What is a time base corrector?
The world of television and broadcast-quality video changed forever in 1973 with the introduction of the digital time-base corrector (TBC).
Two American companies — Consolidated Video Systems and Television Microtime — announced similar products almost simultaneously.
The biggest advancement came with the digital time-base corrector. … As a friend of mine put it, with it you “could take unstable poor-quality ½-inch videotape and turn it into stable poor-quality ½-inch videotape.” — John J. Godfrey, Emmy award-winning editor and a supervising engineer at WNET/13 in New York in 1983.
Digital TBC allowed indie artists and activists, who used portable, affordable video equipment since mid-1960s, to overcome the steep technological and regulatory hurdles that had prevented access to the broadcast TV.
Professionals took notice of TBC as well. As early as 1973, small television stations started switching from big and expensive 2-inch videotape recorders to cheaper 1-inch helical machines and even to ¾-inch system, which was introduced by Sony in 1969 as a home video format.
By 1975 big networks were using ¾-inch recorders for shooting and editing news and documentaries, and aired these tapes, passing the signal through a digital TBC.
So, what exactly is time base corrector and why it is so important?
Videotape recorder combines electronics and mechanics to record video on magnetic tape as a series of stripes, either across a 2-inch tape, or at a very gentle slope on narrower tape. The first approach is called “transversal”, the second is “helical”. Both systems use a rotating drum with magnetic heads affixed to it. Transversal machines are known as “quads” because they have four heads. Helical machines can have one, two or more heads depending on the design and features.
Minute instability of tape travel speed and head drum rotation cause jitter, when things that should happen with strict periodicity, do not. For example, individual picture lines, which make up a video frame, may start at a slightly different time and may have slightly different duration, which results in shaky or wavy picture. More serious timing errors cause skew, tearing, color streaking, flagwaving, hue shift, picture roll, and even complete loss of synchronization.
Each video line is identified with a horizontal synchronization pulse, or H-sync. Each frame is identified with a vertical synchronization pulse, or V-sync. The timing reference to which the pulses are synchronized is known as time base.
To make sure that the picture is broadcastable, the periodicity errors of synchronization pulses, or time base errors, must be contained within very narrow margins. The best 2-inch machines are capable of controlling up to 1 µs of synchronization instability.
Helical machines can have timing errors an order of magnitude higher than quads, even more so when a portable recorder is tumbled around. The video produced by these machines was not broadcast-legal until digital TBC had been invented. TBC electronically corrects image instability, making the video suitable for broadcast.
The basic function of a digital TBC is to read incoming video line by line, digitize it, and store in a memory buffer. Originally, the memory could fit just a couple of lines, but grew to 8, 16, 32 lines and finally to a field or even a whole frame.
TBC strips away synchronization pulses from the incoming video and generates new ones either from a built-in high-precision clock, or from an external one, adjusts duration of individual lines, and writes out the buffered lines with precise intervals between them. Visually, this achieves a perfectly rectangular frame with straight edges.
As long as the buffer is not completely emptied and neither is overflown, a TBC is able to output perfectly timed video without loss of data. If the videotape consistently plays too fast, then the buffer would overflow, and the TBC would skip a line or even a whole field or frame. If the tape plays too slow, the TBC would repeat a line or even a whole frame.
To ensure the consistent tape speed, the mechanism of a typical VCR has two servos: capstan servo for stabilizing linear tape speed, and head drum servo for stabilizing reading speed.
At any given time tape can move slightly faster or slightly slower than required, but its speed does not drift too far from nominal, because the capstan servo in the VCR adjusts tape speed with every frame using 30 Hz pulse from the control track.
A professional-grade TBC can output a correction signal (“Advanced Sync”) to a compatible videotape machine, which in turn would adjust its playback speed to avoid overflow or underflow of the buffer of the TBC. This sort of ad-hoc correction is not possible with consumer-grade VCRs, so one must rely on the VCR speed control system, matched with a larger correction buffer of a TBC.
