SMPTE time code
Encyclopedia
SMPTE timecode is a set of cooperating standards to label individual frames of video or film with a time code
Time code
A timecode is a sequence of numeric codes generated at regular intervals by a timing system.- Video and film timecode :...

 defined by the Society of Motion Picture and Television Engineers
Society of Motion Picture and Television Engineers
The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers (SMPTE , founded in 1916 as the Society of Motion Picture Engineers or SMPE, is an international professional association, based in...

 in the SMPTE 12M specification. SMPTE revised the standard in 2008, turning it into a two-part document: SMPTE 12M-1 and SMPTE 12M-2, including important new explanations and clarifications.

Timecodes are added to film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

, 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 audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 material, and have also been adapted to synchronize music
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...

. They provide a time reference for editing, synchronization
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....

 and identification. Timecode is a form of media metadata. The invention of timecode made modern videotape editing possible, and led eventually to the creation of non-linear editing system
Non-linear editing system
In video, a non-linear editing system is a video editing or audio editing digital audio workstation system which can perform random access non-destructive editing on the source material...

s.

Basic concepts

SMPTE timecodes (icon) contain binary coded decimal hour:minute:second:frame identification and 32 bits for use by users. There are also drop-frame and color framing flags and three extra 'binary group flag' bits used for defining the use of the user bits. The formats of other varieties of SMPTE time codes are derived from that of the longitudinal timecode.

Time codes may use a number of frame rate
Frame rate
Frame rate is the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...

s. Common ones are:
  • 24 frame/sec (film
    Film
    A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

    , ATSC, 2k, 4k, 6k)
  • 25 frame/sec (PAL
    PAL
    PAL, 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...

     (Europe, Uruguay, Argentina), SECAM, DVB, ATSC)
  • 29.97 (30 ÷ 1.001) frame/sec (NTSC
    NTSC
    NTSC, 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...

     American System (US, Canada, Mexico, Colombia, etc.), ATSC, PAL-M (Brazil))
  • 30 frame/sec (ATSC
    ATSC
    ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....

    )


In general, SMPTE timecode frame rate information is implicit, known from the rate of arrival of the timecode from the medium, or other metadata encoded in the medium. The interpretation of several bits, including the "colour framing" and "drop frame" bits, depends on the underlying data rate. In particular, the drop frame bit is only valid for a nominal frame rate of 30 frame/s: see below for details.

More complex timecodes such as vertical interval timecode
Vertical interval timecode
Vertical 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...

 can also include extra information in a variety of encodings.

Discontinuous timecode, and flywheel processing

Timecodes are generated as a continuous stream of sequential data values. In some applications 'wall clock' time is used, in others the time encoded is a notional time. After making a series of recordings, or after crude editing, recorded timecodes may consist of discontinuous segments.

In general it is not possible to know the linear timecode (LTC
Linear timecode
Linear Timecode 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...

) of the current frame until the frame has already gone by, by which time it is too late to make an edit. Practical systems watch the ascending sequence of the timecode, and infer the time of the current frame from that.

As timecodes in analog systems are prone to bit-errors and drop-outs, most timecode processing devices check for internal consistency in the sequence of timecode values, and use simple error correction schemes to correct for short error bursts. Thus, a boundary between discontinuous timecode ranges cannot be determined exactly until several subsequent frames or discontinuous sequences of them have passed.

For this reason, most videotape editing attempts to keep the timecode of the recorded material continuous, so that multiple edits may be repeatedly over-recorded onto the same piece of videotape.

Although it would be possible in all-digital systems to eliminate the need for the flywheel algorithm by adding a frame delay to allow the timecode to be decoded prior to the processing of the frame, this is not done in most practical systems because
  • it would introduce an unnecessary frame delay in the signal processing path, and
  • there would still be a need to compensate for timecode errors in signals derived from analog video or audio systems.

Drop frame timecode

Drop frame timecode dates to a compromise invented when color NTSC
NTSC
NTSC, 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...

 video was invented. The NTSC re-designers wanted to retain compatibility with existing monochrome TVs. Technically, the 3.58 MHz (actually 315/88 MHz = 3.57954545 MHz) color subcarrier would absorb common-phase noise from the harmonics of the line scan frequency. Rather than adjusting the audio or chroma subcarriers, they adjusted everything else, including the frame rate, which was set to 30/1.001 Hz.

This meant that an "hour of timecode" at a nominal frame rate of 29.97 frame/s was longer than an hour of wall-clock time by 3.59 seconds, leading to an error of almost a minute and a half over a day, as the timecode was calculated in a manner that assumed the frame rate was exactly 30 frame/s.

