VITC(Vertical Interval Timecode)

erolroniberaha · August 9, 2004

There are two things I don't get about Vertical Interval Timecode both in Arri 535 and Aaton Xtr Prod. Firstly in 535B,when the video signal is being sent to the video recorder, there are 2 choices of position. As the signal is written directly into the tape that can not be seen on the monitor, how do these position change as I change the number from 6 to 22? I mean when I watch the footage with a vcr that can read timecode, will the place of the timecode change as I dial different numbers? Also in Aaton Xtr Prod, it is stated that I can record VITC in even or odd fields. What will be the difference or the benefit choosing one over the other?

Regards,

Erol Roni Beraha

Istanbul

Preston Herrick · August 9, 2004

I could be wrong but I believe what you are referring to, where VITC is concerned, is the option of selecting which line in the Vertical Interval you want to insert the timecode data. If that is the case, then it has no affect on where the timecode is displayed on playback. Therefore you can probably choose whichever one you want. However, I would find out if there is a standard or default setting - the other one being an alternate.

Selecting a screen position on playback is usually a function (option) of the VCR.

John Sprung · August 9, 2004

This will probably be kind of a long answer. To understand VITC, we have to know about the vertical interval, and to understand that we need to look at the whole analog TV scanning system. All this stuff was carved in stone shortly before WWII, and it is the way it is because of the technological constraints of the analog vacuum tube world of that time.

To start with, we have an oscillator producing a sawtooth wave at 15,734 Hz to drive the horizontal scan (that was very fast and hard to do back then). To keep things simple, this thing runs continuously. Then we have to add vertical deflection to the horizontal deflection to get the electron beam to cover the whole surface of the CRT. That fast horizontal oscillator has to keep going while the vertical deflection jumps back up from the bottom to the top to start the next field, so there are many of its cycles that can't be used in the picture, and have to be blanked. In NTSC, there are 525 "lines" -- really cycles of the horizontal oscillator -- per frame, of which 483 are active picture. The other 42 are blanked for the big move from the bottom to the top again, and are called the vertical blanking interval. (BTW, interlaced systems have always used an odd number of total "lines" -- 383, 405, 441, 525, 625, 809, 1125 -- because you need a half line's worth of time to make field 2's lines land in between rather than on top of field 1's lines)

As time went by, technology improved and it became possible to do that vertical retrace much faster. The electron beam was consistently back up above the top of picture before line 10 or so, which made it possible to use the rest of the interval for a variety of things, because it was hidden under the edge of the plastic at the top of the TV set. Vertical Interval Test Signal and Vertical Interval Reference Signal (VITS & VIRS) were used to keep things in reasonable shape in the pre-satellite days when network TV travelled from coast to coast (here in the U.S.) using 32 terrestrial microwave links.

Closed captions on line 21 have always been the most stable and consistent item in the interval. Most decoders are hard-wired for line 21 only, and won't work if the captions are anywhere else. I forget now where VITS and VIRS used to be. VITC traditionally was on lines 12 and 14, later 17 and 19 became more common as some editing systems needed the extra time. VITC readers now are more sophisticated, and look at the whole interval until they find something that they recognize. Or they can be set to look at certain lines. That way you can have continuous VITC matching the longitudinal code in one place, and use another set of lines for VITC matching the original production sound, or other handy things.

You can see all this vertical interval stuff on an underscan monitor. Or on an ordinary TV set, if you set the vertical hold off by just enough to get a slow roll, you can see it as a bunch of dancing dots just above the actual picture.

Unless somebody from post production tells you otherwise, probably the safest thing to do is put VITC on lines 17 and 19 of field 1. (Perhaps that was all you really needed from all of this.)

-- J.S.

erolroniberaha · August 10, 2004

Unless somebody from post production tells you otherwise, probably the safest thing to do is put VITC on lines 17 and 19 of field 1. (Perhaps that was all you really needed from all of this.)

Actually,when I do someting that is written in the manual or anything in real life I want to know why I should follow the instructions. Thank you for your explanation. I have to reread and digest now

Regards

Erol Roni Beraha