Speedlock

Speedlock was a software protection system used on the ZX Spectrum and the Amstrad CPC, written by David Aubrey-Jones and David Looker in 1983. The two programmers had become frustrated at the slow loading times of the computer's tape loading system, and realised it would be possible to write a better one. The prototype sat unused for about a year, but was finally picked up by Ocean Software on "Daley Thompson's Decathlon", released in late 1984. It was subsequently used by U.S. Gold and for several titles by Ultimate Play the Game, amongst others. Speedlock was also ported to the Amstrad CPC in 1985.
The system used several advanced features of the Spectrum's architecture, such as the memory refresh register and parity branch instructions of the Z80 processor, which made it harder to create illegitimate copies without the protection. Early versions used a set of audible "clicking" lead tones when loading a program. Later versions did not include this, but instead had a counter showing the time left to finish loading the program, similar to those of "Technician Ted" and "Fairlight".
Some companies created software to bypass protection schemes, including Speedlock, for the purpose of backing up or transferring to Microdrive, the Spectrum +3, or other proprietary disk systems.