Linear timecode
Encyclopedia
Linear 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'. In the industry "LTC" is pronounced "Litsy" except in the UK where it is pronounced "ell-tee-cee".
Made up of 80 bits per frame, where there may be 24, 25 or 30 frames per second, LTC timecode varies from 960 Hz (binary zeros at 24 frames/s) to 2400 Hz (binary ones at 30 frames/s). and thus is comfortably in the middle of the audio frequency range. 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:
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.
, least significant bit first.
There are thirty-two bits of user data, usually used for a reel number and date.
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
, 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
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
relationship with any video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
or film
Telecine
Telecine is transferring motion picture film into video and is performed in a color suite. The term is also used to refer to the equipment used in the post-production process....
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).Made up of 80 bits per frame, where there may be 24, 25 or 30 frames per second, LTC timecode varies from 960 Hz (binary zeros at 24 frames/s) to 2400 Hz (binary ones at 30 frames/s). and thus is comfortably in the middle of the audio frequency range. 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. Values are stored in binary-coded decimalBinary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...
, least significant bit first.
There are thirty-two bits of user data, usually used for a reel number and date.
| Bit | Weight | Meaning | Bit | Weight | Meaning | Bit | Weight | Meaning | Bit | Weight | Meaning | Bit | Value | Meaning |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 00 | 1 | Frame number units (0–23, 24, or 29) |
16 | 1 | Seconds units (0–59) |
32 | 1 | Minutes units (0–59) |
48 | 1 | Hours units (0–23) |
64 | 0 | Sync word, fixed bit pattern 0011 1111 1111 1101 |
| 01 | 2 | 17 | 2 | 33 | 2 | 49 | 2 | 65 | 0 | |||||
| 02 | 4 | 18 | 4 | 34 | 4 | 50 | 4 | 66 | 1 | |||||
| 03 | 8 | 19 | 8 | 35 | 8 | 51 | 8 | 67 | 1 | |||||
| 04 | User bits field 1 |
20 | User bits field 3 |
36 | User bits field 5 |
52 | User bits field 7 |
68 | 1 | |||||
| 05 | 21 | 37 | 53 | 69 | 1 | |||||||||
| 06 | 22 | 38 | 54 | 70 | 1 | |||||||||
| 07 | 23 | 39 | 55 | 71 | 1 | |||||||||
| 08 | 10 | Frame number tens |
24 | 10 | Seconds tens (0–59) |
40 | 10 | Minutes tens (0–59) |
56 | 10 | Hours tens |
72 | 1 | |
| 09 | 20 | 25 | 20 | 41 | 20 | 57 | 20 | 73 | 1 | |||||
| 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 | |
| 12 | User bits field 2 |
28 | User bits field 4 |
44 | User bits field 6 |
60 | User bits field 8 |
76 | 1 | |||||
| 13 | 29 | 45 | 61 | 77 | 1 | |||||||||
| 14 | 30 | 46 | 62 | 78 | 0 | |||||||||
| 15 | 31 | 47 | 63 | 79 | 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 NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
standard. - Bit 11, the color framingColor framingIn video engineering, color framing refers to the color frame sequence of fields in a composite video signal through which the video frame timing and chrominance subcarrier signal timing—in particular, that of the color burst -- cycle through all possible phase relationships.The exact nature of the...
bit, is set to 1 if the time code is synchronized to a (color) video signal. The frame number modulo 2 (for NTSC and SECAMSECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
) or modulo 4 (for PALPALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
) should be preserved across cuts in order to avoid phase jumps in the chrominance subcarrierChrominance subcarrierChrominace subcarrier refers to a separate subcarrier signal that carries the color information during transmission of a video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of the color composite video signal...
. - 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 oscilloscopeOscilloscopeAn oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
. - 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.
See also
- Vertical interval timecodeVertical interval timecodeVertical Interval TimeCode is a form of SMPTE timecode embedded as a pair of black-and-white bars in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal...
- Burnt-in timecodeBurnt-in timecodeBurnt-in timecode is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image...
- MIDI timecodeMIDI timecodeMIDI time code , or MIDI time division, embeds the same timing information as standard SMPTE timecode as a series of small 'quarter-frame' MIDI messages. There is no provision for the user bits in the standard MIDI time code messages, and SysEx messages are used to carry this information instead...
- CTL timecodeCTL timecodeCTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or striping, reference SMPTE timecode onto a videotape....
- AES-EBU embedded timecode
- Rewritable consumer timecodeRewritable consumer timecodeThe Rewriteable Consumer Timecode is a nearly frame accurate timecode method developed by Sony for 8mm and Hi8 analog tape formats. The RC timecode is written by the video camera directly to analog tape tracks and records the hour, minute, second and frame for each frame of video recorded to tape...
- VTR
- Manchester CodeManchester codeIn telecommunication and data storage, Manchester code is a line code in which the encoding of each data bit has at least one transition and occupies the same time...
- Biphase Mark Code