Time base corrector

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 (VTR) or videocassette recorder (VCR) cannot be mixed with other, more time stable devices found in television studios and post-production facilities. Most broadcast quality VCRs have simple time base correctors built in though external time base correctors (TBCs) are often used.

Time base correction counteracts errors by buffering the video signal as it comes off the videotape at an unsteady rate, and releasing it at a steady rate. TBCs 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. A single master clock or "sync generator" provides the reference for all of the devices' clocks.

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 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. 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.

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.

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