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originally posted by: chr0naut
originally posted by: stormcell
It's a term from computing. When you are designing some bit of code or hardware, you often have a specification based only on a series of inputs, and a series of outputs. The actual purpose could be anything, an automatic ignition controller, real-time language translation, a mathematical function for encryption. But automagically, that code or hardware module does that particular task just like a magicians trick box turns eggs into chickens. Data goes in, answers come out. So they call it a "black box". That gets extended to flight recorders which take in all the inputs from the aircraft sensors, and save a recording in storage.
Initial flight recorders did not use magnetic tape or digital memory, because it was too prone to external disruption.
Recorders, therefore, used a beam of light, hitting a vibrating mirror to write the oscillations on to a rotating drum driven by a clock like mechanism with photographic film wrapped around it.
This recorded the sound optically, was rugged, mechanically and electronically simple (fewer points of failure), was easily reloaded and cheap.
Of course, the film handling and optical sealing was important and so the boxes were usually opened only in a darkroom.