Component cable and stuff

[parsec]

I've been lurking here for a while and thought I'd introduce myself, and at the same time ask for some help.

 

I live in Sweden and work as a software developer. This career started back in the 80's with - guess what - a TI-99/4A. In recent months I contracted a severe case of the retro-flu and consequently started surfing eBay, hunting for a TI. I ended up with two specimens, one of which was boxed, and both PAL versions. I also started writing a space shooter game in Extended Basic and, after realizing (again) how slow this was, went on to assembly using Asm994A/Win994A. I'm now waiting for some cartridges to be delivered from US so that I can test my game on the real thing. A game screenshot is attached below, this is from the XB version that I am now halfway into porting to assembly.

 

Anyway, on to my problem. I want to avoid having to connecting my TI to an old clunky CRT-TV and would instead prefer using a VGA monitor. I looked at a French modification (see here) that would get me composite video out of the RF modulator, which I then could connect to a composite-to-VGA converter. Unfortunately the two modulators I have (models SIEL PHA2036 and PHA2030, made in Italy) are not built the same way. My modulators do not have any obvious connect points like the modulator in the instructions.

 

I noted however that the modulator input was component video so I thought, "Great, I can just build a component cable and plug it into a component-to-VGA converter". Apparently not so easy. From what I have found on the web there needs to be some additional circuitry slabbed on between the TI-output and TV-input for this to work.

 

So my question boils down to this: Does anyone have a schematic of such a circuitry, or could point me to somewhere where I can find this? I've seen discussions about this topic here on AtariAge but have been unable to find information that clearly illustrates how to build such a thing and which components to use. Would appreciate any help I can get!

[sometimes99er]

Game graphics looks great. Welcome aboard.

[+retroclouds]

Welcome Parsec! It's great hearing you'r into TMS9900 assembly language. The screenshot looks cool and I'm looking forward seeing more

 

You might want to checkout this page.

[matthew180]

So my question boils down to this: Does anyone have a schematic of such a circuitry, or could point me to somewhere where I can find this? I've seen discussions about this topic here on AtariAge but have been unable to find information that clearly illustrates how to build such a thing and which components to use. Would appreciate any help I can get!

 

VGA output has been the Holy Grail of 99/4A users for a long time. Many have tried, but no one has produced a circuit yet. It seems easy, but ultimately everyone discovers what you found out, video signal conversion is not trivial.

 

When I decided to give it a try, I spent a long time looking into various conversion circuits and such, and everything was seriously complex, expensive, or had some sort of draw back. That's when I decided it would be easier to reproduce the 9918A with an FPGA and generated VGA output directly. So, that's what I did. I'm currently working on the board layout and hope to have them available for sale early this year. You can read about it on my thread in this forum, or on my webpage.

 

http://codehackcreate.com/archives/30

http://www.atariage.com/forums/topic/168968-f18a/

[matthew180]

By the way, cool looking game! Let us know if you have any assembly questions, we love to help around here. I have a thread dedicated to assembly discussion, and there are many others in the forum too.

 

http://www.atariage.com/forums/topic/162941-assembly-on-the-994a/

[parsec]

Thanks for the warm welcome! I'm confident I will need help with assembly in due time but I got a good start thanks to you guys who contributed your knowledge and source code here on the forum.

 

retroclouds: Seems like this guy is feeding the component signal from TI to a VGA converter directly, which is amazing considering what I have read elsewhere.

 

matthew180: I'm not so much out to convert the signal, more like adjust the signal levels. From what I understand the TI component video signal levels are not exactly what a modern TV or other equipment expects. That is why I'm surprised that the setup on retrocloud's link actually works. My goal is very similar, to connect TI directly to an existing VGA converter (such as this one). I guess I could experiment a bit and hopefully not fry anything in the process.

 

Looking forward to your F18A, I've been following the development closely and will definitely be getting one when they are ready.

[matthew180]

I think the problem with feeding a VGA converter the signal from the 99/4A is that, the VGA converter is probably expecting a standard interlaced NTSC or PAL signal, and the 9918A only puts out progressive video, i.e. 60 fields (not frames) per second, and only the odd fields I believe. It has been a while since I looked into the details though. There was a thread somewhere, either here or the Y! list, where people were discussing in detail what kinds of monitors and TV's they were able to connect to.

