The RGBtoHDMI interface converts the "digital" RGB video signal from the BBC Micro or Electron to HDMI compatible with most modern TV/Monitors. The interface comprises a Raspberry Pi Zero and a specially designed Hat containing a small CPLD. Custom firmware on the Raspberry Pi, in conjunction with the CPLD, is able to correctly sample each of the Beeb's video modes to give a pixel-perfect rendition.
For more information, see the project Wiki.