Linear timecode
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Linear (or Longitudinal) Timecode (LTC) encodes SMPTE timecode data as a Manchester-Biphase encoded audio signal. 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'. Many professional audio engineers see this use of LTC in sound-effects as an exceptionally lazy, unrealistic sound design technique. In the industry "LTC" is pronounced "Litsy" (Rhymes with itsy-bitsy).
[edit] 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.
LTC timecode is essentially an audio signal around 2 kHz 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.
[edit] 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:
- 0..3: Frame units
- 4..7: user-bits field 1
- 8..9: Frame tens
- 10: 1=drop frame format (frame 0 and 1 omitted from first second of each minute, but included when minutes divides by ten; approximates 29.97 frame/s)
- 11: 1="color frame", i.e. the time code is intentionally synchronised with a color TV field sequence.
- 12..15: user-bits field 2
- 16..19: seconds units
- 20..23: user bits field 3
- 24..26: second tens
- 27: Bi-phase mark-correction bit; set or cleared so every 80-bit word has an even number of zeros. This can be used to assure that there is no net current flow in the time code system. It also allows a technician to read the bi-phase coding timing with an oscilloscope, without the signal inverting itself continually. Modern equipment regenerates the time code to a fixed timing in the video vertical interval, so this is far less needed than before.
- 28..31: user bits field 4
- 32..35: minutes unit digit
- 36..39: user bits field 5
- 40..42: minutes tens digit
- 43: binary group flag bit (with bit 59, 43,59 = 00 = no format for user bits, 10 = eight bit format, 01, 11 are unassigned and reserved).
- 44..47: user bits field 6
- 48..51: hours units
- 52..55: user bits field 7
- 56..57: hours tens
- 58: unused, reserved, should transmit zero and ignore on receive for compatibility
- 59: binary group flag bit (see bit 43 for encoding)
- 60..63: user bits field 8
- 64..79: sync word, should be 0011 1111 1111 1101
There are thirty-two bits of user data. These are usually used for a reel number and date. They can be anything at all, as long as bits 43 and 59 are set to zero.
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.
Longitudinal SMPTE timecode should be played back at a middle-level when recorded on an audio track. Too high or too low will both cause distortion.
