Digi sample
The trick of squeezing digitised sound out of the SID by hammering the volume register faster than the chip was built for — making the C64 talk far beyond what the spec promised.
The SID had no channel of its own for sampled audio, but it had a loophole: writing to the chip's volume register produced an audible click. By writing new volume values thousands of times a second — timed against the raster — a routine could play back a 4-bit PCM stream and make the C64 speak or drum. The method cost essentially all of the CPU's time while it played.
The technique was demonstrated early in games like Ghostbusters (1984), which had the machine shout the title, and was then pushed to extremes in the demoscene. A later bug discovered in the SID's filter even let sampled audio and ordinary music play at once, something MOS had never intended.