||The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (September 2014)|
Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode data in an audio signal, as defined in SMPTE 12M specification. The audio signal is commonly recorded on a VTR track or other storage media. The bits are encoded using the biphase mark code, also known as "FM": 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. 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'.
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).
Made up of 80 bits per frame, where there may be 24, 25 or 30 frames per second, LTC timecode varies from 1920 Hz (binary zeros at 24 frames/s) to 4800 Hz (binary ones at 30 frames/s), and thus is comfortably in the middle of the audio frequency range. LTC can exist as either a balanced or unbalanced signal, and can be treated as an audio signal in regards to distribution. Like audio, LTC can be distributed by standard audio wiring, connectors, distribution amplifiers, and patchbays, and can be ground-isolated with audio transformers. It 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' (aka "crosstalk") from the LTC track to the audio tracks.
- 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 distortion.
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. Values are stored in binary-coded decimal, least significant bit first. There are thirty-two bits of user data, usually used for a reel number and date.
|10||D||Drop frame flag.||26||40||42||40||58||0||Reserved, zero||74||1|
|11||C||"Color frame" flag||27||P||Even parity bit||43||1||Binary group flag||59||2||Binary group flag||75||1|
- Bit 10 is set to 1 if drop frame numbering is in use; frame numbers 0 and 1 are skipped during the first second of every minute, except multiples of 10 minutes. This converts 30 frame/second time code to the 29.97 frame/second NTSC standard.
- Bit 11, the color framing bit, is set to 1 if the time code is synchronized to a (color) video signal. The frame number modulo 2 (for NTSC and SECAM) or modulo 4 (for PAL) should be preserved across cuts in order to avoid phase jumps in the chrominance subcarrier.
- Bit 27, the "bi-phase mark-correction bit" is set to provide an even number of 0 bits in the whole frame, including the sync code. (As there are an even number of bits, this corresponds to the even parity rule of an even number of 1 bits.) This ensures zero net DC bias, and keeps the phase of each frame consistent so that it may be more easily read with an oscilloscope.
- Bits 43 and 59, the "binary group flag" bits, indicate the format of the user bits. 0 indicates no (or unspecified) format, while 1 indicates 4 8-bit characters. Values of 2 and 3 (i.e. bit 59 set) are reserved.
- Bit 58 is unassigned, should always be transmitted as zero, and ignored on reception.
- The sync pattern in bits 64 through 79 includes 12 consecutive 1 bits, which cannot appear anywhere else in the time code. Assuming all user bits are set to 1, the longest run of 1 bits that can appear elsewhere in the time code is 10, bits 9 to 18 inclusive.
- The sync pattern is preceded by 00 and followed by 01. This is used to determine whether an audio tape is running forward or backward.
- Vertical interval timecode
- Burnt-in timecode
- MIDI timecode
- CTL timecode
- AES-EBU embedded timecode
- Rewritable consumer timecode
- Manchester Code
- Biphase Mark Code