Time Code is a method of accurately identifying each frame in a video sequence. It is an international standard jointly developed by the Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union (EBU). VTR transports and associated equipment use SMPTE/EBU timecode values for Frame Accurate cueing.
SMPTE/EBU Time Code values are represented in four (8-bit) bytes, where each byte contains two BCD encoded digits, the tens digit and the unit digit. The bytes represent values for Hours, Minutes, Seconds and Frames accordingly.
NTSC colour signals have an actual frequency of close to 29.97 frames-per-second. As cue points must be located at frame boundries, this figure is normally rounded up to 30 frames-per-second, causing an excess of 108 frames per hour.
To eliminate these 108 frames each hour, two specific time code
frame values are dropped each minute (2x60=120), except every 10th
minute (2x60-2x6=108). The time code values dropped are specified
as the first two frames of a minute. Thus, the time code number
following 01:04:59:29 would be 01:05:00:02,
time code frames 01:05:00:00 and 01:05:00:01
don't exist.
SMPTE/EBU did not define a string representation for timecode values. Today a number of representations are in common usage:
HH:MM:SS:FF and dropframe values as
HH:MM:SS.FF, the dropped dot is a clever mnemonic.
But the format can now could be confused with the decimal fractions i
of seconds in an ISO-8601 time of day profile.HH:MM:SS:FF and dropframe values as
HH:MM:SS;FFHH:MM:SS:FF
string is accompanied by a character N(NDF), D(DF), P(PAL), or F(film)Timecode is used in two different domains. In production, (editing) timecode is a means of identifying video frames, but in (master control) presentation, timecode is used to synchronize activities along a timeline. Timecode is a useful, (good enough) but not perfect bridge between the production and presentation aspects of television. Where this is most noticeable is in timecode math:
01:02:03:05 - 01:02:03:04 = 2 frames
01:02:03:05 - 01:02:03:04 = 1 frame
Until recently, as production and presentation were essentially separate, so this difference was not significant. But as new technologies start to move timecode data across these domain, it is more important to acknowledge and accommodate the differences.
SMPTE Drop-Frame Timecode values were only intended as a handle to
identify specific video frames. As such the only valid values are
in the range 00:00:00:00 to 23:59:59:29.
However, Drop-Frame Timecode notation is also a convieniant way to
express duration. For example, a 900 frame commercial spot is commonly
described as having a duration of 00:00:30:00. This pattern
of use infers values beyond those described by SMPTE. For example, a
four-day telethon could be described as having a duration of
96:00:00:00.
For these long events, sometimes an extra column for days is used,
so the string becomes DD:HH:MM:SS:FF.
Drop-Frame Timecode notation is also commonly used when compare durations. In this usage pattern, the notation is extended to support a concept of sign. For Example a negative value can represent that a timeslot is insufficiently filled (time-under) by programming, while a positive value represents the opposite (time-over) condition.
In these ways, Drop-Frame Timecode notation is used in ways not encompassed by the original SMPTE definitions.