SMPTE Timecode
The 1969 standard for synchronising audio, video, and lighting — still the lingua franca of film and broadcast.
SMPTE timecode (named for the Society of Motion Picture and Television Engineers, who standardised it in 1969) is a numeric label printed onto every frame of video and audio that needs synchronisation. The format:
HH:MM:SS:FF
Hours, minutes, seconds, frames. A typical timecode at the 30-minute mark of a programme is 00:30:00:00.
It's the fundamental sync mechanism for film, TV, broadcast, theatre, and live multimedia. Without it, lining audio to picture is impossible.
How It Works
Each frame of video has a unique timecode value. When you exchange material between systems — shooting on a camera, editing in Avid, sending audio to a Pro Tools mix — every system reads the same timecode and aligns to it.
For audio sessions, timecode is recorded as LTC (Linear Timecode) — a square-wave signal at audio rates, encoding the timecode numbers as a binary stream. You can patch LTC into a track, listen to it, see it, and read it electronically.
Frame Rates
Different video standards use different frame rates:
| Frame rate | Use | |-----------|-----| | 23.976 fps | Modern cinema (digital film). | | 24 fps | Traditional film. | | 25 fps | PAL (UK, Europe, Australia). | | 29.97 fps | NTSC (USA, Japan). | | 30 fps | Some legacy / specialised broadcast. | | 50 fps / 59.94 fps / 60 fps | High frame-rate broadcast and games. |
A timecode without a specified frame rate is ambiguous. 01:00:30:00 at 24 fps is a different absolute time than 01:00:30:00 at 30 fps — there are 24 versus 30 frames per second.
Drop-Frame vs Non-Drop-Frame
29.97 fps NTSC is almost 30 fps but not exactly. The 0.03 frame discrepancy adds up — over an hour, 29.97 fps produces 107,892 frames where 30 fps would produce 108,000. The timecode drifts.
Drop-frame timecode drops two frame numbers per minute (except every 10th minute) to keep the timecode aligned to wall-clock time. The frames themselves don't disappear — the numbering skips two values.
Non-drop-frame timecode doesn't skip. The timecode count drifts from wall-clock time over long programs.
Drop-frame is the broadcast standard for NTSC; almost all professional 29.97 fps work uses drop-frame. The notation distinguishes them:
01:00:30;00(semicolon between seconds and frames) — drop-frame.01:00:30:00(colon throughout) — non-drop-frame.
Common Sync Workflows
Filming + recording audio separately. Camera and audio recorder both record timecode. In post, the editor uses the timecode to align them sample-accurate.
Multiple cameras at one event. Each camera records its own timecode. A "jam-sync" master clock distributes time to all of them, so the editor can switch angles automatically.
Live broadcast. Master clock distributes timecode to cameras, switchers, audio consoles, lighting desks, and pre-recorded video sources.
Audio-to-picture editing. Pro Tools session timecode-locked to the picture editor's video. When the picture cuts, the audio knows where to follow.
How Audio Engineers Encounter It
If you only mix records, you'll rarely deal with timecode. If you do post-production, broadcast, or live multimedia work, timecode is constant.
The most common audio engineer touchpoint:
- OMF / AAF imports. Picture editors send audio session via these formats. Pro Tools, Logic, and Cubase all import OMF/AAF, preserving the timecode-aligned positions of every clip.
- Resync after picture changes. If the picture editor cuts a few seconds, your audio needs to slip by the same amount. The timecode metadata in the new picture file tells you where.
Common Mistakes
Mismatched frame rates. Importing a 24 fps audio session into a 25 fps project. The audio appears at the wrong timecode positions; everything is out of sync. Fix: re-conform audio at the new rate.
Confusing drop-frame and non-drop-frame. Two engineers communicating, one assumes drop, the other non-drop. Their timecode references are subtly different. Fix: always state which.
Forgetting timecode is in addition to sample rate. A 48 kHz / 24 fps session and a 48 kHz / 25 fps session both have the same audio sample rate but different frame numbering. They're not interchangeable for sync.
Treating timecode as a substitute for clock sync. Timecode tells two systems where they should be. Word clock or sample clock keeps them sample-accurate over time. Both are needed for serious multi-system work.