A plain-English guide to Linear Timecode, jam sync, and why film sets rely on it to keep sound and picture locked together.
If you've worked on a set with a separate sound recordist, you've probably heard someone ask "is it jammed?" before the first take of the day. They're talking about LTC, and getting it right is the difference between an editor syncing your footage in seconds, or spending hours lining up waveforms by hand.
Timecode is a running clock, in the format hours : minutes : seconds : frames, that gets stamped onto both your picture and your sound recording so an editor can line them up automatically, frame for frame.
LTC stands for Linear Timecode (sometimes called Longitudinal Timecode). It's one of the two main ways timecode gets carried around a set, defined by the SMPTE ST 12M standard, the same body that defines broadcast timecode generally.
The important part: LTC is an audio signal. It's a stream of encoded pulses that sounds like a harsh, buzzing tone, and it can travel down a normal audio cable, the same way a microphone signal does. That's what makes it so useful on set: any device with an audio input, a sound recorder, a camera, or a phone, can receive it.
The other common type is VITC (Vertical Interval Timecode), which is embedded directly into the video signal itself rather than sent as audio. VITC is more common inside broadcast cameras and switchers. LTC is the one you'll deal with day to day on an independent set, because it's simple to generate, simple to distribute over an audio cable, and doesn't need a video connection at all.
Picture and sound are usually recorded on two separate devices: the camera, and a dedicated sound recorder. Without a shared reference, an editor has to sync every single take by eye and ear, lining up the clap of the slate with the spike in the audio waveform. On a long day with dozens of takes, that adds up to real time in the edit.
If both devices are jammed to the same LTC source before the day starts, every take carries matching timecode automatically. Editing software like Premiere, Avid, and Resolve can then auto-sync picture and sound with no manual work at all.
Jamming is the act of setting a device's internal clock to match an external timecode source, at one specific moment, so both devices then run forward together.
This trips up a lot of people new to timecode. Video doesn't run at exactly whole-number frame rates, 29.97fps rather than a clean 30fps being the classic example, a legacy of how colour NTSC television was originally broadcast. Left alone, that tiny difference means timecode drifts out of sync with real clock time over a long day.
Drop-frame timecode periodically skips a small number of frame labels to correct for this drift, so timecode stays aligned with the real-world clock over time. Non-drop-frame doesn't skip anything, so it's simpler, but it slowly drifts from real time.
The rule that matters on set: whatever you choose, every device needs to match. Mixing drop and non-drop timecode between camera and sound is a common, entirely avoidable cause of sync headaches in post.
Traditional LTC generators are small dedicated hardware boxes, and a decent one costs several hundred pounds. For a lot of independent productions, that's a lot of budget for something that just outputs a clock signal.
True-Slate reads and generates real LTC directly from an iPhone or iPad, over a standard audio cable: it can jam to an existing timecode source, or act as the source itself for a sound recorder or camera to jam from. It's the same standard, just without the dedicated box.
A digital clapperboard and film slate app for iPad and iPhone, with LTC jam sync in and out, an automatic shot log, and exports. £2.99 once, no subscription.
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