Timebase correction

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Timebase correction is a technique to reduce or eliminate errors present in all analog recordings on mechanical media, including video tape recorders and videocassette recorders, caused by mechanical instability. 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, as laws of physics prevent mechanisms from doing this, the timing of the playback always differs from the original signal. The discrete nature of video signals - division into lines and fields - means that any such timing distortion must appear as timebase errors.

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.)

Cameras and other video devices, such as character generators or digital effects devices have highly accurate clocks that can be precisely adjusted (using a waveform monitor and a vectorscope) 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 together. A single master clock or "sync generator" provides the reference for all of the device's clocks to lock to.

Tape machines, being mechanical (and tape, being subject to stretching and other wear) simply cannot be as accurate as clock driven devices. As mentioned, instability does exist in audio tape machines though higher precision in the mechanical transport can reduce these errors to insignificant levels. Video, however, deals with frequencies at least two orders of magnitude higher than audio signals and errors introduced by the tape transport or the recording tape are simply too large to correct by mechanical means. Early TV pioneers dealt with this problem by inventing the time base corrector.

A time base corrector is simply a big bucket that video can be stored in and then let out at a precise rate. In modern time base correctors the video is digitized and stored in a buffer. A clock chip then releses the video at a steady rate (often this clock is adjusted with reference from the sync generator to be in time with the other devces in then system). If the VTR is sending video too fast or too slow, respectively filling or emptying the buffer, an "advanced synch" signal is sent to the VTR to actually speed it up or slow it down to prevent the buffer from over or underfilling. Using a TBC, a signal originating on tape can be as clock steady as any other device in the system and can be switched or mixed with the other signals.

A variant of the Time Base 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. Sattelites, 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.

As you might imagine, all this adjustment of timing signals requires time and skill. In many of today's video switchers, frame synchronizers are built in, so any video source can be plugged in without worrying about timing.

Video signals are more sensitive than audio signals to mechanical error, due to their wider bandwidth. Even back in 1956, let alone today, 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. In particular, the color information in NTSC is encoded relative to the phase of high frequency color sub-carrier, making the displayed colors extremely sensitive to timebase errors. Timebase correction fixes (or at least reduces) these errors by inserting a variable delay in the video stream, then adjusting this delay in real time so that the output signal appears at precisely the right rate and time.

Since video is written and read in diagonal tracks, using heads on a spinning drum to read or write a moving tape, there are many potential causes of timing errors. Causes of longitudinal error (error in the long direction of the tape) include variations in the rotational rate of the capstan drive and jamming of tape in the machine; transverse error (error in the cross tape direction) is 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.

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 common time bases used. The first is to make the frames, fields and lines come out smoothly and uniformly, at the rates specified by the standards. The reference in this case is an oscillator, either internal or external. The alternative is to align the frames, fields, and lines with some external signal, a procedure called genlocking. This allows sources that are not themselves genlock-capable to be used with production switchers and A/B roll editing equipment. Stand-alone broadcast model timebase correctors typically will genlock the signal to an external sync source, and also allow the brightness, contrast, chrominance, and color phase ("tint" or "hue") to be adjusted. In addition, most broadcast-grade VCRs have simple timebase correctors built in.

A few high-end domestic or semi-professional time base correctors are available at modest cost. These may include controls for adjusting the colour position relative to the luminance (brightness), as well as other colour and picture controls. Furthermore, features such as freeze-frame and NTSC/PAL/SECAM standards coversion may be offered.