Direct From Disk (DFD) is Native Instruments’ term for the technology that allows a virtual instrument to play samples directly from hard drive instead of loading them into RAM. This allows for playing longer samples than will fit in the computer’s memory, among other things. A small amount of RAM is still required to “preload” a bit of the sound before it starts playing, to compensate for the time it takes for the computer to find the sample on the disk and begin playing it – this is known as the “preload buffer.”
