You have an old PC with a nonstandard RGB video out and you need to bring it to a modern PAL TV set. That’s the problem [svofski] had, so he decided to use an Altera-based DE1 board to do the conversion. Normally, you’d expect reading an RGB video signal would take an analog to digital converter, which is not typically present on an FPGA. Instead of adding an external device, [svofski] used a trick to hijack the FPGA’s LVDS receivers and use them as comparators.
The scheme does take a few discrete components to level shift the input signal and to provide an RC integrator. The integrator is used as a digital to analog converter, allowing the FPGA to compare the incoming signal with an output voltage. Once the analog signal is digitized, it is relatively straightforward to convert it to any format you want. Going back to the analog domain is as simple as a pulse width or pulse density modulation scheme and an RC filter (or you could use a simple R2R DAC).
The result is a very low parts count project that gets the job done. Of course, this is a complete hack of the LVDS I/O in the FPGA. If you want to hear more about the real use of LVDS, see the video below.