Basic Operational Requirements of VTR
Videotape recorders need precise alignment to ensure consistent results. Test signals of known level should therefore be recorded at the beginning of every tape. On replay these signals can be used to check the replay systems to ensure that the videotape process is a ‘unity’ system, i.e. the original recorded signal and replay signal are identical. Colour bars are used for this purpose, derived from a test signal generator in the studio complex or when using lightweight cameras, from the camera’s own ‘bars’ generator. Line-up tone is used in a similar way to ensure consistency of audio levels. It is generated within a sound-mixing console or from a lightweight camcorder’s own line-up tone generator.
Idents
Some form of identification is needed for video when recording ‘takes’ or for complete programme tapes. It is normal practice to use a clock for this. Not only tape numbers, programme titles, detail etc. but counts into programmes are derived from this clock. It performs the same functions as a film leader on telecine. It can be generated electronically, or from a clock placed physically in front of a camera. It is worth noting that for location recording a film-type clapper board can be used. This gives the editor readily recognisable starts to, and information on, specific shots.
Pre-roll
Every VTR requires time for its mechanical and electronic systems to settle when it is made to play or record. This is known as pre-roll, especially when editing. Five seconds is the normal time, but this can be shortened to one or two seconds when playing into a studio. Beware of shorter pre-rolls if music is being reproduced as significant ‘wow’ occurs even though the pictures appear stable. About ten seconds are needed at editing, so ensure that at least this amount of stable material is recorded before calling ‘Action’.
Timebase correctors
As VTRs are essentially mechanical devices, the video signal on replay does not conform to the standards of timing stability required by the electronics elsewhere within the television system. Picture sources need to be synchronous at vision mixers (page 76).
If a VTR is required to play synchronously through a vision mixer, a timebase corrector (TBC) must be used. This corrects the timing variation produced by the VTR. It is safe to say that the more this device costs the better will be its performance. Beware of cheap timebase correctors.
It is good practice to identify your programmes or tape inserts with a clock (mechanical or electronically generated). It ensures accurate cueing and proper identification
Clapper board
Inherited from the film industry, the clapper board is as effective in identifying shots on location on video, except that the board need no longer be clapped.
Synchronising the VTR
If a VTR output needs to be combined with other video sources, e.g. another VTR in an edit suite or a camera in a studio, a timebase corrector is required. The timebase corrector (TBC) needs to be referenced to local pulses and in turn provides an advanced reference to the VTR so that the video output arrives at the mixing point at exactly the right time. Professional machines have TBCs built into the machine.
Non-broadcast VTRs need a separate TBC which may be more expensive than the VTR.