Fable
Sheet music and a player piano
Old Jin, a composition professor at the conservatory, spent half a year writing a piano suite, with exquisite scores, complex harmonic arrangements, and tempo markings precise down to every measure. The department printed the score as a textbook and distributed it to piano majors at twenty conservatories nationwide, asking each student to play it once and send back a recording.
Three months later, twenty recordings came back. Old Jin watched them all and found that not one was played exactly as written in the score. Some were half a beat slow at tempo changes, some forgot to make dynamic changes in repeated passages, and some pressed the pedal in the wrong places. The same score produced twenty different results from twenty people.
Old Jin complained to Old Wen, a professor at the engineering school: the score is written so clearly, why is the result so different when played. Old Wen said: because your score is a set of instructions, not a machine. Instructions tell a person what to do, but do not guarantee he can do it, does it right, or does it the same way. You need a music box.
Old Wen showed Old Jin around his laboratory: a self-playing piano. Old Jin converted the score into a digital format and input it, and the piano played by itself, exactly the same every time. Tempo changes were precise, dynamics were consistent, and pedal timing was flawless. After listening to the automatic performance of his own work, Old Jin was silent for a long time — it was the first time he heard what it sounded like when his score was executed with perfect accuracy.
Old Jin asked Old Wen: what's the difference between this machine and an ordinary piano. Old Wen said: an ordinary piano only provides sound; how it sounds depends on the person. The player piano adds three things: first, an instruction parser—converting score symbols into execution actions. Second, a state manager—knowing which measure it's playing now and what the next beat is. Third, an exception handler—if a mechanical finger gets stuck, it knows whether to skip or retry. These three things together are called the execution engine.
After returning, Old Jin added a sentence to the preface of the textbook: the score defines what the music should be like, and the performance engine ensures the music actually comes out that way. Learning the score teaches you how to design the process; learning the engine teaches you how to turn the process into a deterministic result. The two courses used to be taught as one; starting with this cohort, they were split into two.
A student asked Old Jin: If I can play the piano myself, do I still need automatic performance? Old Jin said: You play alone, and each time you can play differently—that is manual execution. You want twenty people at the same time, in different locations, executing by the same standard—that is system execution. A score can be photocopied; execution capability cannot.
Old Wen later organized the technical principles of the self-playing piano into a paper titled: "From Precision of Definition to Certainty of Execution." The abstract read: many systems fail because they treat "designed correctly" and "executed correctly" as the same thing. Design correctness depends on talent; execution correctness depends on the engine.