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Old July 1st, 2004, 01:24 PM   #1
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Timecode?

I have made numerous short videos for school and church in the past. Now as I plan to shoot my first short film, I have been reading a lot about readjusting timecodes and other various timecode topics. I am just wondering if someone can enlighten me as to what timecode is used for. I assumed it was for audio, but I'm not positive. Any help is appreciated...
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Old July 3rd, 2004, 06:26 AM   #2
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See this thread for information on what timecode is generally
used for in the world of DV.

In the "film" world it is also used to synchronise audio and
multiple camera's. This is not possible in DV since no camera
accepts timecode in. Only one camera I know of allows you to
actually set timecode.

This can be handy so that hour 1 is tape 1, hour 2 is tape 2 etc.
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Old July 3rd, 2004, 09:25 AM   #3
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Thanks for clearifying that for me.
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Old July 4th, 2004, 04:43 PM   #4
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A little clarification - and history.

Timecode positively identifies each frame of video (on a tape - disk - whatever). While audio isn't recorded in "frames" like video, it's handy to segment audio into tiny snippets of one frame duration so it can be handled in the same way.

With today's digital video and audio, it's easy to add a few data bits to each frame to positively identify it's position in HH:MM:SS:FF. Today's consumer DV camcorders begin at 00:00:00:00, and if they are building on existing timecode continue from that point. That's why timecode starts over again when you leave a gap in the middle of a tape.

Today's digital recorders all run at a very consistent and accurate speed, so if you find a synchronizing point, things probably will stay in sync for an hour or so. Camcorders, DAT, MD. CDs all maintain a very precise speed, so that if timecode is later added. sync can be maintained. (The old Nagra reel-to-reel audio recorders maintained a precise speed by recording an audio tone on a separate track, and used it to control a servo motor.) It's not necessary to have the same timecode to combine elements - but it helps. An offset must be determined by comparing video from different cameras, or comparing video to audio. That's what the old clapboard did for moviemakers - provided an obvious point for synchronization. If adding and subtracting timecode gives you a headache (try it!), you can let your editor do it for you by sliding clips along the timeline. The simpler DV capture programs do not copy timecode from the tape - they just make new. Some allow the capturing of timecode as well, however.

Drop-frame timecode and non-drop-frame timecode. Confusion abounds! Let's go waaaay back to the day's of B&W TV. Television was 30 FPS and life was easy. Then came color. The color was (still is in composite or S-video) modulated onto a 3.579545MHz subcarrier, and 8 to 11 cycles of sample subcarrer are provided as a reference at the beginning of each scan line. But this takes TIME. Not much mind you, but it had co come from somewhere. So the wizards of the day decided that all other timings would stay the same EXCEPT there could then be only (roughly) 29.97 frames per second.

NON-DROP-FRAME
Since it's impossible to go to a fractional frame, this technique simply assumes (wrongly) that there are thirty frames per second. Arithmetic is relatively easy, and the only downside is that a one-hour program is about 3.6 seconds longer than we think it is.

DROP-FRAME
This fixes the above problem. We simply DROP frame 29 from a second every once in a while (that's another story). Adding and subtracting frames is tougher, but the time is right. You can see how we get in trouble if we mix drop-frame and non-drop-frame.

Oh! what webs we weave.

HISTORY
If you've come this far, here's a little background. Early video editors controlled the old 2" quad machines by counting control track pulses (one per frame). After a little tape shuffling, errors would begin to accrue - enter timecode. The SMPTE (Society of Motion Picture and Television Engineers) standardized a timecode system in the early 1970's. Data identifying each frame was formatted so it could be recorded on a spare audio track. There were even some programmable user bits, but we never used them. Since TV sound was only mono, there was a spare audio track on the quad machines, and multi-track audio recorders would simply steal a track. Since it was recorded in a linear fashion, it was called LINEAR timecode. The code could be read forward and backward as the tape was winding, and in play, but as the tape ground to a halt, it could not be read easily. The solution was to put the data in some spare lines at the top of the picture (the vertical interval). This could easily be read by the spinning head in the newer helical scan machines (Like your VHS shows still frames). This was called VERTICAL INTERVAL timecode. Today it's easy to embed the data into a digital signal, so it's just called timecode.

If you actually read all this, let me know!
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Old July 5th, 2004, 12:35 AM   #5
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I read all of it, and thanks for the reply. I don't exactly understand all of it, but I now understand where timecode came from and what it is. Thanks again.
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