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SMPTE TIME CODE
SMPTE (pronounced "simtee") is an acronym for the "Society of Motion Picture and Television Engineers". SMPTE adopts and
sets standards for the motion picture and television industry. SMPTE has adopted standards for both Longitudinal Time Code
(LTC) and Vertical Interval Time Code (VITC). For historical reasons, LTC is more commonly referred to as SMPTE time code
and VITC simply as VITC.
3.1 LTC
Longitudinal time code is an audible timing signal, sounding much like a FAX machine signal, that identifies each frame of a
television picture with a number expressed in an hours, minutes, seconds and frames format.
Being an audio signal, LTC is not recorded in the picture but instead on either an audio channel or on a special time code channel
of a video recorder or other audio recorder. The LTC signal can be recorded simultaneous with recording of the video signal or
can be recorded later during post-production (post-recorded).
The LTC signal itself is an 80-bit serial binary code that repeats once each video frame. Of these 80 bits, 32 are reserved for the
time information. An additional 32-bits, known as user bits, are available for including miscellaneous information along with the
time information. These user bits are organized as eight, 4-bit hexadecimal digits. (A hexadecimal digit can contain the values
from 0 to 9 and from A to F.) The remaining 16 bits of time code form a special "sync" pattern which is used to locate and
decode the time code and user bit information.
When a tape with time code is played back, the time code signal can be read by appropriate equipment to provide a precise frame
number identification for video editing and other purposes.
Depending upon the playback capabilities of the video recorder, the VLR-100 can read and decode LTC signals at speeds as slow
as 1/30th times play speed up to speeds as high as 100 times play speed.
3.2 VITC
Vertical interval time code is a visual timing signal that is recorded as a series of varying width white dots located in the vertical
interval of a standard television picture. VITC is typically recorded on two non-adjacent lines of the vertical interval of each
composite video field. In addition to containing the same time code and user bits information as does LTC, VITC also indicates
which video field is currently displayed.
The major advantages of VITC over LTC are:
1. VITC does not use an audio channel or a special time code channel on the video recorder.
2. VITC can be read from search speeds down to still frame and pause. Thus, it provides a very accurate and precise means
of identifying each video frame (and possibly field, depending upon the video recorder).
However, there are a few disadvantages to VITC. Because VITC is actually part of the video signal, it must either be recorded
when the original video signal is recorded, or when a copy of the original tape is made. Thus, it cannot be "post recorded"
without going down one generation. Recording in the field can be difficult using composite video camcorders unless a VITC
generator is an integral part of the camcorder. Sometimes the VITC signal gets stripped from the vertical interval by various
types of video equipment, such as time-base correctors, field and frame stores, etc.
3.3 Drop Frame Time Code
Drop frame time code is a form of SMPTE time code which is used when it is important that the timing information stored in the
time code be an accurate representation of real time.
US color television standards for composite video were developed to be compatible with earlier black and white television. This
required a slight slowing of the video frame rate from 30 frames-per-second to 29.97 frames-per-second. Because it is this frame
rate that increments the SMPTE time code numbers, these time code numbers fall behind real clock time by about 108 frames per
hour, or 3.6 seconds. Drop frame time code was developed to adjust for this error and make SMPTE time code more closely
match real time.
The technique for producing drop frame time code involves advancing the frame number ahead by two frames at the start of each
minute, except on minutes 00, 10, 20 , 30, 40, and 50. Thus, excluding these tens-of-minutes exceptions, instead of the frame
