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CGA to SCART adaptor

FishFinger

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This is something I made the other day, and I thought people here might find it handy. It's a little adaptor that lets you connect a CGA card to the SCART socket on a TV.

Many CGA cards already have a composite output, but the quality is often dubious at best, and they all use NTSC colour, which may not be supported by your TV if you live outside the US.


(click for big)

It uses a few resistors to convert the TTL RGBI signals down to 0.7V RGB, and a circuit based on this VGA-SCART adaptor, which uses a 74HC86 (quad XOR) to generate a composite sync signal from the CGA's seperate HSYNC and VSYNC.

As well as connecting to the CGA port it also needs +5V and +12V, which aren't available on the CGA connector, so it needs a molex connector for plugging into the PSU. (I left this off the diagram for clarity).

NOTE:
- Although SCART itself has pins for RGB input it may not be supported on all TVs - mostly only very old, or very cheap/crappy ones though.
- Some TVs with multiple SCARTs may only support RGB on one of them.
- The display is still 60Hz, so your TV must support that too (all TVs in the US do, and most non-US TVs made in the last 15 years or so should as well)
 
Last edited:

per

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Great!

However, note that color 6 should be brown, not dark yellow as it will appear with this adapter.
 

FishFinger

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Yeah, I couldn't think of any way around that, not without making things three times as complicated anyway.
 

FishFinger

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I took a few pictures to show the results. These are on a cheap 14" TV.

Colour Test:


80-column text:


80-column text (close-up):
 

FishFinger

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I don't think so.

According to the wikipedia page 160x200 is really 320x200, but it exploits the smearing effect you get with NTSC composite video to give the illusion of 160x200 with a more varied palette. The adaptor connects to the 9-pin RGB socket, so any effects that rely on composite/ntsc trickery won't work.

You can always just connect the composite output directly to the TV with an RCA cable though.
 

tomasont

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I'll be the first to admit that my electronics knowledge is pretty rusty, so maybe I'm looking at this wrong. I'm not following how your resistor array works for the RGB signal. It look to me like your RGB signals will be about 2V when intensity is low, and about 5V when intensity is high. What am I missing here?
 

vwestlife

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Looks good, but unfortunately SCART is unknown here in North America, so we just have to live with composite. I've found that 80-column text mode is actually readable on a color composite monitor or TV, as long as you keep the CGA in black & white mode. As soon as you turn on the color burst, the image gets unreadably smeared and rainbow-fringed. But 40-column text mode and color graphics modes work perfectly, and like "digger" mentions, many games were designed to take advantage of composite video artifacts in order to produce extra colors on the screen.

The same applies to other vintage computers as well. The Apple II high-resolution graphics mode is based entirely on composite color artifacting, and many Atari 800/XL/XE games use it in order to produce high-res color as well; if you play these games in an emulator on your PC, they will show up as black & white unless the emulator is able to simulate the artifacting effect.
 

FishFinger

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I'll be the first to admit that my electronics knowledge is pretty rusty, so maybe I'm looking at this wrong. I'm not following how your resistor array works for the RGB signal. It look to me like your RGB signals will be about 2V when intensity is low, and about 5V when intensity is high. What am I missing here?

The TV inputs are 75 ohms internally, so the resistors effectively form a potential divider with the CGA output on one leg, and 75 ohms to ground on the other:
image1f.gif

You end up with approx 0.7v for high intensity, and 0.5v for low.


Looks good, but unfortunately SCART is unknown here in North America, so we just have to live with composite. I've found that 80-column text mode is actually readable on a color composite monitor or TV, as long as you keep the CGA in black & white mode. As soon as you turn on the color burst, the image gets unreadably smeared and rainbow-fringed. But 40-column text mode and color graphics modes work perfectly, and like "digger" mentions, many games were designed to take advantage of composite video artifacts in order to produce extra colors on the screen.

There are limitations here too. Pretty much any TV made in the last 15 years will handle 60Hz refresh just fine, but 99% of them won't do NTSC colour, so if you use the CGA composite output you get black and white whether colour is enabled or not. That's actually what made me build this adaptor in the first place - the composite image was good enough, but there was no way for me to get colour from it.
 

vwestlife

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There are limitations here too. Pretty much any TV made in the last 15 years will handle 60Hz refresh just fine, but 99% of them won't do NTSC colour, so if you use the CGA composite output you get black and white whether colour is enabled or not. That's actually what made me build this adaptor in the first place - the composite image was good enough, but there was no way for me to get colour from it.
That's because PAL TVs are looking for the color subcarrier at 4.433618 MHz, while the NTSC color subcarrier is at 3.579545 MHz -- generated in the PC by taking the 14.31818 MHz clock crystal and dividing it by 4. If you replaced it with a 17.73447 MHz crystal (if there is such a thing!) then the CGA's composite output would generate so-called "NTSC 4.43", which many modern European TVs will reproduce in color. However, this would also overclock your PC from 4.77 MHz up to 5.91 MHz! (The CPU clock speed is derived from the clock crystal's frequency divided by 3.)
 

