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Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media. Without time base correction, a signal from a videotape recorder (VTR) or videocassette recorder (VCR), cannot be mixed with other, more time-stable devices such as character generators and video cameras found in television studios and post-production facilities.
Time base correction counteracts errors by buffering the video signal as it comes off the videotape at an unsteady rate, and releasing it after a delay at a steady rate. A sync generator provides the timing reference for all devices in the system. By adjusting the delay using a waveform monitor, the corrected signal can be made to match the timing of the other devices in the system. If all of the devices in a system are adjusted so their signals meet the video switcher at the same time and at the same rate, the signals can be mixed.
Though external TBCs are often used, most broadcast-quality VCRs have simple time base correctors built in. Some high-end domestic analog video recorders and camcorders also include a TBC circuit, which typically can be switched off if required.
As far back as 1956, professional reel-to-reel audio tape recorders were mechanically stable enough that pitch distortion could be below an audible level without time base correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals and difficulty in synchronizing with other devices. [1] A video signal consists of not only picture information, but also sync and subcarrier signals. Sync allows the image to be framed up square on the monitor and allows the combination and switching of two or more video signals. The subcarrier is involved in reproducing colors accurately. [lower-alpha 1]
Implicit in the idea of time base correction is that there must be some target time base that the corrector is aiming for. There are two time bases commonly used.
Some TBCs featured a Drop Out Compensation (DOC) logic that enabled videotape flaws caused by oxide drop-out to be temporarily corrected. The DOC logic required dedicated cabling between the videotape player and the TBC in which irregularities were detected in portions of the video image. Previously captured and stored lines of video would then be superimposed over the flawed video lines.
A variant of the time base corrector is the frame synchronizer which allows devices that cannot be steered by a sync signal to also be time base corrected or timed into a system. Satellites, microwave transmitters and other broadcast signals as well as consumer VTRs cannot be sent a sync signal. The synchronizer accomplishes this by writing the incoming digital video [lower-alpha 3] into a frame buffer memory using the timing of the sync information contained in that video signal. A frame synchronizer stores at least a full frame of video. Simultaneously the digital video is being read back out of the buffer by an independent timing system that is genlocked to the house timing reference. If the buffer over or underfills, the Frame Sync will hold the last good frame of video until another full frame's worth of video is received. Usually, this is undetectable to viewers.
Physically there are only 4 types: dedicated IC, add-in cards for prosumer/professorial VTR/VCRs, desktop standalone units, and dedicated rack mount units.
In the broadcast world, 1U Rack-mount type time base correctors were common. They are intended to be easily slid out of a rack on rails to be serviced, as these units were meant for 24/7 workloads. They typically contained little to no SMD hardware, only operator serviceable through-hole electronics, making these units quite heavy.
A modern 5th and final type of TBC being achieved in the late 2010s is software-defined. The python project LD-Decode [3] (and its extended versions VHS-Decode [4] and CVBS-Decode [5] ) implement this "software time base correction" method. The programs take in radio-frequency captures of analogue media signals, then de-modulates and corrects the signal in software.
The TBC programs stores the corrected signals in a ".TBC" file, containing a digital, lossless, 4fsc copy of the signal at 16 bits per sample [6] – not unlike the older D-3 digital videotape. The data within can be combined luminance and chrominance, or separated. ld-analyze, a tool from the LD-decode project, allows frame by frame analysis, closed captioning & VITC timecode readout using the TBC file. [7] TBC files can have their chroma decoded to a uncompressed YUV [lower-alpha 4] or RGB video stream, then encoded into a lossless compressed FFV1 .mkv video file for archival via tools like tbc-video-export.
TBC file streams can also be directly played back to analouge TV systems via a DAC.
Linear 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 biphase mark code : a 0 bit has a single transition at the start of the bit period. A 1 bit has two transitions, at the beginning and middle of the period. This encoding is self-clocking. Each frame is terminated by a 'sync word' which has a special predefined sync relationship with any video or film content.
Phase Alternating Line (PAL) is a colour encoding system for analog 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 per second, and associated with CCIR analogue broadcast television systems B, D, G, H, I or K. The articles on analog broadcast television systems further describe frame rates, image resolution, and audio modulation.
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) systems, which, in turn, were replaced by flat-panel displays of several types.
Vertical Interval Timecode 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.
The VHS is a standard for consumer-level analog video recording on tape cassettes, introduced in 1976 by the Victor Company of Japan (JVC). It was the dominant home video format throughout the tape media period in the late 1970s, 1980s, and 1990s.
