VGA to SCART RGB converter
This is a simple circuit to connect your VGA graphics card to your TV. The advantage of using RGB over S-Video is superior contrast and clarity. The schematic of this circuit is taken from
Tomi Engdahl's site. The circuit basically XORs together your horizontal and vertical synchronization signals to create a composite sync output, and adjusts output for input polarity. By setting the resolution and timings on your VGA card correctly, it is possible to generate a signal that the TV will accept. This circuit works for both PAL and NTSC signals, just adjust the timings in software accordingly. To see which timings to use,
this site has a list. If it doesn't work, experiment a bit. The resolution should normally be 720x576 for a PAL tv. The refresh rate should be 50 Hz interlaced. Use Powerstrip to set the timings after you've set the resolution to 720x576.
Schematic
Board
The board is made in
Eagle PCB. It's small enough to fit inside a D-sub case, which is very neat. The resistor packages are 0603. It was the only (well easiest anyway) way to make this circuit fit inside a D-sub case.
If you want to etch this board yourself, you can
download the Eagle schematic and board files. EAGLE PCB can be found at the
Cadsoft website. Note that the board file has an unconnected via, the top left one. Connect your +5V supply here.
Component list:
Name Value Package
C1 22 µF 0603
C2 22 µF 0603
R1 2.2 kOhm 0603
R2 2.2 kOhm 0603
R3 2.2 kOhm 0603
R4 2.2 kOhm 0603
R5 2.2 kOhm 0603
R6 47 Ohm 0603
R7 47 Ohm 0603
R8 47 Ohm 0603
R9 120 Ohm 0603
Q1 BC847C SOT-23
Q2 BC847C SOT-23
IC1 74LS86 SO-14
You will also need a D-Sub case, a 15 pin high density D-sub connector, a SCART connector and some cable.
Photos
These are some photos of my finished circuit.
The black wires are the +5V/GND supply. These can more easily be taken from pin 9 (+5V) and pin 5 (GND) if your graphics card is VESA compliant. Most cards are. If in doubt, use a voltmeter to see if you have +5V on pin 9. I was too lazy to check and just soldered a wire there instead and used the internal molex connector in the pc. This turned out to be more work than simply check with the voltmeter. The logic of my decision still eludes me.
I used shielded ethernet cable which has an impedance of approx. 100 Ohm. The VGA card and TV are both 75 Ohms. The impedance can be lowered somewhat by sending the red, green and blue signals in separate pairs and ground the other wire in the pair. I doubt you will run into any problems due to this though. I used 2 m cable but I suspect up to 10 m cable can be used without problems.
I scoped the composite sync signal at the TV end of the scart cable with the TV on to check that I got a nice signal. Photos below. Click to enlarge.
Horizontal sync pulses. Vertical scale is 500 mV/div and horizontal is 10 µs/div.
Vertical sync pulses. Vertical scale is 500 mV/div and horizontal is 15 ms/div.
