BBC Engineering September 1973
|From Manual Splicing to Time Code Editing
G. R. Higgs, B.Sc(Eng.)
Video Tape Editor. Television Recording Department
The development of video tape editing from physical splicing of the tape to electronic editing of programme material was straightforward and understandable, but the continued development of electronic editing is perhaps less straightforward. The aim of complete control and total frame accuracy has now been achieved with the introduction of time code as applied in the most recent equipment, and this has inevitably led to greater complexity. Time code has, however, other uses and is applied over a wide range not necessarily concerned with editing. Developments in video tape editing systems are still in progress with an ever-widening choice of hardware appearing on the market, and this article traces the course of these developments in the Video Tape Section at the BBC Television Centre in London.
|Physical splicing of tape
Video tape recording of television programmes in the BBC dates from about 1958. The early requirement was rapid turn round of material (e.g. for sports programmes covering more than one event) or recording of programmes at times more convenient than their scheduled transmission hours.
Naturally it was not long before the desire to modify the existing recording before transmission initiated the first steps in 'editing' of the video tape. These first edits were made by cutting and joining the tape during black level - typically a fade down (edit), and fade up - and were not undertaken lightly. With the crude facilities then available an edit could take up to half an hour and was an irreversible operation similar to cutting film negative.
Techniques and equipment improved and editing a picture to make a 'camera cut' became commonplace. On video tape, the sound and vision are physically displaced by 15 frames (0.6sec) with the vision head preceding the sound head as the film gate precedes the sound head in a 'comopt' or 'commag' film projector. The 0.6sec of sound track preceding the splice must therefore carry the sound associated with the picture following the splice; this section of track can be transferred by copying it on to a separate ¼ in. tape and re-laying it on the sound track of the video tape preceding the cut. This is possible because a broadcast video tape machine can record 'audio only' on the sound track while replaying the vision.
This facility has other uses as may be seen later.
There was a period of consolidation and extension of these techniques to a point where complex operations were undertaken by physical cutting and splicing of the video tape. Programmes began to be written, planned and recorded for editing to create production transitions not possible with 'live' or 'non-stop recording' productions.
Physical edits are butt joins made using a specialised metalfoil adhesive tape about ¼ in. wide and only 0.00025 in. thick, positioned across the back of the join. With modern microscope splicing blocks, joins were regularly made with normal accuracies of about 00005 in., positioned during the field blanking period of the recorded signal.
As manufacturers improved the quality of the machines, the video head assemblies became increasingly more delicate and sensitive, and the thickness of the splicing tape at physical joins now became a significant factor. Good edits would replay satisfactorily but problems were encountered with their ability to withstand many playings over a period of time. Slight disturbances were also sometimes noticeable on copies of programmes at points where the original had appeared to replay an edit quite satisfactorily.
The advent of colour increased demands on the system and reduced tolerances still further.
During this period some experience was being gained with an electronic editor fitted to a monochrome machine. This was very much aided by the interest of one or two drama directors keen to make it work and willing to accept the inevitable development problems and set-backs.
| Electronic editing
A normal machine switched to record at a given point during replay produces a totally unacceptable disturbance (loss of vision, syncs and colour) for anything from 2 to 10sec. If it can be made to switch from replaying to recording in such a manner that the resulting transition appears as a camera cut, an electronic 'in' edit will have been made. The device added to a machine to achieve this is called an Electronic Editor (Ampex) or Electronic Splicer (RCA), and it controls the transition in such a way that erasure, servo switching, signal switching and audio changeover take place in the correct sequence at precisely the correct times to produce on the tape a 'synthetic' camera cut. The process can be reversed to produce an 'out' edit-a transition front recording to replay of material already recorded on the tape.
Electronic edits immediately dispose of:-
(a) splicing of tape and associated tape handling problems,
(b) the problem of staggered vision and sound heads. Since the electronic edit is time-controlled the vision and sound
changeovers can be initiated simultaneously.
The electronic editor demands first-class performance from virtually all the machine's sub-sections: the signal system, audio system, control system and particularly the servo system. It can be set to control both sound and vision together or 'vision only', leaving the sound unaltered.
|Positioning the Edit Manually
Electronic editing is usually carried out during the copying of the originally-recorded material on to a fresh tape so as to assemble it in programme order. Editing from live sources or others not on tape is done, but not commonly. Timing the incoming material is normally achieved by setting back the editing and replay machines the same time from the chosen transition points and starting both simultaneously.
