Speedlock is the name of a disc and cassette copy protection designed by Dave Looker (DJL) and Dave Aubrey-Jones. Earlier versions mentioned DJL Software later versions mentioned "Speedlock Associates".
Cassette
It was a modified version of the Spectrum's cassette routines with protection added.
It is often recognised for it's blue/black stripes in the border during loading.
Later versions had multicoloured stripes for the leader/pilot tone, and then blue/black stripes for the data.
As the protection system continued to evolve, programs such as Soft-Lok and the Bonzo Blitz were upgraded to break the protection.
Disc
Speedlock was also available in disc version. One version called "8k Speedlock", relied on the fact that the CPC's hardware could reliably read but not write 8k sectors.
There were two methods to get around this:
1. A hardware modification to the Amstrad CPC and a special copy program
2. Executioner's utility Xexor included a 'brain file' solution to crack individual games before copying them.