David recently asked about Interplanetary sound on the blog comments, so I thought it'd be a good time for a movie audio primer.
One of the first things people ask about my 16mm camera is "does it record sound?" Though there have been a few 16mm cameras that record sound, most 16mm shooters (and 35mm shooters, for that matter) resort to double-system sound. The camera captures only images, and audio is recorded on a separate machine. These days, the ideal sound machine would be something like this recorder from Sound Devices. Sound Devices stuff is expensive, though, so I use an old Tascam DA-P1 portable DAT machine.
Video shooters who are particular about their audio might also want to consider double-system sound. Even though most video cameras are capable of recording audio, the quality of said audio probably isn't as high as that of a dedicated sound recorder.
So you have your film or video footage captured from one machine and audio captured from another. How do you join them together? Well, that's what the slate is for.
The slate includes two small sticks connected by a hinge. At the start of the scene, at some point after both the camera and audio recorder are rolling, someone slaps the two slate sticks together. During editing, you can look through the camera footage frame by frame until you see the point at which the sticks meet. Listening to the audio, you can find the moment where the sticks meet and make a loud clicking sound. In your editing software, you set the "click" on the audio track to occur at the moment the camera recorded the sticks making contact, and your sound and picture will be synchronized for that take.
This probably sounds tedious, but sync work goes pretty quick after you get used to it. Modern technology has brought us "smart" slates and camera systems that can automate this sync process, but modern technology demands modern prices. Doing it the old-fashioned way is much cheaper and just as efficient, if a bit slower.