also break down and get your hands on a PAL TI-99/4A. Then you'd just need a display capable of displaying the PAL output or trying your cheapo converter to see if it works fine with the PAL console to give you a good VGA signal on an unmodified console. That would meet your primary goal of keeping a console stock through and through. PAL gives no issues with TI software either, unlike what happens with some of the other retro systems--there is one version and it works on both types of console.

[Fredrik Öhrström]

...PAL gives no issues with TI software either, unlike what happens with some of the other retro systems--there is one version and it works on both types of console.

 

I have wondered about this. It seems like most software (in particular GPL software) runs well with both 60Hz and 50Hz, perhaps the games are slightly easier when played on a PAL system because the run slightly slower? :-) But if you push the machine to its limits, it must be possible to write assembly software that is so tied to the frame rate that they will not work in both locales at the same time?

[Asmusr]

 

I have wondered about this. It seems like most software (in particular GPL software) runs well with both 60Hz and 50Hz, perhaps the games are slightly easier when played on a PAL system because the run slightly slower? :-) But if you push the machine to its limits, it must be possible to write assembly software that is so tied to the frame rate that they will not work in both locales at the same time?

 

Indeed. If you use a 50Hz console for testing your game you might get in trouble.