Dual-System Sound

The camera captures its own reference track while a recorder, phone or wireless system captures the real audio at the same time. It exists because camera preamps are usually the weakest link in the chain, and because a microphone needs to sit near the mouth rather than wherever the camera happens to be standing. The cost is sync. Clap at the start of every take or let the software match the waveforms, and check the recorder is genuinely rolling, because a flat battery gives you a take with no usable sound.