Atari external sync test. A Boss DR-55 drum machine sends clock pulses to a modified Atari 800XL. The Atari is running a simple interrupt handler that first randomizes the pitch of one of the Atari's synthesizer channels, then emits a gate pulse from one of the Atari's joystick ports. The gate pulse triggers a simple decay envelope circuit that the Atari's audio output is plugged into.