VT Edit Systems
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.
Control track editing
Using accurate tape timers to control the edit sequence allows predetermined points to be marked, rehearsed, changed and performed relatively easily. This is a very common form of controlled editing. Microcomputers store both play and edit machine tape times. The computer ‘pre-rolls’ both machines for either 5 or 10 seconds (dependent upon preset switches), then runs them from pause, allowing the pre-programmed edit to take place.
These controllers usually operate with only two machines, and rely upon accurate reading of tape counters; these are prone to slippage, resulting in edits which are inaccurate by an order of a few frames.
Time-code editing
On an unused audiotrack or special track ‘time of day’ or ‘elapsed time’ information can be recorded along with the original programme. This is known as time code and gives the microcomputer a totally frame-accurate reference to use, instead of the tape counter. Time code is the key which unlocks the door to computer control of all functions required during post-production. Three or more machines can be controlled via a computer keyboard, vision effects can be triggered by the computer, edit details can be remembered on magnetic storage, lists made up from original edit sessions can be recalled and manipulated, and re-edits can be performed automatically. The problem of serial access of shots, i.e. removal of a sequence from the middle of a programme necessitating that all the following sequences need to be re-edited, is partially alleviated. ‘Off-line’ editing (rough cutting on cheap machines) becomes possible, loading data into the computer and then fine cutting ‘on-line’ using high-quality machines.
Time code is a series of pulses at audio frequency, used to indicate time of day or elapsed time. It requires a time-code generator when recording and a time-code reader when replaying the tape. Time code can be ‘burnt in’ on the picture and viewed without a separate reader on any television monitor— useful for reviewing takes and preparing editing schedules.
1. Basic two-machine editing suite.
2. Typical three-machine editing suite.