|One aspect of the televising of the 1966 World Cup football matches that attracted a good deal of interest was the ease and speed with which slow-motion recordings of the highlights were presented, sometimes only seconds after the event. This article describes how the complex equipment that achieved these results was developed and built in a relatively short period.|
|A SLOW - MOTION VIDEO TAPE
by P. RAINGER, B.Sc., C.Eng., A.M.I.E.E.
|IT IS well known that television standards in the United Kingdom provide 50 fields /s; in contrast with film, which has only 24 fields (or rather frames), there is sufficient information to permit the slow-motion reproduction of a recording obtained from a standard television camera. There have been a number of attempts to make slow-motion replay equipment. A Japanese system was developed and used for the 1964 Olympic Games, but, as this was designed for operation on 60 field standards, it is not applicable to the United Kingdom 50 field system. The machine was also large and complex. Other systems have been tried abroad with varying success. It was therefore suggested that some form of slow-motion video tape recorder (v.t.r.) should be constructed in Britain, using available standard v.t.r. equipment and employing techniques which required the minimum of new development. In this case, maintenance would be assisted by the spare parts and the extensive experience which existed for basic machines of this type.
|Standard record and replay|
|After some consideration, it was decided that the ideal machine should be able to replay in slow motion standard transverse-track 2in tapes. The machine should have the following facilities :
Simulated slow-motion reproduction was obtained by step-printing motion-picture film. The results were studied with a view to determining what degree of slow motion could be tolerated without causing the signal to appear to be unduly jerky. Our conclusions were that a slow-motion reproduction of one quarter to one fifth of the normal speed would be valuable as a programme facility; this only exhibited a jerky motion on a few very difficult pictures. Although the Japanese used a 5:1 slow-motion reproduction, they were able to work on a 60 field/s television system; therefore it seemed reasonable to settle on a 4:1 slow-motion reproduction in the United Kingdom.
Preliminary studies also demonstrated clearly that the repetition of a complete picture of 625 lines gave an unacceptable result, and the slow-motion equipment would have to operate on single fields, i.e. 312½ lines. A study of the technical difficulties which may arise suggested that both 4:1 and 5:1 reproduction could be obtained equally easily, but that it would be relatively difficult to change the speed of reproduction once the design had been completed. It was therefore decided that the machine should be designed to produce a fixed 4:1 reduction in the speed of movement.
In order to obtain a 4:1 slow-motion reduction, it was proposed that we should replay a standard tape in an intermittent manner. The signal leaving the video tape recorder would therefore follow the cycle shown in Table 1.
|Table 1. Record-replay cycle for v.t.r. and disc store|
gaps in the Table are filled with a signal from the magnetic-disc field
store; the cycle involves the use of two heads and two alternate tracks
on the disc|
It was further proposed that the gaps in the sequence shown in Table 1 should be filled with the signal from a magnetic-disc field store. The field leaving the video tape recorder was therefore recorded on the disc store and repeated a total of four times before the next field arrived. In order to achieve the record, erase and replay cycle for the disc store as detailed in Table 1, it is necessary to use two heads and to record two tracks alternately on the magnetic disc. In order to minimise the cyclic variations which were likely to occur, all four fields would in fact be obtained by replaying from the disc store; the original signal from the v.t.r. is not seen at any part of the cycle.
The only mechanical work carried out was the modification of the v.t.r. to provide intermittent motion of tape. It was decided to use a series of tape rollers (Fig.1) to guide the tape into an omega-shaped path. In order to reduce the inertia of the system to a minimum, the tape guides are pumped-air bearings and do not rotate in any way. The two guides B and B' are linked together and attached to the arm A, the guides E and F being stationary. The tape is pulled by the conventional capstan mechanism at one quarter of its normal speed, and the arm A oscillates backwards and forwards, driven by an eccentric coupled to a synchronous motor. This synchronous motor is driven by a 12½ c/s signal, so that, when the tape velocities due to the combined action of the capstan and the oscillating arm are in the same sense, they add together, to cause the tape to travel past the video heads at the required normal speed of 15 in/s (Fig.2).
