Timecode is not Sync!
Timecode is not Sync... Sounds weird, but unfortunately this is usually the case. Most cameras and audio recorders use timecode only to get a timestamp for the file meta data when record is pressed.
This means that the camera uses an internal oscillator as reference for everything that needs to be clocked, including the shutter and the rate at which frames are captured. The speed of this oscillator is usually somewhat in tune but can't exactly be determined.
When you hit record, the camera quickly checks what the current timecode frame is and stores it to the file meta data, similar to someone checking their watch to read the current time of day.
Aside from that, the external timecode and especially the speed of it are ignored.
This leads to two problems:
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The exact point within a timecode frame where the recording starts is random, because the internal clock of the camera is not aligned with the external timecode. The camera may start the recording at the beginning of a timecode frame or at the end or somewhere in between. For a project rate of 25 fps, a timecode frame is 40 ms long. This is the general inaccuracy that every video clip has when it is shot on a camera without any kind of synchronisation. You can imagine that this can be an issue, for example when you try to capture video for 3D. |
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The second problem affects long continuous recordings like music concerts or documentary shots. If the sensors between the cameras are not properly synced, they will work at slightly different speeds and eventually drift against each other. Effectively, some cameras may record more frames than others in the same amount of time. When you try to align such video material during editing, the video clips will show different real-time events on the same position of the editor timeline. |
Should I care?
Luckily, these phenomena are not always relevant. When you shoot with only a single camera and/or your takes are no longer than a few minutes, you don't necessarily need to use external Genlock to synchronise the set, even though it is considered professional to do it anyway.
On the other hand, external synchronisation is absolutely mandatory for 3D productions, long recordings with multiple cameras or virtual production workflows with LED video screens. How you can achieve synchronisation between your recording devices can be learned here.