The Electronic Editor has a turn on (or turn off) sequence lasting 15 frames. When an 'in' (or 'out') edit is initiated the actual transition on the tape occurs 15 frames later. On an 'in' edit this arises because the existing material must be erased from the tape before new information can be recorded and the video erase head is situated 15 frames upstream from the video record/replay head assembly. Thus initiating the 'in' edit immediately turns on the erase: the wiped tape moves on, and 15 frames later video recording - and sound if required - begin when the wiped tape reaches the recording heads. For an 'out' edit the erase is turned off immediately but the record,/replay head must go on recording until the 15 frames of wiped tape between the two heads have been filled in, and the record/replay head switches to replay at the moment when the 'old' recording reaches it.
Early electronic edits were positioned by marking the back of the tape 15 frames before the required edit point and pressing the record button as the mark reached the vision head. A rather hair-raising experience at the time but nevertheless sequences such as complete intercut telephone conversations were edited in this manner! A rehearsal of the proposed edit could only be made by observing two monitors or by switching the monitoring at the proposed point. After a satisfactory 'rehearsal' the process was repeated but with the record button pressed 15 frames early!
Control of the Electronic Editor
The first 'programmed' electronic editing on BBC video tape machines was 'Editsure'. Both machines were set back about 20sec from the desired points and started simultaneously. Editsure counted reference frame pulses and after a count of 485 started the Electronic Editor. The 'in' edit then occurred exactly 20sec from the start point. The Editsure control panel contained 'Play', 'Rehearse' and 'Edit' buttons. If 'Rehearse' was pressed rather than 'Edit' then only the monitoring (vision and sound or vision only as required) was switched at a count of 500.
This system gave repeatability on each run-up (within the I or 2 frame tolerance of machine lock-up) and was a great step forward. The system also provided that when the Editor was in the 'Vision only' mode it was possible to initiate' sound record' manually. Artistically this was a great improvement. Many of the problems of VT editing are concerned with the sound track and there are numerous occasions when the desired sound transition point is not coincident with the vision. 'Out' edits could still only be made by pressing the editing machine 'stop' button whereupon the 'out' edit occurred 15 frames later.
Editsure was extensively used, particularly on the 405-line system. On this system the video tape cue track was not available because the tape width was required for video information. This arose due to the longer line time of the 405-line system (98usec as against 64usec) coupled with the poorer stability of the video head assemblies and associated servo systems. With the change to 625-lines and more stable machines the cue track became available for clean effects, guide tracks or control signals for electronic editing purposes.
The device called Editec, which was marketed by Ampex Corporation, was the first commercially-produced means for controlling the Electronic Editor to enable the edit point on the edit tape to be made precisely on the desired frame. Editec achieves this by storing edit-point information in the form of edit cues recorded on the cue track. It is important to realise that Editec controls the Electronic Editor; it does not directly control the machine to make the edit. The cues are l0msec bursts of 4kHz tone (i.e. 40 cycles at 4kHz) and are keyed onto the cue track by pressing a cue button. They are recorded by the normal cue track record/replay head at a time coincident with the required edit point. Replayed by this head they would be useless for initiating Editor turn-on since the edit would occur 15 frames later. They are therefore read off by an auxiliary 'cue read-off' head situated 'upstream', just before the video erase head assembly. This is the only practical position but means the cues are now read off 33 frames before the point in programme time at which they were recorded. Therefore after cue read-off Editec counts 18 frames then initiates the Editor turn-on cycle. Fifteen frames later the edit occurs precisely at the required frame, that is just as the edit cue reaches the cue track record/replay head.
As a by-product of Editec having to 'mark time' for 18 frames, the edit point can be altered to occur earlier than the time defined by the edit cue, without changing the cue position on the tape. If Editec is set to minus 7, it counts only 11 frames before starting the Editor, the edit occurring 26 frames after cue read-off instead of 33, that is 7 frames early. The maximum shift possible is obviously minus 18 frames. For operational symmetry Editec can also delay the edit up to 18 frames. There is of course no technical reason why the delay cannot be as long as required.
