CLK, or Clock Signal, is an emulator written and maintained by Thomas Harte. It first acquired support for CPC emulation in late 2017. It was originally available only for macOS but now also has a command-line based Linux/SDL port.
It emulates the [[CPC old generation|old generation]] hardware only but seeks to be cycle perfect in its attempts to do so, right down to producing a 1.79Mhz 1Mhz audio feed and simulating a spinning platter and a fixed head for disk access. Since it has some known incompatibilities, it does not yet fully succeed.
Unlike many emulators, it is not frame-centric, emulating a CRT surface and reproducing its current state in time with the host machine.