Linear (or Longitudinal) Timecode (LTC) is an
encoding
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
of
SMPTE timecode
SMPTE timecode ( or ) is a set of cooperating standards to label individual frames of video or film with a timecode. The system is defined by the Society of Motion Picture and Television Engineers in the SMPTE 12M specification. SMPTE revised ...
data in an
audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
, as defined in SMPTE 12M specification. The
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
* Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
* Digital audio, representation of sou ...
signal is commonly recorded on a
VTR
A video tape recorder (VTR) is a tape recorder designed to record and playback video and audio material from magnetic tape. The early VTRs were open-reel devices that record on individual reels of 2-inch-wide (5.08 cm) tape. They were us ...
track or other storage media. The bits are encoded using the
biphase mark code
Differential Manchester encoding (DM) is a line code in digital frequency modulation in which data and clock signals are combined to form a single two-level self- synchronizing data stream. In various specific applications, this method is also ca ...
(also known as ''FM''): a 0 bit has a single transition at the start of the bit period. A 1 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
In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...
' which has a special predefined
sync relationship with any
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
or
film
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
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 into 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 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.
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 distortion.
Longitudinal timecode data format
![Timecode-audacity](https://upload.wikimedia.org/wikipedia/commons/6/68/Timecode-audacity.png)
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.
* 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
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
standard.
* Bit 11, the
color framing
In 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 ...
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
SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog color television system that was used in France, some parts of Europe and Africa, and Russia. It was one of th ...
) or modulo 4 (for
PAL
Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
) should be preserved across cuts in order to avoid phase jumps in the
chrominance subcarrier The chrominance subcarrier is a separate subcarrier signal that carries the color information during transmission of a composite video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of th ...
.
* Bits 27, 43, and 59 differ between 25 frame/s time code, and other frame rates (30, 29.97, or 24). The bits are:
** "Polarity correction bit" (bit 59 at 25 frame/s, bit 27 at other rates): this bit is chosen to provide an even number of 0 bits in the whole frame, including the sync code. (Since the frame is an even number of bits long, this implies an even number of 1 bits, and is thus an
even parity
A parity bit, or check bit, is a bit added to a string of binary code. Parity bits are a simple form of error detecting code. Parity bits are generally applied to the smallest units of a communication protocol, typically 8-bit octets (bytes), ...
bit. Since the sync code includes an odd number of 1 bits, it is an odd parity bit over the data.) This keeps the phase of each frame consistent, so it always starts with a rising edge at the beginning of bit 0. This allows seamless splicing of different time codes, and lets it be more easily read with an
oscilloscope
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
.
** "Binary group flag" bits BGF0 and BGF2 (bits 27 and 43 at 25 frame/s, bits 43 and 59 at other rates): these indicate the format of the user bits. Both 0 indicates no (or unspecified) format. Only BGF0 set indicates four 8-bit characters (transmitted
little-endian
In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most si ...
). The combinations with BGF2 set are reserved.
* Bit 58, unused in earlier versions of the specification, is now defined as "binary group flag 1" and indicates that the time code is synchronized to an external clock. if zero, the time origin is arbitrary.
* 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.
See also
*
Vertical interval timecode
Vertical Interval Timecode (VITC, pronounced "vitsee") is a form of SMPTE timecode encoded on one scan line in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal.
With one exception, VITC ...
*
Burnt-in timecode
Burnt-in timecode (often abbreviated to BITC by analogy to VITC) is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image. BITC is sometimes used in conjunction with "real" machine-re ...
*
MIDI timecode
MIDI time code (MTC) 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 ...
*
CTL timecode
Control track longitudinal timecode, or CTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or ''striping'', reference SMPTE timecode onto a videotape.
Similar to the way VITC timecode is embedded in the verti ...
*
AES-EBU embedded timecode
AES3 is a standard for the exchange of digital audio signals between professional audio devices. An AES3 signal can carry two channels of pulse-code-modulated digital audio over several transmission media including balanced lines, unbalanced ...
*
Rewritable consumer timecode The Rewriteable Consumer Timecode (RCTC, RC Timecode, or RC Time Code) is a nearly frame accurate timecode method developed by Sony for 8mm and Hi8 analog tape formats.
*
VTR
A video tape recorder (VTR) is a tape recorder designed to record and playback video and audio material from magnetic tape. The early VTRs were open-reel devices that record on individual reels of 2-inch-wide (5.08 cm) tape. They were us ...
*
Manchester code
*
Biphase mark code
Differential Manchester encoding (DM) is a line code in digital frequency modulation in which data and clock signals are combined to form a single two-level self- synchronizing data stream. In various specific applications, this method is also ca ...
References
{{Reflist
External links
LGPL library to en/decode LTC in software
Broadcast engineering
Electrical engineering
Timecodes
Recording devices
Film and video technology