Time base correction 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 or videocassette recorder cannot be mixed with other, more time stable devices found in video studios. Most broadcast-grade VCRs have simple time base correctors built in though external time base correctors ("TBC"s) are often used.
Time base correction counteracts errors by buffering the video signal and releasing it at a steady rate. "TBC"s also allow a variable delay in the video stream. By adjusting the rate and delay using a waveform monitor and a vectorscope, the corrected signal can now 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 together. A single master clock or "sync generator" provides the reference for all of the devices' clocks.
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Video Correction
As far back as 1956, professional reel-to-reel audio tape recorders relying on mechanical stability alone had no audible pitch distortion, and no need for timebase correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals. A video signal consists of picture information but also sync and subcarrier signals which allow the image to be framed up square on the monitor, reproduce colors accurately and, importantly, allow the combination and switching of two or more video signals. Unlike audio, video signals cannot be mixed at all if they are out of time with each other. Video signals are more sensitive than audio signals to mechanical error, due to their wider bandwidth. In particular, the color information in
A video recorder uses heads on a spinning drum to read and write diagonal tracks onto a moving tape. This has many potential sources of timing errors. If the mechanism ran at an absolutely constant speed, and never varied from moment to moment, or from the time of recording to the time of playback, then the timing of the playback signal would be exactly the same as the input. However, imperfection being inevitable, the timing of the playback always differs to some extent from the original signal. Longitudinal error (error arising from effects in the long direction of the tape) can be caused by variations in the rotational rate of the capstan drive, stretching of the tape medium and jamming of tape in the machine. Transverse error (error arising from effects in the cross-tape direction) can be caused by variations in the rotational speed of the scanning drum and differences in the angle between the tape and the scanning heads (usually addressed by video "tracking" controls). Longitudinal errors are similar to the ones that cause wow and flutter in audio recordings. Since these errors are not so subtle and since it is standard video recording practice to record a parallel control track, these errors are detected and servos are adjusted accordingly to dramatically reduce this problem.
Methods
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. The first method is to make the frames, fields and lines come out smoothly and uniformly, at the rates specified by the standards using an oscillator for time reference. The alternative to this method is to align the frames, fields, and lines with some external signal, a procedure called genlocking. Genlocking allows sources that are not themselves genlock-capable to be used with production switchers and A/B roll editing equipment. Stand-alone broadcast model time base correctors typically will genlock the signal to an external sync reference, and also allow the brightness, contrast, chrominance, and color phase ("tint" or "hue") to be adjusted.
A variant of the timebase corrector is the frame synchronizer which allows devices that cannot be "steered" by an advanced sync signal to also be time base corrected and/or timed into a system. Satellites, microwave transmitters and other broadcast signals as well as consumer VTRs cannot be sent an advance sync signal. A frame synchronizer stores at least a full frame of video. If the buffer over or under fills, 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.
See also
References
Extensive guide to TBCs and their selection
A digital synchronizer for a video-tape recorder, Bucciarelli, F.V.; Proceedings of the IEEE, Volume 61, Issue 4, April 1973 Page(s):506 - 507
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