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Vertical Interval Time Code

aka VITC (say VIT-see)

There are two major problems associated with longitudinal time code. Firstly, it is not frame-accurate (not even close...) when you're moving at very slow speeds, or stopped, as would be the case when shuttling between individual frames (remember that it's possible to view a stopped frame because the head keeps rotating and therefore moving relative to the tape). Secondly, the LTC takes up too much physical space on the videotape itself.

The solution to both of these problems was presented by Sony back in 1979 in the form of Vertical Interval Time Code or VITC. This information is stored in what is known as the ``vertical blanking interval'' in each field (we won't get into what that means... but it's useful to know that there is one in each field rather than each frame, increasing our resolution over LTC by a factor of 2. It does, however, mean that an extra flag will be required to indicate which field we're on.

Since the VITC signal is physically located in the vertical tracks on the tape (because it's recorded by the rotating head instead of the stationary one) it has a number of characteristics which make it differ from LTC.

- Firstly, it cannot be played backwards. Although the video can be played backwards, each field of each frame (and therefore each word of the VITC code) is being shown forwards (in reverse order).

- Secondly, we can trust that the phase of the signal is reliable, so a different binary encoding system is used called Non-Return to Zero or NRZ. In this system, a value of 80 IRE (an IRE is a unit equal to 1/140th of the peak-peak amplitude of a video signal, usually 1 IRE = 7.14 mV, therefore 140 IRE = 1 V) is equal to a binary ``1'' and 0 IRE is a 0.

- Thirdly, as was previously mentioned, the VITC signal contains the address of a field rather than a frame.

- Finally, we don't need to indicate when the word starts (as is done by the sync bits in LTC) since every field has only 1 frame associated with it. sonce both the field and the word are read simultaneously (they cannot be read seperately) we don't need to know when the word starts... it's obvious.

Unlike LTC, there are a total of 90 bits in the VITC word, with the following assignments:

Table 10.9: INSERT CAPTION HERE

Information	Number of Bits
Time Address	26
User Information	32
Sync Groups	18
Cyclic Redundancy Code	8
Status Information	5
Unassigned (Field Mark Flag?)	1

In many respects, these bits are the same as their LTC counterparts, but we'll go through them again anyway.