This little board allows to connect your 15khz compatible monitor to your CPC 6128. This board converts the original CSYNC signal into a compatible with HSYNC and VSYNC required by the standard HD15 signal.
15Khz supported monitors here (look for the 15 kHz RGB Support column):
This is not a scandoubler or upscaler. You can use this with any scandoubler or upscaler like the GBS8220 or the OSSC.