To correct this, drop frame SMPTE timecode was invented. In spite of what the name implies, no video frames are dropped (skipped) using drop-frame timecode. What's actually being dropped are some of the timecode "labels". In order to make an hour of timecode match an hour on the clock, drop-frame timecode drops frame numbers 0 and 1 of the first second of every minute, except when the number of minutes is divisible by ten (i.e. when minutes mod 10 equals zero). This achieves an "easy-to-track" drop frame rate of 18 frames each ten minutes (18,000 frames @ 30frame/s) and almost perfectly compensates for the difference in rate, leaving a residual timing error of roughly 86.4 milliseconds per day, an error of only 1.0 ppm.

That is, drop frame TC drops 2 frames every minute, except every tenth minute, achieving 30×0.999 = 29.97 frame/s. The error is the difference between 0.999 and 1/1.001 = 0.999000999000999….

For example, the sequence when frames are dropped:

01:08:59:28

01:08:59:29

01:09:00:02

01:09:00:03

For each tenth minute

01:09:59:28

01:09:59:29

01:10:00:00

01:10:00:01

While non-drop time code is displayed with colons separating the digit pairs—"HH:MM:SS:FF"—drop frame is usually represented with a semi-colon or period (.) as the divider between all the digit pairs—"HH;MM;SS;FF", "HH.MM.SS.FF"—or just between the seconds and frames—"HH:MM:SS;FF" or "HH:MM:SS.FF". The period is usually used on VTRs and other devices that don't have the ability to display a semi-colon.

Drop frame timecode is typically abbreviated as DF and non-drop as NDF.

Color framing and timecode

A color framing
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 all possible phase relationships.The exact nature of the...

 bit is often used to indicate field 1 of the colour frame, so that editing equipment can make sure to edit only on appropriate field boundaries in order to prevent picture corruption.

Studio operations and master clocks

In television studio
Television studio
A television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...

 operations, longitudinal timecode is generated by the studio master sync generator, and distributed from a central point. Central sync generators usually derive their timing from an atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

, either using network time, or GPS. Studios usually maintain two or three clocks, and automatically switch over if one fails.

A recent development is to mount small GPS-synchronized SMPTE timecode generators on each camera, which eliminates the distribution network for portable set-ups and shooting on location.

Longitudinal SMPTE timecode is widely used to synchronise music. A frame rate of 30 frame/s is often used for audio in America, Japan, and other countries which which rely on a 60Hz mains frequency and use the NTSC
NTSC
NTSC, 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...

 television standard. The EBU (European Broadcasting Union
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...

) standard frame rate of 25 frame/s is used throughout Europe, Australia and wherever the mains frequency is 50 Hz, and the PAL
PAL
PAL, 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...

 or SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....

 television standards are used.

SMPTE timecode media

  1. Linear timecode
    Linear timecode
    Linear Timecode 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...

    , a.k.a. "longitudinal timecode" and "LTC" : suitable to be recorded on an audio channel, or by audio wires. This is how it is distributed within a studio to synchronize recorders and cameras. To read LTC, the recording must be moving, meaning that LTC is useless when the recording is stationary or nearly stationary. This shortcoming led to the development of VITC.
  2. Vertical interval timecode
    Vertical interval timecode
    Vertical 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...

    , a.k.a. VITC : recorded directly into the VBI (vertical blanking interval) of the video signal on each frame of video. The advantage of VITC is that, since it is a part of the playback video, it can be read when the tape is stationary.
  3. CTL timecode
    CTL timecode
    CTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or striping, reference SMPTE timecode onto a videotape....

     (control track longitudinal): SMPTE timecode embedded in a videotape's control track.
  4. Visible time code, a.k.a. burnt-in timecode
    Burnt-in timecode
    Burnt-in timecode is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image...

     and BITC - the numbers are burnt into the video image so that humans can easily read the time code. Videotapes that are duplicated with these time code numbers "burnt-in" to the video are known as window dubs.
  5. Film labels, such as Keykode
    Keykode
    KeyKode is an Eastman Kodak Company advancement on edge numbers, which are letters, numbers and symbols placed at regular intervals along the edge of 35 mm and 16 mm film to allow for frame-by-frame specific identification...

    .

History

Longitudinal and vertical-interval timecodes were developed in 1967 by EECO, an electronics company that developed video recorders, and later video production systems. EECO assigned its intellectual property to permit public use.

See also

  • Linear timecode
    Linear timecode
    Linear Timecode 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...

  • Vertical interval timecode
    Vertical interval timecode
    Vertical 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 timecode
    Burnt-in timecode
    Burnt-in timecode is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image...

  • MIDI timecode
    MIDI timecode
    MIDI 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...

  • AES-EBU embedded timecode
  • Rewritable consumer timecode
    Rewritable consumer timecode
    The 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...

  • 2-3 pulldown
  • Field dominance
    Field dominance
    In video engineering, field dominance refers to the choice of which field of an interlaced video signal is chosen as the point at which video edits occur. There are only two choices for field dominance in normal interlaced video: odd or even. Interlacing divides the frame into two fields, scanning...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK