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Vertical Interval Timecode (VITC, pronounced "vitsee") is a form of
SMPTE timecode
SMPTE timecode ( or ) is a set of cooperating standards to label individual frames of video or film with a timecode. The system is defined by the Society of Motion Picture and Television Engineers in the SMPTE 12M specification. SMPTE revised t ...
encoded on one scan line in a video signal. These lines are typically inserted into the
vertical blanking interval
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...
of the video signal.
With one exception, VITC contains the same payload as SMPTE
linear timecode
Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode data in an audio signal, as defined in SMPTE 12M specification. The audio signal is commonly recorded on a VTR track or other storage media. The bits are encoded using the bi ...
(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
Interlaced video (also known as interlaced scan) is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This ...
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
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negati ...
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
Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the be ...
.)
* Bit 14 is set to 1 if
drop frame numbering is in use; frame numbers 0 and 1 are skipped during the first second of every minute, except multiples of 10 minutes. This converts 30 frame/second time code to the 29.97 frame/second
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
standard.
* Bit 15, the
color framing
In video engineering, color framing refers to the color frame sequence of fields in a composite video signal through which the video frame timing and chrominance subcarrier signal timing—in particular, that of the color burst -- cycle through ...
bit, is set to 1 if the time code is synchronized to a (color) video signal. The frame number modulo 2 (for NTSC and
SECAM
SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog color television system that was used in France, some parts of Europe and Africa, and Russia. It was one of th ...
) or modulo 4 (for
PAL
Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
) should be preserved across cuts in order to avoid phase jumps in the
chrominance subcarrier The chrominance subcarrier is a separate subcarrier signal that carries the color information during transmission of a composite video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of th ...
.
* Bits 35, 55, and 75 differ between 25 frame/s time code, and 30/29.97 frame/s.
[ This is an example of the 25 frame/s bit assignments, which are different from the more commonly seen 30/29.97 bit assignments.] The bits are:
** Field flag (bit 35 for 29.97/30 frame/s, bit 75 for 25 frame/s): This is an additional least-significant bit for the frame number, distinguishing the two interlaced fields in one video frame. It is set to 0 during the first field of a frame, and to 1 during the second. This bit replaces the "polarity correction" bit in linear timecode.
** "Binary group flag" bits BGF0 and BGF2 (bits 55 and 75 for 29.97/30 frame/s, bits 35 and 55 for 25 frame/s): These indicate the format of the user bits. Both bits zero indicates no (or unspecified) format. Only BGF0 set indicates four 8-bit characters (transmitted
little-endian
In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most si ...
). The combinations with BGF2 set are reserved.
* Bit 74 ("Binary group flag 1") was previously unassigned, but is used to indicate that the time code is synchronized to an external clock. If zero, the timecode start time is arbitrary.
*The checksum in bits 82–89 is a simple bytewise XOR of the previous 82 bits (''including'' the sync bits, so bit 82 is the XOR of bits 74, 66, ..., 2), which can be described as a CRC with generator polynomial ''x''
8+1. (Preset to zero, no inversion.)
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.
See also
Related technologies and standards
*
Linear timecode
Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode data in an audio signal, as defined in SMPTE 12M specification. The audio signal is commonly recorded on a VTR track or other storage media. The bits are encoded using the bi ...
*
Burnt-in timecode
Burnt-in timecode (often abbreviated to BITC by analogy to VITC) is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image. BITC is sometimes used in conjunction with "real" machine-re ...
*
CTL timecode
Control track longitudinal timecode, or CTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or ''striping'', reference SMPTE timecode onto a videotape.
Similar to the way VITC timecode is embedded in the verti ...
*
MIDI timecode
MIDI time code (MTC) embeds the same timing information as standard SMPTE timecode as a series of small 'quarter-frame' MIDI messages. There is no provision for the user bits in the standard MIDI time code messages, and SysEx messages are used to ...
*
AES-EBU embedded timecode
AES3 is a standard for the exchange of digital audio signals between professional audio devices. An AES3 signal can carry two channels of pulse-code-modulated digital audio over several transmission media including balanced lines, unbalanced lin ...
*
Rewritable consumer timecode The Rewriteable Consumer Timecode (RCTC, RC Timecode, or RC Time Code) is a nearly frame accurate timecode method developed by Sony for 8mm and Hi8 analog tape formats.
References
{{Video formats
Timecodes
Film and video technology