Vertical interval timecode
Encyclopedia
Vertical Interval TimeCode (VITC, pronounced "vitsee") is a form of SMPTE timecode embedded as a pair of black-and-white bars in a video signal. These lines are typically inserted into the vertical blanking interval
of the video signal. There can be more than one VITC pair in a single frame of video: this can be used to encode extra data that will not fit in a standard timecode frame.
VITC contains the same payload as SMPTE linear timecode
frame embedded in a new frame structure with extra synchronization bits and an error-detection checksum. The VITC code is always repeated on two adjacent video lines, one in each field. This internal redundancy is exploited by VITC readers, in addition to the standard timecode "flywheel" algorithm
.
A video frame may contain more than one VITC code if necessary, recorded on different line-pairs. 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 Linear timecode
(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. 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:
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.ion bit" is set the same as in linear timecode, providing odd parity over the 64 time code and user bits.
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
of the video signal. There can be more than one VITC pair in a single frame of video: this can be used to encode extra data that will not fit in a standard timecode frame.
VITC contains the same payload as SMPTE linear timecode
Linear timecode
Linear Timecode 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 biphase mark code, also known as "FM": a zero bit has a single transition...
frame embedded in a new frame structure with extra synchronization bits and an error-detection checksum. The VITC code is always repeated on two adjacent video lines, one in each field. This internal redundancy is exploited by VITC readers, in addition to the standard timecode "flywheel" algorithm
SMPTE time code
SMPTE timecode is a set of cooperating standards to label individual frames of video or film with a time code defined by the Society of Motion Picture and Television Engineers in the SMPTE 12M specification...
.
A video frame may contain more than one VITC code if necessary, recorded on different line-pairs. 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 Linear timecode
Linear timecode
Linear Timecode 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 biphase mark code, also known as "FM": a zero bit has a single transition...
(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. 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:
| Bit | x0 | x1 | x2 | x3 | x4 | x5 | x6 | x7 | x8 | x9 |
|---|---|---|---|---|---|---|---|---|---|---|
| 0x | 1 | 0 | 1 | 2 | 4 | 8 | User bits 1 | |||
| Frame number (0–23, 24, or 29) |
||||||||||
| 1x | 1 | 0 | User bits 2 | |||||||
| 10 | 20 | D | C | |||||||
| 2x | 1 | 0 | 1 | 2 | 4 | 8 | User bits 3 | |||
| Seconds (0–59) |
||||||||||
| 3x | 1 | 0 | User bits 4 | |||||||
| 10 | 20 | 40 | P | |||||||
| 4x | 1 | 0 | 1 | 2 | 4 | 8 | User bits 5 | |||
| Minutes (0–59) |
||||||||||
| 5x | 1 | 0 | User bits 6 | |||||||
| 10 | 20 | 40 | F1 | |||||||
| 6x | 1 | 0 | 1 | 2 | 4 | 8 | User bits 7 | |||
| Hours (0–23) |
||||||||||
| 7x | 1 | 0 | User bits 8 | |||||||
| 10 | 20 | 0 | F2 | |||||||
| 8x | 1 | 0 | Vertical parity (g(x) = x8+1) | |||||||
| Sync | Timecode | User bits | ||||||||
- 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 NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
standard. - Bit 15, the color framingColor framingIn 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 all possible phase relationships.The exact nature of the...
bit, is set to 1 if the time code is synchronized to a (color) video signal. The frame number modulo 2 (for NTSC and SECAMSECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
) or modulo 4 (for PALPALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
) should be preserved across cuts in order to avoid phase jumps in the chrominance subcarrierChrominance subcarrierChrominace subcarrier refers to a separate subcarrier signal that carries the color information during transmission of a video signal. It is modulated and synchronized using the colorburst signal and then attached to the back porch of the color composite video signal...
. - Bit 35, the "bi-phase mark-correctThe 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.ion bit" is set the same as in linear timecode, providing odd parity over the 64 time code and user bits.
- Bits 55 and 75, the "binary group flag" bits, indicate the format of the user bits. Their weighted sum is a number from 0 to 3. 0 indicates no (or unspecified) format, while 1 indicates 4 8-bit characters. Values of 2 and 3 (i.e. bit 75 set) are reserved.
- Bit 74 is unassigned, should always be transmitted as zero, and ignored on reception.
- The CRC in bits 82–89 is a simple bytewise XOR of the previous 82 bits (including the sync bits), which can be described as a CRC with generator polynomial x8+1.
See also
Related technologies and standards- Linear timecodeLinear timecodeLinear Timecode 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 biphase mark code, also known as "FM": a zero bit has a single transition...
- Burnt-in timecodeBurnt-in timecodeBurnt-in timecode is a human-readable on-screen version of the timecode information for a piece of material superimposed on a video image...
- CTL timecodeCTL timecodeCTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or striping, reference SMPTE timecode onto a videotape....
- MIDI timecodeMIDI timecodeMIDI time code , or MIDI time division, 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 carry this information instead...
- AES-EBU embedded timecode
- Rewritable consumer timecodeRewritable consumer timecodeThe Rewriteable Consumer Timecode is a nearly frame accurate timecode method developed by Sony for 8mm and Hi8 analog tape formats. The RC timecode is written by the video camera directly to analog tape tracks and records the hour, minute, second and frame for each frame of video recorded to tape...