A Time Base Corrector
is a device which takes a video input and does any or all of the following before outputting the signal:
Make adjustments to the technical parameters of the signal, e.g. voltage level, horizontal and chroma phase, etc.
Synchronize video parameters with other equipment.
Correct defects in a video signal, e.g. replace a weak synch pulse with a more stable one.

Professional TBCs use a frame store or synchronizer system. This stores either a full field or (preferably) a full frame of video in a buffer and releases it at a very precise moment. Synchronisation is important when mixing different vision sources together, to avoid picture disturbances caused by out-of-synch transitions.

Effects
Some TBCs provide digital effects such as mosaic, posterization, multi-picture, etc