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When you send a file from Final Cut Pro to Soundtrack Pro, the conversion process sets up your Soundtrack Pro project so that it closely matches the settings from your Final Cut Pro sequence. For example, the timecode will be either non-drop or drop frame, depending on the setting used in the source Final Cut Pro sequence. Similarly, the sampling rate will be set to match the rate used in Final Cut Pro.
The sampling rate is the audio equivalent of frames per second in the video world. In essence, a sample is a discrete recording of a sound at a moment in time, and the sampling rate determines how many “moments in time” play per second. The higher the sampling rate, the more closely the sound mimics an analog waveform. In theory, a higher sampling rate produces a more accurate reproduction of the sound, but in practice, you rarely need to use a sampling rate higher than 48 kHz. The reason is simple: only the most discerning of human ears can hear the difference between a 48 kHz file and a file recorded at a higher sampling rate (dogs can hear the difference, though, so if your recording depends on having an audible dog whistle in it, go with the higher sampling rate). Note that Final Cut Pro works only in 48 kHz.