Patent GB-482049-A (GB)
Title: Improvements in or relating to apparatus including a scanning disc for transmitting television and like purposes
IPC: H04N3/04
Filed: 1936-09-22
Abstract: 482,049. Television. GENERAL ELECTRIC CO., Ltd., ESPLEY, D. C., and WALTER, D. O. Sept. 22, 1936, Nos. 25746 /36, 27350 /36, 28409 /36, 314 /37, and 24934/37 (the fifth Provisional Specification being divided out of the first Provisional Specification). [Class 40 (iii)] The object is imaged on a Nipkow. disc by optical means whose magnification is varied in time, so that the image is smaller when being scanned by holes nearer the centre of the disc than when it is being scanned by holes further from the centre of the disc, thus compensating for distortion arising from the different linear speeds of the holes. The invention is described with reference to an interlaced scanning of films such as described in Specifications 465,184 and 466,051, in which the film is traversed at constant speed and scanned by a Nipkow disc having, e.g. two, spirals of holes ; each spiral scans the same frame in turn, the individual scans being displaced by one line to provide for interlacing. In such an arrangement, the paths traced by the holes of the inner spiral are shorter than those traced by the outer spiral, so that the alternate lines are shorter, Fig. 1b. By varying the size of the image projected on the disc, this distortion is compensated. The film 11, Fig. 3, is imaged on the disc 12 by an optical system comprising lens components 2, 4, 3. Shutters 9, 10, comprising glass segments. rotate about an axis 6 to interpose a glass plate alternately in the planes 9, 10 thus slightly altering the magnification of the system without altering its focal planes. This correction introduces a further defect ; since the magnification is varied radially of the disc as well as circumferentially, the lines produced by the inner spiral will be further apart than those produced by the outer spiral and will not satisfactorily interlace with lines produced by the outer spiral. This may be compensated by : (1) variation of the pitch of the inner spiral ; (2) variation in the speed of traverse of the film, as by cams &c. ; or (3) subsidiary optical means such as a cylindrical lens to reduce magnification the radial direction, e.g. the glass plates interposed in the optical system may be curved cylindrical lenses. The result of this compensation is a picture as shown in Fig. 2d. The keystone distortion may be removed by a mask so shaped that teh image on the disc is of keystone shape in the reverse sense. The mask may be placed immediately before the disc. Fig. 5 ; or a double mask, Fig. 6, may be placed at the gate, each of the two trapezoidal portions corresponding to a different magnification. The keystone distortion may also be compensated by changing the magnification continuously, as by tapering the glass plates interposed in the optical system. The change in the magnification produces a change in the mean brightness of the picture, and this may be compensated by : (1) use of a graded filter, e.g. a wedge as disclosed in Specification 466,780 ; (2) interposition of a filter in the optical system when the magnification is reduced, e.g. one of the glass plates interposed may absorb, the required amount of light; (3) variation in the intensity of the light source, e.g. the nicker produced by a lamp energized with alternating current may be used, when the frequency of alternation is suitably related to the frame frequency (4) variation in the size of the scanning holes ; or (5) variation in the amplification of the picture signals. Since the middle and end holes of the spiral scan the black strips between successive frames on the film, the picture gate of Fig. 6 may be made slightly smaller than two frames and may be bridged at the centre by a bar, preferably bevelled, which will hold the film flat, Fig. 10 (not shown). Thus the centre and end holes will give a black signal. Alternatively, these holes may be blocked. Specifications 464,831 and 466,966 are referred to.