Linear timecode

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Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode data as a Manchester-Biphase encoded audio signal. The audio signal is commonly recorded on a VTR track or other storage media. Each frame is terminated by a 'sync word' which has a special predefined sync relationship with any video or film content.

A special bit in the linear timecode frame, the 'biphase mark correction' bit, ensures that there are an even number of AC transitions in each timecode frame.

The sound of linear timecode is a jarring and distinctive noise and has been used as a sound-effects shorthand to imply 'telemetry' or 'computers'. In the industry "LTC" is pronounced "Litsy" except in the UK where it is pronounced "ell-tee-cee".


Generation and Distribution

In broadcast video situations, the LTC generator should be tied-in to house black burst, as should all devices using timecode, to ensure correct color framing and correct synchronization of all digital clocks. When synchronizing multiple clock-dependent digital devices together with video, such as digital audio recorders, the devices must be connected to a common word clock signal that is derived from the house black burst signal. This can be accomplished by using a generator that generates both black burst and video-resolved word clock, or by synchronizing the master digital device to video, and synchronizing all subsequent devices to the word clock output of the master digital device (and to LTC).

LTC timecode is essentially an audio signal around 2400 Hz in frequency. This signal can be distributed by standard audio wiring, connectors, distribution amplifiers, and patchbays, and can be ground-isolated with audio transformers. LTC can also be distributed via 75 ohm video cable and video distribution amplifiers, although the voltage attenuation caused by using a 75 ohm system may cause the signal to drop to a level that can not be read by some equipment.

Care has to be taken with analog audio to avoid audible 'breakthrough' from the LTC track to the audio tracks.

LTC care:

  • Avoid percussive sounds close to LTC
  • Never process an LTC with noise reduction, eq or compressor
  • Allow pre roll and post roll
  • To create negative time code add one hour to time (avoid midnight effect)
  • Always put slowest device as a master

Longitudinal SMPTE timecode should be played back at a middle-level when recorded on an audio track, as both low and high levels will introduce distrortion.

Longitudinal timecode data format

The basic format is an 80-bit code that gives the time of day to the second, and the frame number within the second.

The bits of the longitudinal SMPTE code:

The basic format is an 80-bit code that gives the time of day to the second, and the frame number within the second.

The bits are encoded as biphase: a zero bit has a single transition at the start of the bit period. A one bit has two transitions, at the beginning and middle of the period. This encoding is self-clocking.

There are thirty-two bits of user data, usually used for a reel number and date.

See also

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