Typical Uses of Timecode

Applications that provide video capture and editing functionality will typically require control of external devices. These applications need to identify and index video and audio frames through references to SMPTE timecode. Linear editing system computers generally control three or more tape machines, as well as a video switcher and possibly a digital disk recorder. The controlling computer must execute commands at precise times and therefore must get videotapes cued to specific places at specific points in time.

Applications typically use timecode in a number of different ways including, but not limited to the following:

• Tracking of video and audio sources throughout the editorial process so an edit decision list, or EDL, can be generated for archival or export to another system. To create an EDL, perform the following steps:
  1. Shoot the video.
  2. Capture into a nonlinear offline system that uses some form of intraframe-only compression (MJPEG, DV, and so on).
  3. Edit the material and generate an EDL and offline edited master.
  4. Import the EDL to an online system and do an "auto-assembly" using the original source material to generate the final master, adding titles and effects where required.
• Synchronizing audio to video. In feature film production, audio is usually recorded on a separate tape recorder along with timecode. Specially equipped film cameras can also record timecode on the film in-between the sprocket holes. After the filmed image is electronically transferred to videotape, the timecode is used to align the audio with the picture in a process known as "synching the dailies". If the audio and video timecodes are different, VITC and LTC can sometimes be used together, one for video timecode and the other for audio timecode.
• Synchronization and triggering of multiple devices such as ATRs, digital disk recorder or players, VCRs, or other similar devices. This is a much broader class of synchronization than previously described, and is most commonly seen in linear editing and nonlinear editing systems, closed-captioning systems, and subtitling systems.
• Using the undefined bits in the timecode, called userbits. Often information such as dates, ASCII codes, or film industry information is contained in the userbits; however, use of userbits is limited only to the ingenuity of the user.

It quickly becomes apparent that timecode makes many things possible when properly handled. Unfortunately, there is also a lot that can go wrong, either because of poor technique or hardware malfunctions. Some things to look out for on timecoded tapes are:

• Unstable or drifting timecode relative to video or audio.
• Poor timecode field integrity. This means an LTC word begins in the middle of a frame rather than at the beginning, or VITC is not updated on a true frame boundary. The net result is an ambiguous reference.
• Unintentional VITC/LTC mismatch.
• Intermittent dropouts.
• Missing timecode.
• Poor timecode signal quality.
• Incremental frame offset from incorrectly made copies.