DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

SMPTE 258M, Television – Transfer
of Edit Decision Lists.
00:00:00;00 and 00:00:00;01. Editing equipment treats
the duplicate codes as a timecode interruption.
Editing
Timecode is basic to video editing. An edit is denoted
by its in point (the timecode of the first field or frame to
be recorded) and its out point (the timecode of the first
field or frame beyond the recording). An edited
sequence can be described by the list of edits used to
produce it: Each entry in an edit decision list (EDL)
contains the in and out points of the edited material,
the in and out points of the source, and tape reel
number or other source and transition identification.
An edited tape is invariably recorded with continuous
“nonbroken” timecode. Nearly all editing equipment
treats the boundary between 23:59:59:29 and
00:00:00:00 as a timecode discontinuity; consequently,
it is conventional to start the main program segment on
tape with the code 01:00:00:00. If the tape includes
the usual 1.5 minutes of bars and tone leader, then the
tape will start near timecode 00:58:30:00.
Linear timecode (LTC)
Timecode was historically recorded on studio videotape
and audiotape recorders on longitudinal tracks
having characteristics similar or identical to those of
audio tracks. This became known as longitudinal timecode
(LTC). The word longitudinal became unfashionable,
and LTC was renamed linear timecode – thankfully,
it retains its acronym. LTC is interfaced in the studio as
an audio signal pair using a three-pin XLR connector.
Each frame time is divided into 80 bit-cells; at 30 Hz
frame rate, the bit rate of timecode data is nominally
2.4 kb/s. LTC is recorded using the binary FM technique,
also known as Manchester code: Each bit cell has
a transition at its start; a 1-bit has a transition in the
middle of the cell and a 0-bit does not. This coding is
immune to the polarity reversals that sometimes occur
in audio distribution equipment.
384 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES