These last days, I have been working on a small project : have an old "Pong" video game (here, from Univox) video output adapted to a TRS-80 monitor. Most "Pongs", including this one, are usually connected to a television through the antenna plug. As usual on this kind of devices, the video signal can easily be found right before the UHF modulator.

However, this one has an interesting "feature" : the video signal is inverted (!) and sits between 0.5V and 1.5V (1Vpp). The TRS-80 monitor expects a 2Vpp signal, between 0V and 2V. I also thought that it would be nice to be able to make a composite output too, so I started to think about a generic solution to perform this kind of operation (here, invert the signal, change its amplitude when needed and adjust it to the right reference voltage).

I thought that this kind of operation could be done with a Operational Amplifier (Op Amp, for short). So here is what I did :

• From the positive voltage coming from the power supply of the device, generate a negative voltage. It has been done using this 555-Timer based voltage inverter circuit.
  
• Regulate both the positive and negative voltages, to feed up the Op Amp with proper voltages. This has been done using LM1117 and LM2990 LDO regulators.
  
• Finally, use the Op Amp to perform a voltage substraction and amplification. I used the MAX477, which is a bit overkill, as it has a 300Mhz bandwidth (!). But at least, there is no signal distortion issue, unlike with the basic 741 one I used for a first try :)
  
  

Here is the result :



There are probably better and cheaper solutions, but I'm quite happy with it, as I managed to design the circuit myself, provided that my knowledge about analog electronics is close to nothing :) Please let me know if you're interested in the schematics.

Note : the 555 voltage inverter circuit schematic has a small mistake in it : the R1 resistor needs to be connected between the 6 and 7 pins, V+ being connected to the 8 pin.

My Hector HRX in a FPGA documentation has been updated as well (the document is still in French). You can find the updated documentation here.