The LEDs offer detailed information about individual robot. In contrast, the audio system can give the user an overview of the activities of the entire swarm, and can be monitored while looking elsewhere. This is a somewhat nostalgic approach: many veteran software engineers reminisce fondly of a time when they could debug programs by listening to the internal operations of their computers on nearby radios. Once the user learned what normal execution sounded like, deviations were quickly noticed, focusing attention on the offending part of the program. This resurrected approach to debugging has proven to be very effective on the swarm, and our modern interpretation.
Each robot has a 1.1 watt audio system than can produce a subset of the general MIDI instruments. This allows the Swarm to play any MIDI file, but we have found that single note work best for debugging. There are four parameters to vary per note: instrument, pitch, duration and volume. Care must be taken to blend these selections into a group composition that is intelligible. Good note selections for correctly operating program produces chords, tempos and rhythms. Once a user has become attuned to variations in these elements (especially tempo and rhythm), he or she can spot bugs from across the room in seconds that would only be apparent after careful analysis of the combined execution traces from all the robots.
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