Colorburst is an analog and composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the back porch (beginning) of each scan line, a television receiver is able to restore the suppressed carrier of the chrominance (color) signals, and in turn decode the color information. The most common use of colorburst is to genlock equipment together as a common reference with a vision mixer in a television studio using a multi-camera setup.
Composite video is an baseband analog video format that typically carries a 405, 525 or 625 line interlaced black and white or color signal, on a single channel, unlike the higher-quality S-Video and the even higher-quality component video.
Genlock is a common technique where the video output of one source is used to synchronize other picture sources together. The aim in video applications is to ensure the coincidence of signals in time at a combining or switching point. When video instruments are synchronized in this way, they are said to be generator-locked, or genlocked.
SMPTE timecode 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 the standard in 2008, turning it into a two-part document: SMPTE 12M-1 and SMPTE 12M-2, including new explanations and clarifications.
A video tape recorder (VTR) is a tape recorder designed to record and playback video and audio material from magnetic tape. The early VTRs were open-reel devices that record on individual reels of 2-inch-wide (5.08 cm) tape. They were used in television studios, serving as a replacement for motion picture film stock and making recording for television applications cheaper and quicker. Beginning in 1963, videotape machines made instant replay during televised sporting events possible. Improved formats, in which the tape was contained inside a videocassette, were introduced around 1969; the machines which play them are called videocassette recorders.
U-matic or 3⁄4-inch Type E Helical Scan or SMPTE E is an analogue recording videocassette format first shown by Sony in prototype in October 1969, and introduced to the market in September 1971. It was among the first video formats to contain the videotape inside a cassette, as opposed to the various reel-to-reel or open-reel formats of the time. The videotape is 3⁄4 in (19 mm) wide, so the format is often known as "three-quarter-inch" or simply "three-quarter", compared to open reel videotape formats in use, such as 1 in (25 mm) type C videotape and 2 in (51 mm) quadruplex videotape.
Film-out is the process in the computer graphics, video production and filmmaking disciplines of transferring images or animation from videotape or digital files to a traditional film print. Film-out is a broad term that encompasses the conversion of frame rates, color correction, as well as the actual printing, also called scannior recording.
1-inch Type C Helical Scan or SMPTE C is a professional reel-to-reel analog recording helical scan videotape format co-developed and introduced by Ampex and Sony in 1976. It became the replacement in the professional video and broadcast television industries for the then-incumbent 2-inch quadruplex videotape open-reel format. Additionally, it replaced the unsuccessful type A format, also developed by Ampex, and primarily in mainland Europe, it supplemented the type B format, developed by the Fernseh division of Bosch.
A PCM adaptor is a device that encodes digital audio as video for recording on a videocassette recorder. The adapter also has the ability to decode a video signal back to digital audio for playback. This digital audio system was used for mastering early compact discs.
1-inch Type B Helical Scan or SMPTE B is a reel-to-reel analog recording video tape format developed by the Bosch Fernseh division of Bosch in Germany in 1976. The magnetic tape format became the broadcasting standard in continental Europe, but adoption was limited in the United States and United Kingdom, where the Type C videotape format met with greater success.
2-inch quadruplex videotape was the first practical and commercially successful analog recording video tape format. It was developed and released for the broadcast television industry in 1956 by Ampex, an American company based in Redwood City, California. The first videotape recorder using this format was built the same year. This format revolutionized broadcast television operations and television production, since the only recording medium available to the TV industry until then was motion picture film.
Audio-to-video synchronization refers to the relative timing of audio (sound) and video (image) parts during creation, post-production (mixing), transmission, reception and play-back processing. AV synchronization can be an issue in television, videoconferencing, or film.
The 1/4 inch Akai is a portable helical scan EIA and CCIR analog recording video tape recorder (VTR) with two video record heads on the scanning drum. The units were available with an optional RF modulator to play back through a TV set, as well as a detachable video monitor. The Akai Electric Ltd. VTR plant was in Tokyo, Japan.
IVC 2 inch Helical scan was a high-end broadcast quality helical scan analog recording VTR format developed by International Video Corporation (IVC), and introduced in 1975. Previously, IVC had made a number of 1 inch Helical VTRs. IVC saw a chance to make a VTR that would have the quality of the then-standard 2 inch Quadruplex videotape format but with the advantages of helical scan. They then developed a VTR using this technology, the IVC Model 9000.
Broadcast-safe video is a term used in the broadcast industry to define video and audio compliant with the technical or regulatory broadcast requirements of the target area or region the feed might be broadcasting to. In the United States, the Federal Communications Commission (FCC) is the regulatory authority; in most of Europe, standards are set by the European Broadcasting Union (EBU).