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Synchronization
Timecode (positional references)
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Timecode (positional references)
The position of any device is most often described using timecode. Timecode represents time
using hours, minutes, seconds, and frames to provide a location for each device. Each frame
represents a visual film or video frame.
Timecode can be communicated in several ways:
LTC (Longitudinal Timecode) is an analog signal that can be recorded on tape. It should
be used for positional information primarily. It can also be used for speed and phase
information as a last resort if no other clock source is available.
VITC (Vertical Interval Timecode) is contained within a composite video signal. It is
recorded onto video tape and is physically tied to each video frame.
MTC (MIDI Timecode) is identical to LTC except that it is a digital signal transmitted via
MIDI.
Timecode standards
Timecode has several standards. The subject of the various timecode formats can be very
confusing due to the use and misuse of the shorthand names for specific timecode standards
and frame rates. The reasons for this confusion are described in detail below. The timecode
format can be divided into 2 variables: frame count and frame rate.
Frame count (frames per second)
The frame count of timecode defines the standard with which it is labeled. There are 4
timecode standards:
24
fps Film (F)
This frame count is the traditional count for film. It is also used for HD video
formats and commonly referred to as “24 p”. However, with HD video, the actual
frame rate or speed of the video sync reference is slower, 23.976 frames per
second, so timecode does not reflect the actual realtime on the clock for 24p HD
video.
25
fps PAL (P)
This is the broadcast video standard frame count for European (and other PAL
countries) television broadcast.
30
fps non-drop SMPTE (N)
This is the frame count of NTSC broadcast video. However, the actual frame rate
or speed of the video format runs at 29.97 fps. This timecode clock does not run
in realtime. It is slightly slower by 0.1 %.
30
fps drop-frame SMPTE (D)
The 30 fps drop-frame count is an adaptation that allows a timecode display
running at 29.97 fps to actually show the clock-on-the-wall-time of the timeline
by “dropping” or skipping specific frame numbers in order to “catch the clock up”
to realtime.
Confused? Just remember to keep the timecode standard (or frame count) and frame rate
(or speed) separate.