invented a floppy controller which (along with the Disk II drive itself) imposed two constraints:
* Between any two one bits, there may be a maximum of one zero bit.
* Each 8-bit byte must start with a one bit.

The simplest scheme to ensure compliance with these limits is to record an extra "clock" transition before each data bit according to differential Manchester encoding or (digital) FM (frequency modulation). Known as 4-and-4 encoding,