Movies for TV - Early Television Foundation

KINESCOPE RECORDING 127
situation will improve. Another requirement is that the magazine
be capable of holding enough film for a half-hour show. Most of the
cameras used for 16 mm recording hold 1200 feet of film which
is sufficient for thirty-three minutes of continuous running. In the
event of a show lasting more than this time, it is a simple matter to
arrange
to switch to another machine at the end of the first reel
in the same way that a motion picture projector is switched during
projection.
But even now it is too soon to discuss the methods used, for there
are two more small matters to consider. First, due to the small
amount of time available for film pull-down, it is necessary for it
to be moved very rapidly. This means that the wear on the film is
in the mechanical de-
higher, and it also calls for greater precision
tails of the movement. Secondly, if 1 200 feet of film are used for one
take rewinding it presents a slight headache. The size of a roll of
this length is about ten and one half inches in diameter. Yet when
rewinding commences, the core or the spool center is only approxi-
mately two inches. It is at once obvious that the peripheral speed
will increase as the film winds onto the take-up due to the unavoid-
able increase in diameter. Some form of slipping clutch is therefore
required to provide a varying speed from start to end.
Practical Conversion Since a field is 1/60 of a second, it fol-
lows that half a frame is 1/120, or 72 if expressed in terms of
shutter action or blanking. A shutter which is open for 288 or 1/30
of a second is not hard to design; this leaves half a field, or 72, for
closed time. If the sequence commences with the shutter open and
an odd field is followed by an even, both will be recorded on the
first film frame. Then the shutter closes for 1/120 of a second and
the first half of the next odd field is lost, the whole of the even field
is recorded and also half the following odd field (two film frames).
The shutter then removes half of this odd field and records a full
even field plus half an odd field before it closes again (three film
frames). The fourth film frame records in this order half an even
field, a whole odd field, and half an even field the second half
being lost under the shutter. Now the whole cycle repeats itself.