Time base correction (TBC) 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 TBCs are often used. Some high-end domestic analog video recorders and camcorders also include a TBC circuit, which typically can be switched off if required. 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. Video correction As far back as 1956, professional reel-to-reel audio tape recorders relying on mechanical stability alone were stable enough that pitch distortion could be below audible level without 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 and difficulty locking to downstream devices. 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. Types Physically there are only 4 types, dedicated IC, add-in cards for prosumer/professorial VTR/VCRs, desktop standalone units & dedicated rack mount units. In the broadcast world, 1U Rack-mount type time base correctors were common & intended to be easily slid out of a rack on rails to be serviced, as these units were meant for 24/7 workloads and typically contained little to no SMD hardware, only operator serviceable through-hole electronics, making these units a substantially heavy device. Software Time Base Correction A modern 5th and final type of TBC being achieved in the 2010s is