ABSTRACT
For any tape-based edit system to work it is necessary to be able to identify each video frame uniquely. This identifier is then used to ‘mark’ the points of interest on the tape, such as the point a new recording is to start. The identifier used is timecode. This code is recorded in one of two ways: longitudinally down the tape (LTC) on a separate track, or in the vertical interval of the video signal (VITC), typically on lines 19 and 21. Timecode is recorded as 4 bytes of binary coded decimal
digits, representing hours, minutes, seconds and frames. Also
recorded in the timecode data-space are various flags and userbit data. The timecode runs from 00:00:00:00 to 23:59:59:24. Timecode is normally recorded concurrently with the video
recording. The edit control system has to assume that the timecode is consistent and consecutive. That is, the same timecode must not occur at more than one place on the tape and the time value must get greater as the tape is played. If, for example, the timecode jumped from 10:05:23:00 to 08:51:16:12, as shown in Figure 4.7.1, and the tape was in still at point P, the control system would be unable to locate to Clip A as it would assume it was after Clip B.
