Timecode reading/generating on AG7760/AG7750 VTRs

Timecode is stamping each frame of video with a fixed code, so that frame can always be found again, precisely.  As opposed to just counting how many frames have passed since you zeroed the counter, which can slip out of place any time the counter has trouble reading the control track.  It's used for peforming precise, repeatable, edits; and for things like automation of tape playback in television studios (playing back tapes from the right part of the tape, at the right moment, and handling advert interruptions, automatically).

An optional AG-F700 time code reader/generator can be installed into the AG7650 and AG7750 editing VTRs (it's fitted directly above the cooling fan).  It allows the reading and generation of time code on the second linear audio tracks (LTC), and within the vertical blanking period (VITC).  You can choose whether it records LTC and/or VITC, or none, they're all independently controlled.

LTC can be read during some slow and fast playback speeds, and normal playback speeds, but not once the tape speed is slowed beyond a certain point, or sped up beyond another point.  The machine will fake timecode at that time, outputing what it thinks the timecode should be, by counting the frame pulses on the control track, making corrections, later on, if it has to, once the tape speed is within the range that it can use (you can tell when the reader is faking timecode, as the colon between the seconds and frames will disappear).  This makes the precise entering of edit points, while slowly jogging a tape frame by frame, a bit error prone.  You also lose a valuable audio track, to use it for timecode, instead of audio.  Which is rather a waste, considering that you can use VITC, instead of LTC.  About the only advantage to using LTC, on these machines, is being able to laydown a timecode track onto a tape that didn't already have any timecode recorded on it (providing that doesn't involve wiping out an audio track that you need to keep).

VITC can be read during still playback, right up to some fast search speeds, so long as there isn't a noisebar through the vertical interval period.  Whenever the machine can't read the VITC, it'll fake it by counting the frame pulses on the control track, to update the timecode with a guessed value, until the VITC becomes readable, again.  So barring bad tape playback, VITC is an accurate way to set your edit points.  But, because it's a part of the video signal, it has to be recorded with the video signal, originally (it can't be inserted, later on).

The generator can be configured to insert VITC on any two (different) lines of video, from lines 10–22.  The manual warns to avoid using lines 10 or 11 on NTSC equipment, as bad tape skew can make reading the timecode impossible.

VITC is inserted into two lines of video per field (the line numbers that you select, in the first field, and their counterparts in the second field).  This allows timecode to verified for errors, and for timecode to still be readable even if there is a small tape dropout.  It's preferred to have a gap between the lines used, rather than use consecutive lines, so that a minor tape dropout isn't too likely to wipeout both lines of timecode.

According to several different (non-authoritative) sources that I've read, it's usual for NTSC video to have VITC on lines 16 & 18, and for PAL video to use lines 19 & 21.  An Australian Broadcasting Corporation technical document for program delivery asks for VITC on lines 18 & 20 (lines 331 & 333 on the next field), with lines 21 & 334 used for closed caption data.  And line 23 is commonly used for Wide Screen Signalling.

Certain lines are traditionally preferred so that timecode fits into unused lines in the vertical interval (other lines may already be in used for VITS), and so that any manually configured timecode readers won't need re-configuring to playback different tapes.

During tape playback, you can find out which lines have timecode on them, by putting the local/remote switch on “remote,” and holding down the counter “reset” button.  The front-panel, and on-screen-display, will list the two video line numbers.

You will only see the generator inserting timecode into the vertical interval during an edit, or while recording, or while holding the record button down in stop mode, or while holding down one of the insert edit buttons while in stop mode.  Otherwise, the vertical interval period will be the same as the video going into the machine.


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