A TBC built into a consumer-grade VCR or a camcorder does not always withstand irregular frame sync pulses from an old or damaged tape, causing jerks or picture roll. In this case, you need a more powerful TBC, which restores not only H-sync pulses, but V-sync pulses as well, ensuring that downstream devices keep working, and the picture remains stable. Some call such a device “Frame TBC” as opposed to “Line TBC”, which deals primarily with H-sync pulses.
Mind you, “Frame TBC” and “Line TBC” are non-standard terms. They are thrown around web forums, and even those who use them cannot provide a comprehensive explanation, spitting out inanities instead.
Assuming that the functionality described by these terms is complementary, “Line TBC” is a shorthand for “8-line TBC”, “16-line TBC” , “32-line TBC”, etc and describes a TBC that corrects H-sync pulses only, lining up individual lines in a frame. This is what any normal TBC does.
“Frame TBC” can be thought of as a special case of frame synchronizer. An ordinary frame synchronizer adjusts V-sync pulses of one or more videos to a reference video or a sync generator. A “Frame TBC” synchronizes a single video channel to a built-in sync generator.
The combination of “Line TBC” and “Frame TBC” ensures both horizontal and vertical synchronization, which results in a rectangular and stable picture.
Case in point, the TBC-1000 from DataVideo. It description reads, “the TBC-1000 is a digital full frame synchronizer that replaces all sync signals from the source.”
Keep in mind, that because TBC digitizes incoming analog video for buffering, and then converts it back to analog for output, the video inevitably degrades somewhat after it passes through a TBC. Different TBCs have different A/D converters, and some may work better than other. For example, early TBCs used 8-bit sampling, newer TBCs have 10-bit or 12-bit sampling.
By the early 1980s most professional VTRs were outfitted with a built-in TBC. The requirements for home video machines have always been less stringent than for professional ones, which is why most consumer-grade VCRs do not employ a built-in TBC and show a slightly wavy picture even when functioning properly.
For a short period of time in the end of the 1980s, some high-end VCRs adopted a built-in TBC, only to drop it just a couple of years later. Was it related to the additional cost, or the manufacturers were threatened by the movie industry, which worried about high-quality home copies of Hollywood movies?
Whatever the case, the waviness was not noticeable when the video was watched on an old-school flickery CRT TV set. But by the early 1990s better TV sets appeared, first progressive-scan CRTs with 100 Hz or 120 Hz refresh rate, then plasma panels, then LCD screens, and the jitter became noticeable.
So, by the end of the 1990s, a built-in TBC returned to high-end VCRs and became the norm on camcorders.
Today, a dedicated TBC, even a consumer-grade one, costs an arm and leg, so many video archivists look for alternatives. Some of these are:
- a VCR with a built-in TBC;
- a camcorder with a built-in TBC to play VHS-C and 8-mm home videos;
- a Digital8 or DV camcorder with a built-in analog-to-digital converter (passthrough mode) and a TBC, to convert any analog video into a stabilized DV video;
- a DVD recorder with TBC-like functionality, to use in pass-through mode as a TBC;
- finally, some computer video capture cards have TBC-like functionality and are able to stabilize shaky video at least to some extent.
As standalone TBCs go into disrepair, “TBC-like” computer-based converters and stabilization software become more important in the quest of preserving old analog video recordings.
Glossary (by Tektronix)
- Clock — reference timing source in a system. A clock provides regular pulses that trigger or synchronize events.
- Clock Reference — a special time stamp that conveys a reading of a time base.
- Frame Synchronizer — a digital buffer, that by storage, comparison of sync information to a reference, and time release of video signals, can continuously adjust the signal for any timing errors. A digital electronic device which synchronizes two or more video signals.
- Time Base — the notion of a clock; it is equivalent to a counter that is periodically incremented. Literally means timing reference to which things are synchronizing.
- Time Base Error — Aperiodicity of the synchronizing signals.
- Time Base Corrector (TBC) — device used to correct for time base errors and stabilize the timing of the video output from a tape machine.
