In the bad old days of control track editing, nothing was repeatable. An edit done with control track/tape timer was not very accurate due to the timer count not being tied or synchronised tightly to the frame at the edit point. Rewinding and fast forwarding the tape caused slippage of the counter in relation to the video & audio material. And batch digitisation and generating EDL lists were in the future.
What was needed was a method of providing a digital code that was unique to each and every frame laid down on the tape. The code invented for this purpose was Bi-Phase Mark LTC (Longitudinal Time Code). LTC could be laid down on an audio track or on its own dedicated track.
This code is both robust and reliable. It counts in hours, minutes, seconds and frames.
The count range is: H M S F to H M S F
00:00:00:00 23:59:59:24 (after this the count resets to all zeros again)
Note: the frame count is from 00 to 24 and then back to 00.
- The code can be read from 1/5th normal playing speed to over 50x playing speed when in tape shuttle mode in either direction.
- The LTC cannot be read with speeds less than 1/5th normal playing speed and certainly not in pause mode.
- VITC (Vertical Interval Time Code) was invented for this purpose. VITC is recorded at the top of the picture frame, before the start of active picture information. The lines normally used are 19 & 21. This code can be read in pause mode and slow speed.
- Switching between LTC & VITC is normally done automatically by the VCR TC reader electronics, depending on tape transport operation. This can be over-ridden in a lot of broadcast VCRs by forcing the reader to read LTC or VITC instead of being in auto mode.
- In record mode the TC is laid down by a TC generator. This generator (which generates both LTC & VITC) can be synchronised/locked up in a number of different ways.
Generator Regenerative (Regen) mode
This mode is used when it is required that the TC be continuous on the tape. In this mode, when the VCR runs up to record, the generator locks to the off tape TC before the record point to enable continuous regenerated TC from the generator at the record/edit point and onwards.
Generator Preset Mode
This mode is usually used in one of two situations: a) When it is required to pre-stripe a blank tape b) When Time of Day (TOD) TC is required
Time of Day TC is done by setting the generator to increment freely approximately with the time of the day. In this situation it is undesirable for the generator to lock to off-tape TC.
Generator External Mode.
This is used when it is desired that the generator be locked to something, say another VCR’s TC, or even studio master TOD TC.
For more information, get in touch with us on 03332 400 222 or email broadcast@Jigsaw24.com.
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