Cheap RGB to Video converter for vintage computers

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By Alexey

Guardian (2238)

Alexey's picture

13-04-2015, 17:49

Hello dear MSX fans,

The idea of creating a converter for connecting vintage computers to modern VGA monitors is not new. There are plenty of such projects on Internet. However, most of them are made for specific hardware, for example for Amiga. It's impossible to connect most of MSX computers to such converter without splitting the synchronization signal first. My idea is to create a universal converter that supports PAL and NTSC and that could be used for any vintage computer that has R, G, B and either composite or separate (horizontal+vertical) synchronization signals. The output from such converter is a high quality S-Video signal, that can be easily converted to VGA with a cheap 15 euro Chinese-made Video-to-VGA converter. The quality of composite video is acceptable, but nothing compared to S-Video.

So, the prototype based on AD724 chip is now ready and is working fine. I called this device "Zefisha" (don't ask why and what this means). The quality of S-Video output is indeed great, PAL and NTSC seems to work correctly, both sync signal types are supported. There's a switch that allows to chose either composite or separate sync signals. The standard 15-pin VGA female connectors are used for input, the output has RCA and S-Video connectors. In order to connect a vintage computer to the converter, you have to make a special cable from computer's RGB port to the male VGA connector. The pinout for the VGA connector matches the real VGA's pinout for convenience.

I tested the converter with Yamaha YIS805 and YIS503, Panasonic FS-A1, Sony F1-XV and Amiga 500. The Atari ST 1040, Commodore C64 and C64C, Sinclair ZX Spectrum +3 will be tested next - as soon as I make the special cables for them. You can see the photos of the assembling process as well as the future circuit board pictures here:

http://www.finnov.net/~wierzbowsky/zefisha

The original plan was to use the EL1883 chip to split composite signals, but this failed for unknown reason. And due to the lack of the oscilloscope I could not diagnose the problem. So I decided to use the good old EL1881 chip in a DIP case instead. For making connections between components I used the "wiring pen" for the first time. Seems to work just fine as long as your soldering iron's temperature is above 400 C. Be advised that using such pen is recommended only in properly ventilated areas as some gas is emitted when you solder the wire.

I will order several circuit boards for the convertor soon and the next prototype will be assembled on proper board. The board is designed to support both AD724 and AD725 chips. For AD725 you can install an optional Chroma filter. For AD724 you will need to solder a wire to AD72x chip's pin 12 (Select). The wire should be connected to power for 4FSC mode (14,3 and 17,7 mHz) or to be connected to the ground for the FSC mode (3,5 and 4,5 mHz). The potentiometers are installed on R, G, B and Composite video lines to adjust the signal levels. The selection between PAL and NTSC is possible with 2 jumpers. If you wish, you can use one oscillator only for either PAL or NTSC format and you will need to set the jumpers correctly.

So if you have questions about this project or you will be willing to purchase the board or the assembled converter, feel free to contact me. I am not going to charge 80-130 euro for such device like some MSX hardware makers do. I believe that it's unethical to make business on people's nostalgia for the old 8-bit computers. But I am not reproaching those guys in any way - they can sell their hardware for whatever price they feel right...

Alexey

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By raymond

Champion (373)

raymond's picture

17-04-2015, 08:32

Yes, I am interested. I am now trying to get my GBS-8220 to work. And I get an image, but is has a wave in it (see this thread on msx.org: GBS 8220 So if this device is producing a nice steady video signal then I am very interested!

By sd_snatcher

Prophet (3020)

sd_snatcher's picture

17-04-2015, 12:39

I thought all European TVs featured the SCART connector and that it was easy to just build a cable to connect your MSX to it via RGB. Isn't that the case?

By NYYRIKKI

Enlighted (5300)

NYYRIKKI's picture

17-04-2015, 13:01

The problem with almost all converters is that in the end they produce wrong kind of MSX picture. (Too sharp, too wide, wrong sync, bad progressive scan decoding etc.)

Maybe you could try this monitor test tool and tell how it worked out on your MSX2 setup?

By Alexey

Guardian (2238)

Alexey's picture

17-04-2015, 13:49

Timo, this converter produces almost ideal picture with S-Video. I actually did not believe my eyes when I saw that for the first time. The composite video picture is indeed overexposed and has slight distortions. But with S-Video it's almost perfect. I improved the converter a bit by adding a 1kOhm potentiometer into the Composite In circuit - this allows to reduce distortions. The 100Ohm potentiometers on RGB entry allows to regulate the brightness to some extent. The 1kOhm potentiometer on CVBS exit allows to change the brightness of composite video output.

By Louthrax

Prophet (2072)

Louthrax's picture

17-04-2015, 13:58

I'm definitively interested by this product !

By Alexey

Guardian (2238)

Alexey's picture

17-04-2015, 15:47

Sorry guys, but I am not ready to sell this product yet as it requires some minor tuning and circuit board manufacturing. I will announce here when I can provide the board and parts for sale. I prefer not to assemble this converter myself as I have no time for this. The board and electronic components will cost around 30 euros + shipment.

Please note, that this board does not directly produce the VGA signal. It produces S-Video and Composite video signals that can be converted to VGA with a cheap Chinese-made convertor. I myself prefer a monitor with S-Video connection (Dell has those).

Alexey

By marcoo

Expert (97)

marcoo's picture

17-04-2015, 15:52

I us a scart to hdmi converter from conrad it works perfect

By Alexey

Guardian (2238)

Alexey's picture

17-04-2015, 16:11

I only use old 4:3 monitors where HDMI is not available. In fact, MSX screen fits better in such monitors. Widescreen is no use for vintage computers IMO. But it's the matter of taste...

By Alexey

Guardian (2238)

Alexey's picture

18-04-2015, 20:07

The components on the board have been completely rearranged taking into account the recommendations from AD725 chip manufacturer. The 1k potentiometer was added on CSync line so that output from Philips's SCART connectors worked properly. Tested with Atari 1040 - the video seems stable, though it's a bit too bright because of while background. The Sinclair output is still a problem. The CSYNC output voltage is only 1V, this may not be enough. I don't have oscilloscope to check what is output on that pin yet, but soon enough I will get it. Maybe the output is just not working properly...

New layout of the board:
http://www.finnov.net/~wierzbowsky/zefisha/gerber.jpg

Alexey

By Alexey

Guardian (2238)

Alexey's picture

21-04-2015, 10:53

The boards have been ordered after some minor final changes were made. Estimated delivery in 2 weeks.

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