Memphis is simple a fixed delay locked in at 115 milliseconds. Why fixed and why 115 miliseconds? Well Memphis is designed to be a quick way to get a rockabilly sound.
Back then Ampex tape machines were really all there was to record. The echo was created by the use of the record and playback head which just happen to be 1.5" apart. The tape moved at 15 inches per second so it took the tape about, you guessed it, 115 millisecond to travel between the heads.
Many of the of the those songs sync up nice to the 115 milliseconds and the did not even have a sync to host button or even a host for that matter. It sounds pretty good on vocals of any type. The color scheme was a adopted form the Sun record label in Memphis.