With Editec a second cue can control the Editor turn-off in such a way that a new piece of vision (and sound if required) can be 'inserted' into existing material.
Editec has various facilities. Rather than merely altering the edit point relative to the cue one can actually erase the cue and record a new cue at the new edit point selected. Cues can be selected out of a series of cues, selectively erased, or made to operate external equipment, and the machine can be set to record automatically a sequence built up from any number of 'takes' of anything from 1 to 36 frames duration. After the machine stops and is rewinding for the next 'take', the picture determining the vision input to the machine is altered to produce the required effect in the 'animation'. As each pass takes about 20sec, a I min animated sequence made 8 frames at a time takes just over an hour. If the machine is manually restarted each time, of course the time taken becomes indeterminate.
The manufacturers of 'On time' (EECo - Electronic Engineering Company) developed a system whereby the replay machine is control led during run-up to ensure that both replay and edit machines reach their selected frames at the same time. With hindsight this may seem a rather simple, fundamental requirement but its realisation involves logic of considerable complexity. Its eventual development can be ascribed to:
External operations during editing
Other Applications of Time Code
|Use of Video Disc in Editing
The facility of slow motion (forward or reverse), freeze and (with certain limitations) up to double-speed replay from the video disc make it a powerful addition to video tape editing capabilities. The BBC Video Disc control logic has been extended in such a way that it can be operated by external command signals. These are distributed by the local remote control facilities system. With Editec this means starting the disc at any pre-selected speed by commands originating at either cue read-off or edit-point time. Start time can be varied over a range of up to 10seconds by a variable delay incorporated in these modifications.
A time-code editing channel can, by nature of its equipment, originate commands through the remote control system at any time, independently of edit points.
A second development of the BBC video disc is a 'field counter'. The fields recorded can now be precisely selected and the video disc can always be recued accurately to any one of the 1800 fields (36sec at normal speed) recorded. These features mean that the disc can match the accuracy of time code, making possible repeatably precise tape/disc edits. The facility to make subsequent changes in the disc operational mode is now under development. These commands maybe originated from an editing channel or the field counter itself to reverse or freeze.
The channel also has a black edge caption facility which might be extended from its present manual 'cut in', 'cut out' operation to external control as above.
Imaginative use of the video disc and facilities allied with mixing during video tape editing allows a wide range of effects and sequences to be achieved.
As stated earlier the 'continuous information' aspect of time code gives rise to many benefits and may be expected to give rise to more in the future.
Already centralised time-of-day code can be used to start programme transmissions precisely. In the United States time code is used to control programme junctions with automatic slide changing, mixing, overlay, TK & VT running etc., to create complex presentations far too involved for manual, real-time operation.
Systems available now dispense with pre-wired programmers and fixed options by using computer control of equipment to instructions given by keyboard and light pen on a visual display unit. The computer editing programmes are on computer tape, thus extending operations means only playing-in an extended programme. The unit can ask for decisions and point out errors as required. Additionally all edit decisions made can be stored and recalled. This at present is the limit of 'on-line' editing.
'Off line' editing involves making programme decisions using low-cost equipment such as helical scan machines or monochrome `disc packs'. The decisions made are stored on punched paper or magnetic tape that is used later to control assembly of the programme on broadcast VTRs. This moves the decision making time off broadcast VTR channels. The disadvantages of having to pre-load these units with material, the obvious need to rehearse or simulate completely the intended edit coupled with the great flexibility to which producers are accustomed (particularly with respect to sound track editing) give rise to much duplication of effort and equipment. They are perhaps more suited to short, intensivelyedited sequences, such as titles, special effects, and commercials.
Video tape editing can be seen to be reaching a stage where production requirements are no longer restrained by equipment capabilities but by equipment capital cost. Limitations are no longer intrinsic in the medium; only in the cost of the time which it is realistic to expend on a programme.
The development of ever-improving small, light, high quality cameras, recorders and less expensive format machines promises to widen the horizons of electronic programme production. It seems likely that video tape editing in the BBC will evolve further in the mixture of 'on' and 'off' line techniques that has already begun. This suits the nature of our requirements and makes possible many specialised services and flexible combinations of these to master the occasional unique problem' programmes.
Inevitably video tape must cease to be regarded only as a technical production facility and come to be considered a creative medium.