THANKS TO a process known as digital sampling, it has been possible since the late '70s to take any sound -- from a garbage disposal full of turnips to the purest tones of an opera diva -- and turn it into a "virtual" musical instrument.
Most computers cannot manipulate data unless they are in digital form -- a stream of binary, 0-or-1, on-or-off signals -- which can be processed electronically. But sound is a smoothly continuous (analog) process, and has to be reconfigured for computer use. This is done in the same way that a movie camera records motion: Instead of capturing the whole action, it takes a consecutive series of still photos. If they are taken at small enough intervals, and displayed in rapid enough sequence, they will appear to recreate the original motion.
Similarly, digital sampling takes multiple "pictures" of a sound wave at intervals. As the sound enters the system, its strength and frequency are converted to voltages. (The same method is used to depict wave forms on an oscilloscope.) Those voltages have a certain value at any given instant, and "snapshots" of those values are recorded tens of thousands of times per second. The higher the sampling rate, the greater the fidelity.
Once the sound has been converted into digital form, it can be processed by the computer: combined with other sounds, transformed into another pitch, played back over itself, time-delayed to produce "echo," and so forth.
Thus the computer composer could take the sound of a Ford truck horn, combine it with a mockingbird warble, and substitute the amalgamated result for, say, the cello part in his concerto.
These capabilities raise excruciatingly difficult legal and moral questions. For example, Aretha Franklin certainly owns the rights to her own voice singing "Amazing Grace." But does she have rights to the individual notes? And what would happen if a digital sampler copied enough of her notes, rearranged them to produce "Hail to the Redskins," and then sold the result? The resolution of such problems will shape the future of the performing arts.