Oberheim redesigned its DX drum machine to include MIDI, creating the Stretch DX and adding 4 voices in the process. The DX stored its sampled sounds on EPROM (Erasable Programmable
Read-Only Memory) chips. These chips could be taken out and replaced by other chips purchased from Oberheims sampled library. You could also, through products like the Oberheim Prommer, sample your own sounds and burn them into your own EPROM chips.