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Syncing Sound

Contributed By Fred Ginsburg

When you are planning your production and what equipment you will use, always consider before hand how you intend on mating sound to picture afterwards. This will depend upon your budget and the type of equipment you are using.

In the early days of Eclair NPR and Arriflex 16BL film cameras, sync between the camera and recorder was maintained by a "sync cable" that linked the two together. A 60 cycle sync pulse was generated by the camera motor and this signal was recorded on the sound recorder as an indicator of exact film speed. Due to motor and battery variances the film speed tended to vary slightly and the 60 cycle sync pulse would reflect these speed changes.

The eventual advent of the crystal controlled camera motors eliminated any film speed variance and so the physical linking of the camera and the recorder became unnecessary. Both the camera and the recorder are running at precisely controlled rates. What this means is that the crystal sync pulse system is intended to replicate or reproduce a recorded event in precise real time - no longer and no shorter than elapsed real time. It does not matter what the frequency sync pulse is used during the record process, so long as that same frequency is used during the playback process.

reprinted courtesy of Dr. Fred Ginsburg

For example take a one minute scene recorded at 24fps on the film camera and 60 Hz on a Nagra. When the same film is later projected at 24fps and the sound played at 60 Hz, they will represent one minute of real time. Instead of being locked by a sync pulse, they are locked by minutes and seconds of real time.

Now that the recorder and camera are recording events in real time, sync can be achieved by creating an event that is simultaneously recorded as an image and a sound that can be used to sync picture and sound later. This is done with the rather low tech solution, the slate. The sound of the sticks striking is then aligned with the first frame of the sticks coming together on screen to create sync.

A more complicated and expensive option is to record with audio timecode. Most of the film cameras on the market are not yet equipped with their own time code generators. In order to make the time code visible on the film, the industry uses a clapstick slate (Denecke TC-1) that features a bright display of the time code coming from the Nagra. Originally, the manufacturer assumed that a mic cable would link the slate to the time code output from the Nagra. In short time, though, the industry adopted the practice of using a wireless transmitter and receiver system (Comtek). Daily rental of the Denecke time code slate and the Comtek wireless system is approximately $75 per day.

Here is what happens when audio is recorded with SMPTE timecode. Timecode is recorded along with production sound, on a Nagra IV-STC stereo recorder or a sophisticated DAT such as the HHB or Fostex PD4. During the transfer to video process, the film negative is rolled down in the telecine to the head of the shot or the clapstick slate and the time code is entered into a computer. That computer system then searches for the matching time code on the audio tracks. Then the controlling computer pre-rolls (backs up a few seconds) both the telecine and the audiotape machine, then both machines are put into FORWARD PLAY. It takes a moment for the telecine to reach normal speed. The computer monitors the time code of the audio, and adjusts the speed accordingly so as to achieve lipsync. Picture and audio are then transferred together onto the videotape for future editing. If the camera is more modern, matching (jam sync’d) timecode may or may not be recorded on the film by means of in-the-camera keycode and an Aaton master clock module. A Denecke slate is filmed at the head of each scene, displaying a visual timecode as well as providing an old fashioned clapstick marker.

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