What is a time base corrector?
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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.
With the digital wide-windowed time base corrector one “could take unstable poor-quality ½-inch videotape and turn it into a stable poor-quality ½-inch videotape.”
Standalone 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.
As early as 1973, small television studios started switching from big and expensive 2-inch quads to cheaper 1-inch helical machines and even to ¾-inch U-Matic system, which was originally introduced by Sony in 1969 as a home video machine.
The conversion to cheaper video recording formats was hastened by a host of other technologies, like timecode, compact color cameras, editing consoles. By 1975 big networks were using U-matic recorders for shooting and editing, and were dubbing the programs to quad or even aired them directly, passing the signal through a digital TBC.
So, what exactly is time base corrector and why it is so important?
All mechanical devices have jitter. To get a broadcastable TV picture, the errors must be contained within the given standard. The best 2-inch machines were capable of controlling up to 1 µs of jitter thanks to a milled heavy head drum rotating perpendicularly to tape.
Helical 1-inch, ¾-inch and ½-inch machines could have timing errors an order of magnitude higher than quads, even more when a portable recorder was tumbled around.
Stand-alone TBC corrects mechanical errors electronically, stabilizing picture from inherently less stable helical machines, making them suitable for broadcast.
Small timing errors cause flicker and line jitter, vertical lines look wavy, the image gets blurry and loses detail. These defects were considered acceptable for VHS and similar consumer video formats, but not for broadcast. Timing errors cause skew, flagwaving, tearing, picture roll and even complete loss of synchronization.
The basic feature of TBC is to read incoming video line by line, digitize it, and save into a buffer. Originally, the buffer was just one and half lines long, but grew to 8, 16, 32 lines and finally to a whole field or a frame, as computer memory was getting cheaper. Each line is identified with a synchronization pulse.
The TBC generates new synchronization pulses either from a built-in high-precision clock, or from an external one, and writes out the buffered video line by line, also ensuring the correct line duration. Visually, this results in a perfectly rectangular frame with straight edges.
As long as the buffer is not completely emptied and neither is overflown, the TBC is able to output perfectly timed video without loss of data. If the videotape is consistently played too fast, then the buffer would overflow, and the TBC would skip a line or even a whole field or frame. If the tape is played too slow, the TBC would repeat a line or even a whole frame.
Some TBCs can output a correction signal to a compatible videotape machine, which in turn would adjust its playback speed to avoid overflow or underflow of the buffer of the TBC.
Because TBC digitizes incoming analog data for buffering, and then converts it back to analog for output, there is an inevitable loss of information when video is passed through a TBC. Different TBCs have different A/D converters, and some may work better than other.
TBC may have additional functions, and it may be necessary to use one or another TBC depending on tape.
- Many TBCs have built-in processing amplifier (“proc-amp”) to adjust parameters like brightness and color.
- Some TBCs have dropout compensation circuitry to replace a lost piece of picture with a duplicate made from an adjacent piece of picture.
- As the capacity of TBC buffer grew, they became able to store a whole field or frame, which allows to replace not just a single line in case of a dropout, but a whole frame.
- Some full-frame or infinite window TBCs have a built-in synchronizer to lock up two or more VCRs for A/B-roll editing. A frame synchronizer adjusts the timing between two independent video signals without necessarily correcting them, this is useful for seamless switching between several inputs in a studio environment.
- Because TBC converts analog video to digital and back, it can function as a transcoder, converting one video format to another.
By the end of the 1990s some higher-end consumer-grade VCRs as well as camcorders were equipped with built-in TBC. Also, some computer-based analog-to-digital converters have TBC-like functionality, being able to stabilize badly timed video.
As standalone TBCs go into disrepair, “TBC-like” computer-based converters become more important in the quest to preserve old analog video recordings.
