If one opens a standard MIDI file in QuickTime Player, chooses "Get Movie Properties" from the Movie menu, and selects "Music Track" and "Instruments" from the popup menus, the resulting instrument selection window will let you open an on-screen MIDI keyboard. Just double-click any instrument, and you'll see something like the image at left.
As this dialog does not allow you to record or alter the sequence contained in the MIDI file, it is not particularly useful from a composition standpoint. However, it does give you a chance to easily try out all the different timbres built into the Quicktime MIDI module. There are far more sounds available than you might expect, and several of the built-in timbres are provided under license from Roland USA.
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