CLEAN-EDIT FUNCTIONS
In a basic videorecorder, invoking the pause function will merely stop the tape transport. When the pause key is released once more it is not possible for the tape to instantaneously attain normal speed, so that in effect, video tracks and (where applicable) control tracks on the tape become ‘bunched up’ at the pause point. The result is
particularly noticeable on record pause because during subsequent replay severe picture and sound disturbance results from the mistracking and momentary servo instability at the point where the tracks are disordered. At best, a second or so of programme is lost while the blanking system waits for a new servo lock.
To prevent these effects home-base videorecorders have a syscon equipped for clean assemble edit. This normally takes the form of a ‘back-space’ system, as illustrated in Fig. 16.7. The upper shaded section of the diagram represents the tape travelling towards the right in record mode. At point (A) the pause key (start-stop key on camera) is pressed, whereupon the tape rewinds for about 1–1.6 seconds then comes to rest in the record-pause state (B). It is still laced up. When the next recording segment is ready (scene changed for camera, com- mercial break finished, or alternative video source selected from tuner or auxiliary video player) the pause key is once again pressed (C). Tape transport restarts immediately but the syscon does not yet switch the electronics to record. A ‘play’ period (D) of up to one second is permitted, during which time the capstan runs up to normal speed and its servo attains lock. At this time the capstan servo is directed by the syscon to lock incoming vertical syncs to existing replayed control track pulses in readiness for the changeover from old to new programme material.
A few frames (tracks) before the end of the old recording a change- over to record mode (E) is made in signal circuits, servos and control- track pulse routeing, and this is arranged to take place during a field blanking period. The result is a completely smooth changeover from old to new material, with regard to both video tracks and control pulses. The presence of old and new tracks simultaneously on tape momentarily can give rise to crosstalk, especially where the old scene is highly coloured or contrasted and the new one lower-key. For the
short period of over-recording the luminance writing current is increased by about 2 dB in order to provide a more effective erasing action – the full-width erase head is too far removed from the drum to take any part during this period. The resulting edit is very effective, though under some circumstances a burst of spurious colour appears momentarily at the edit point – at low colour-under frequen- cies the recording-erase process is less effective. Some domestic machines incorporate separate flying erase heads on the drum itself for use during in-camera assemble edits.
Although the backspace record-pause cycle takes less than two seconds in total, the number and speed of output manipulations by the syscon at this time are many and high. In a typical machine the following parts are accessed and instructed at least once: loading motor, mode switch, reel brakes, capstan motor, servo circuits, recording f.m. amplifier, video signal routeing, user indications and (where applicable) flying-erase head switching.
Insert edit
The process just described will give continuity of replay servo synchronisation in circumstances of assemble edit, where each new sequence follows on from the previously recorded material, i.e. when the camera stop/start trigger is used to select each ‘live’ sequence in turn; or in a post-production situation where a master recording is being assembled from one or more 1st-generation tapes.
Where it is required to modify an existing recording by inserting new material, the method is not effective at the end of the inserted section because video and control tracks will be disordered at the point of reversion to the original material. The insert-edit system is designed to overcome this problem. Here only the video and audio tracks are erased during the insert recording, leaving the original control track to ‘master’ the head-drum phase and capstan speed. The continuity of the control track ensures a disturbance-free transition at each end of the inserted section.
In formats using tracking tones within the vision tracks, other methods are used to achieve edit control.