[Opry99er]

I definitely like the graphics!!! Can't wait to see it running. In the meantime, there are some very nice ways to couple XB with assembly to get more speed for the very speed-hungry parts of your program.... In the development resources thread of this forum, there is a book called "Compute Introduction to assembly" or something like that. It was written by a guy named Lottrup--- there is a great chapter on the linking of the two languages. Matthew180, Marc Hull, sometimes99er and a few others have done some very cool stuff in this realm...

[+OLD CS1]

Yup. I have recently found that minor signaling differences can make, well, all the difference. I put my project down for a little bit to finish up a couple of programs and keep up with my class this semester, but I am likely to get back to it shortly. (Along with the 3.5"/5.25" combo drive... my attention span requires that I move from project to project randomly.)

 

You are in good company around here, as I have also found.

[parsec]

matthew180: Keep in mind that PAL systems generally have the TMS9928 VDP. These differ from 9918 in that the output is component (YPbPr) instead of composite video. Now, I tried connecting my TI to a component-to-VGA converter using a custom made cable but no luck, there was no detectable signal. I also tried connecting Y to a composite input which should have produced a black and white picture, but no luck there either. There was a signal there but no discernible picture. I measured the voltage on the different pins and Y was around 2.4V which seems way to high, I think it should be at 1V.

 

Opry99er: Thanks for the tips, I've been using Lottrup's book a lot while programming as well as "Beginning Assembly for the TI" and the E/A manual. I'm going to make this game entirely in assembly, more fun this way

[+5-11under]

matthew180: Keep in mind that PAL systems generally have the TMS9928 VDP. These differ from 9918 in that the output is component (YPbPr) instead of composite video. Now, I tried connecting my TI to a component-to-VGA converter using a custom made cable but no luck, there was no detectable signal. I also tried connecting Y to a composite input which should have produced a black and white picture, but no luck there either. There was a signal there but no discernible picture. I measured the voltage on the different pins and Y was around 2.4V which seems way to high, I think it should be at 1V.

 

Opry99er: Thanks for the tips, I've been using Lottrup's book a lot while programming as well as "Beginning Assembly for the TI" and the E/A manual. I'm going to make this game entirely in assembly, more fun this way

You could try the component video mod for the ColecoVision... although it's never been tried for the 9929A PAL yet as far as I know (designed initially for 9928A NTSC). Here's some background (from an old and outdated post, by the way) which may be helpful:

 

As you may know, the ColecoVision uses a TMS9928A Video Display Processor (VDP). The native output consists of 3 signals: Y, B-Y, and R-Y. Y is the luminance, B-Y is the blue color difference (relative to the luminance), and R-Y is the red color difference. Most people who want to output to a YPbPr monitor/TV start by taking the signals from the VDP, adding a load resistor to ground (keep in mind that the ColecoVision already has some loading, if that part of the circuitry is left connected), and connecting the signals to the Y, Pb, Pr inputs of a monitor, either directly, or through a buffer amplifier with unity gain (a buffer may help, because the monitor is likely loaded at 75 ohms to ground - I'm not sure of the implications of loading the VDP with 75 ohms or less). If you do this modification, you will find that, amongst other things, the display is oversaturated with blue. To remedy this, you can replace the load resistors with (500 ohm) potentiometers, tapping the wiper connections to the monitor or buffer amplifiers. This allows control over the Y, B-Y, and R-Y levels. Unfortunately, after adjusting the controls to optimal levels, the colors will still not match what was intended. The colors will be likely muted, especially the yellows.

 

Recall that the blue level was much too high when connected directly to the monitor. In order to get a half-decent display from the method described in the previous paragraph, the B-Y signal needed to be greatly reduced. However, when doing so, not only is the B reduced, but the -Y is also reduced. By adding more -Y (or subtracting more Y) to the scaled down B-Y signal, the proper signals can be nearly attained. The same procedure could be done to the R-Y line, although the improvements would be less striking (note: I haven't tried the R-Y correction yet).

 

In order to perform the mathematics on the B-Y signal, the B-Y and Y signals need to be linked in some fashion. I don't think this can be done with simply a resistor or other passive circuitry between the two lines, because the Y line would be adversely affected. I used a buffer amplifier to keep the Y signal path clean and strong, before tapping it to subtract from the B-Y signal. I used an LMH6734 for the buffer amplifiers, and an LM318 op amp for the subtraction function (I'm sure other "high speed" buffers and op-amps could work fine). I still need to optimize the schematic - I think there's some room for decreasing the parts count I'm currently using. I'll post more details, as well as pictures, at some point, but that's enough for today.