In a single system, audio is captured directly into the camera, and records simultaneously with the image. For a double or dual system, sound is captured independently of the camera and onto a digital audio recorder.
An important thing to remember though, is that when you boost an audio signal using a preamp, you boost the noise of the preamp itself. This is another strike against the single system as in-camera preamps tend to be noisier than their dual system counterparts.
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