NsMn

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Would this actually work with an EGA card, too?
And DID somebody find a way to produce dark brown?

Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...
 

per

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Would this actually work with an EGA card, too?
And DID somebody find a way to produce dark brown?

Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...

As you see about the age of this thread, it's several months old so I don't think any advancements in the design has been implemented.

The color of brown can be implemented, but that will require some transistors/curquits to limit green (to appromaxely 1/2 the color strength in terms of fully on) only if intensity/blue is low and red/green is high. As told earlier, that makes the design qutie a lot more complicated.

EGA uses seperate intensity lines for each color, and it should be easy to implement. Just take the CGA version, and instead of connecting one intensity signal to all of the three color lines, connect one seperate signal to each of the three color lines instead. but certanly, you can connect the adapter as it is with one intensity input to an EGA card (you'll only get 16 colors then, though).
 

FishFinger

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Would this actually work with an EGA card, too?
And DID somebody find a way to produce dark brown?

It should work with EGA with only a small alteration. Instead of connecting the three 1.5K resistors together at pin 6 connect them to each of the secondary RGB pins (2, 6, and 7 iirc)

Is the yellow/brown issue a problem with EGA? I thought EGA solved that by changing the palette rather than hardware trickery in the monitor.


Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...

I'd double check. Most TVs claim to support NTSC, but it seems they really just mean 60Hz, very few of them seem to actually support NTSC colour.

Adding a composite input to the adaptor is a little tricky, as you need to change the voltages on some of the pins. Also some TVs automatically switch to RGB mode if they detect an RGB signal, so you'd need to switch those on/off too. The easiest way would be to either use a separate composite input on the TV if it has one, or a cheap SCART switch with a SCART-composite adaptor if it doesn't.
 

angel_grig

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CGA to SCART in Toshiba T1000

CGA to SCART in Toshiba T1000

Hello to all!I recently made a CGA to SCART adapter for my Toshiba T1000.
I used this schmematic:
adapter-cga-scart.jpg
from this site:http://electroschematics.com/377/cga-scart-adapter/
As you can see in the attached images the picture is perfect in my LCD tv but you must know that there are 2 errors in the circuit:
a)You don't need to connect the circuit to pin 7 of the CGA port-connect it instead to +5v(and -5v respectively to GND)
b)On the SCART side connect the G(green signal) to pin 11 of the SCART plug and not to pin 4 (as shown in the circuit)
And now some photos from my tv:
Image8 (Small).jpg
Image3 (Small).jpg
Image4 (Small).jpg
Image5 (Small).jpg
 
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Great Hierophant

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It should work with EGA with only a small alteration. Instead of connecting the three 1.5K resistors together at pin 6 connect them to each of the secondary RGB pins (2, 6, and 7 iirc)

Is the yellow/brown issue a problem with EGA? I thought EGA solved that by changing the palette rather than hardware trickery in the monitor.

I'd double check. Most TVs claim to support NTSC, but it seems they really just mean 60Hz, very few of them seem to actually support NTSC colour.

When EGA is in a 16-color mode, its output is exactly the same as CGA, RGBI. When in a 16/64 color mode, its output is RrGgBb. Also, I wonder how many SCART TVs can handle the 21.85Khz scan rate of an EGA 350-line mode.
 

gerrydoire

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15Khz RGB CGA Component Video to VGA Converter Scaler

Ebay Item: 250375733782

Has anyone tried one of these?
 

njroadfan

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Jan 21, 2011
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15Khz RGB CGA Component Video to VGA Converter Scaler

Ebay Item: 250375733782

Has anyone tried one of these?

Note, that the unit is for 15.75khz ANALOG RGB, such as the video outputted by the Apple IIgs, Atari ST, Amiga, and Tandy CoCo. CGA and EGA are TTL Digital RGB and require a converter like shown at the beginning of the thread. My PC Transporter in the Apple IIgs actually has such an adapter (small board on top labelled ColorSwitch) to allow CGA video to be shown on the analog RGB monitor the IIgs typically comes with. The adapter is quite a complex circuit compared to the one shown on this thread, but it likely shows brown correctly.

pctrans.jpg
 

FishFinger

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Joined
Sep 19, 2009
Messages
298
Location
UK
Hello to all!I recently made a CGA to SCART adapter for my Toshiba T1000.
I used this schmematic:
View attachment 6258
from this site:http://electroschematics.com/377/cga-scart-adapter/
As you can see in the attached images the picture is perfect in my LCD tv but you must know that there are 2 errors in the circuit:
a)You don't need to connect the circuit to pin 7 of the CGA port-connect it instead to +5v(and -5v respectively to GND)
b)On the SCART side connect the G(green signal) to pin 11 of the SCART plug and not to pin 4 (as shown in the circuit)

I found that circuit when I was looking to build mine, but assumed it was wrong because the PC doesn't put 5V on pin 7.

Which part of the circuit connects to -5V though?
 
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