Atari external sync test. An Atari 800XL is sync'ed with a DR-55 drum machine which is triggering some semi-random notes from the Atari's POKEY chip. This is all being done with an interrupt handler (the sync pulse comes in on the BREAK key) — you can hear the Atari keyclick sounds in this recording because the computer operator is POKEing values into memory from BASIC while the test is running.