Vertical Interval Timecode (VITC, pronounced "vitsee") is a form of SMPTE timecode encoded on one scan line in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal.
With one exception, VITC contains the same payload as SMPTE linear timecode (LTC), embedded in a new frame structure with extra synchronization bits and an error-detection checksum. The exception is that VITC is encoded twice per interlaced video frame, once in each field, and one additional bit (the "field flag") is used to distinguish the two fields.
A video frame may contain more than one VITC code if necessary, recorded on different lines. This is often used in production, where different entities may want to encode different sets of time-code metadata on the same tape.
As a practical matter, VITC can be more 'frame-accurate' than LTC, particularly at very slow tape speeds on analog formats. LTC readers can lose track of code at slow jog speeds whereas VITC can be read frame-by-frame if need be. Conversely, at high speeds (FF/rew.), the VITC is often unreadable due to image distortions, so the LTC is often used instead. Some VCRs have an auto selection between the two formats to provide the highest accuracy.
VITC is 90 bits long: 32 bits of time code, 32 bits of user data, 18 synchronization bits, and 8 bits of checksum. It is transmitted using non-return-to-zero encoding at a bit rate of 115 times the line rate. (The unused 25 bit times are to leave room for the horizontal blanking interval.)
(0-23, 24, or 29)
|80||1||0||CRC bits (g(x) = x8 + 1)|
The exact nature of the color frame sequence depends on the video standard being used. In the case of the three main composite video standards, PAL video has an 8-field (4 frame) color frame sequence, and NTSC and SECAM both have 4-field (2 frame) color frame sequences.
Preserving the color framing sequence of video across edits and between channels in video effects was an important issue in early analog composite videotape editing systems, as cuts between different color sequences would cause jumps in subcarrier phase, and mixing two signals of different field dominance would result in color artifacts on the part of the signal that was not in sync with the output color frame sequence.
To help prevent these problems, SMPTE time code contains a color framing bit, which can be used to indicate that the video material the timecode refers to follows a standard convention regarding the synchronization of video time code and the color framing sequence. If the color framing bit was set in both types of material, the editing system could then always ensure that color framing was preserved by constraining edit decisions between input sources to keep the correct relationship between the timecode sequences, and hence the color framing sequences.
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