In order to replay satisfactorily at least one complete field each cycle, it is necessary to ensure that the tape velocity remains correct over this period of 20ms and is suitably phased, i.e.'tracking' the recorded signal. The eccentric attached to the driving arm A causes a sinusoidal displacement of the tape, and the quasilinear-motion (constant-velocity) part in the centre of the sinewave is used to produce the required signal. In practice, the linear portion of the sinewave does not extend far enough, and a simple 'following-link' accelerator is used to couple the motor to the eccentric. This expands the linear portion of the tape motion to include a number of lines over and above the required field. This is necessary, because it is of the utmost importance that the field-synchronising waveform leaving the video tape recorder should be complete, as this is used to control many other functions in the record-replay process of the slow-motion machine.
was also necessary to keep the inertia of all oscillating parts to a
minimum, and the rollers E, F, B and B' were constructed as fixed guides,
lubricated by a supply of air fed to each bearing surface. The original
design was based on the use of a 20lbf/in² air supply to feed the
air bearings. In practice, the tape lifted off the rollers with only
11bf/in², and the normal operating pressure was adjusted to be
5lbf/in². The only other mechanical alterations to the v.t.r. machlne
involved mounting the control panels and fault indicators for the complete
equipment alongside the normal v.t.r. controls and moving the two tape
spools further apart, to make way for the slow-motion mechanism.
As a result of this switching operation, the output from the field store consists of four even and four odd fields alternately. The field timing at this stage should be a regular 50c/s, but the line rate would have a phase discontinuity or jump of 180° at the beginning of some fields. In order to produce a standard waveform, it is necessary first to ensure that the video signal is based on a line-synchronising signal of a continuous nature. It is therefore necessary to insert a ½ television-line delay in the rather curious cycle shown in Table 2.
|Table 2. Timing cycle of switches S2 and S3|
The half-line delay is simulated by the alternate insertion of a 1-line delay and a 1½-line delay.
practice, a satisfactory half-line delay was not available, and this
effect had to be simulated by alternately inserting a 1-line delay (lH)
and a 1½-line delay (1½H). Large bandwidth quartz delay
lines were employed, operating at 30 Mc/s, and it was necessary to construct
frequency changers to move the f.m. signal up to this band and back
mechanism can give a most interesting picture, providing a new insight
into many sporting events. Although there is some loss in resolution
and some degradation of the signal/noise ratio, these would not appear
to be major problems. Successful operation of this machine depends on
obtaining in the first instance a picture which contains sufficient
information. As photographers well know, exposures of 20ms are not short
enough to stop very rapid motion. The television camera cannot have
exposures shorter than this duration without sacrificing sensitivity,
and all cameras in use today have an exposure time of 40ms in theory,
although in practice the effective exposure time lies between 20ms and
a few hundred milliseconds. Slow-motion reproduction is used to study
fast action. However, fast action will involve movement blur, which,
although acceptable when seen for a fleeting moment, can be quite disturbing
when studied at leisure. Defects of this sort are fundamentally unavoidable
if we use standard television cameras. An improvement in this respect
would require the use of a special slow-motion film camera or the equivalent
nonstandard television camera.|
Similar, but not so important, difficulties occur when using the 'freeze' facility, because, when all movement ceases, the frozen picture is sometimes quite difficult to interpret. The successful operation of this equipment also depends on the synchronous operation of the video tape recorder, oscillating arm and field store. In particular, synchronism must be maintained between adjacent fields to an accuracy within a few tens of nanoseconds. It was hoped that the inertia of the video tape recorder and the field store would be sufficient to maintain reasonable timing at least over the short period of one field, i.e. 20ms.
The experimental results obtained so far suggest that, while it might be possible to reduce the timing errors on the v.t.r. to about l s per field, the field store has much larger variations in its speed. These variations are such as to cause a discontinuity of line synchronism at the beginning of the field, which is not within the range of correction by the variable-delay devices often associated with video tape recorders. In practice, the signals would be acceptable for most driven timebases, but some 'flywheel' time-bases with long time constants would be disturbed in an objectionable manner.
It was therefore necessary to resort to standards-conversion techniques, to ensure that the output synchronising waveform was beyond criticism. In practice, it was most convenient to use an optical standards convertor (i.e. one using a display and camera combination) for this standards-conversion process; the movement blur normally associated with convertors of this type is of negligible importance, because slow-motion techniques have reduced the speed of movement to the point where the convertor can contribute no further loss. There are, nevertheless, the normal losses of resolution etc. associated with this method of standards conversion.
mention has been made of the deficiencies and errors in the vertical
resolution due to creating a sequence of four odd and even fields from
one single field, i.e. 312½ lines. Techniques used in line-store
standards convertors and similar electronic devices have shown that
suitable interpolation between lines can give a much improved result.
Construction of a fully interlaced picture using these techniques has
not been employed in this device. The result is an apparent vertical
hop of fine-detail picture information, which, although somewhat disturbing
on test waveforms, is acceptable on all practical camera signals. The
storage capabilities etc. of the optical convertors employed with this
equipment do much to remove this difficulty, even on the most difficult
credit is due to the engineers who assisted in making this relatively
complex equipment and completing the project in about nine months. In
particular, I would like to acknowledge the efforts of D. P. Robinson
and P. White of the BBC Designs Department and their colleagues throughout
Electronics & Power November 1966 pp 386-389