Introduction - Computer Animation: Algorithms and Techniques - page 19

Source tape

Monitor

Special effects generator

Editing station

Character generator

Output tape

Source tape

FIGURE 1.8
Linear editing system.

versioning ), the source material will be subjected to even more wear and tear because the source mate-
rial has to undergo more handling as it is processed for multiple purposes.
Often, to facilitate the final assemblage of the output sequence and avoid excessive wear of the
original source material, copies of the source material are used in a preprocessing stage in which
the final edits are determined. This is called off-line editing . The result of this stage is an edit decision
list (EDL), which is a final list of the edits that need to be made to assemble the final piece. The EDL is
then passed to the on-line editing stage, which uses the original source material to make the edits and
create the finished piece. This process is referred to as conforming .
To keep track of edit locations, control track pulses can be incorporated onto the tape used to assem-
ble the 30 fps NTSC video signal. Simple editing systems count the pulses, and this is called control
track editing . However, the continual shuffling of the tape back and forth during the play and rewind of
the editing process can result in the editing unit losing count of the pulses. This is something the oper-
ator must be aware of and take into account. In addition, because the edit counts are relative to the
current tape location, the edit locations are lost when the editing station is turned off.
The Society of Motion Picture and Television Engineers time code is an absolute eight-digit tag on
each frame in the form of HHMMSSFF, where HH is the hour, MM is the minute, SS is the second, and
FF is the frame number. This tag is calculated from the beginning of the sequence. This allows an edit-
ing station to record the absolute frame number for an edit and then store the edit location in a file that
can be retrieved for later use.
The process described so far is called assemble editing . Insert editing is possible if a control signal is
first laid down on the output tape. Then sequences can be inserted anywhere on the tape in forming the
final sequence. This provides some nonlinear editing capability, but it is still not possible to easily lengthen
or shorten a sequence without repositioning other sequences on the tape to compensate for the change.

Digital on-line nonlinear editing
To incorporate a more flexible nonlinear approach, fully digital editing systems have become more
accessible [ 17 ][ 33 ] [ 48 ]. These can be systems dedicated to editing, or they can be software systems
that run on standard computers. Analog tape may still be used as the source material and for the final
product, but everything between is digitally represented and controlled 9 (see Figure 1.9 ) .

9 It is interesting to note that the whole process from recording to projection can now be done digitally.