When a VT record machine edits, a sequence of events takes place that makes it change from play to record, in a special way. First the erase head(s) is/are turned on followed by the video and/or audio heads at precisely timed intervals. Electronic control is needed to ensure that this happens correctly, once the machine has reached its correct running speed.
Early editing VTRs only had manual editors, i.e. they edited when a button was pressed by an operator. Simple machines still use this method. It is inaccurate and cumbersome to do more than simply join sections of programmes together. True editing is not possible. The use of an edit controller which times the operation automatically may be one of two basic types.