Patent GB-1191857-A (GB)
Title: Method of and Apparatus for Recording Information on Photographic Films
Inventor: JOHN ROBERT SAWYER
IPC: G11B7/003
Filed: 1967-08-29
Abstract: 1,191,857. Optical recording and playback. BELL & HOWELL CO. 29 Aug., 1967 [30 Aug., 1966], No. 39397/67. Heading G5R. [Also in Division G2] A method of optically recording and reproducing information using a film record carrier of the type having at least one interior layer of photosensitive material comprises recording the information on the interior layer of the film but not on the exterior layer, scanning the exterior layer, and also scanning both layers to derive respective signals, and subtractively combining the signals to obtain an output signal representing the recorded information. The invention extends to apparatus for recording and reproducing information by the method above described. The purpose of the invention is to eliminate from the reproduced information the noise caused by the presence, on the exterior surface of the film, of scratches and adherent dust particles. Thus the signal resulting from the scan of both layers is made up of noise and information components, while the signal from the scan of the exterior layer only is a noise signal only. By subtracting this from the first signal, an information signal free from noise is recovered. As shown, the record carrier is a conventional colour transparency cinematograph film having blue, green and red layers 11, 15 and 19, Fig. 1, respectively, with intervening filters 13, 17, respectively, all on a base 21. Preferably the film base is relatively hard, but it may be soft, or omitted altogether. Fig. 2 illustrates variable-density sound recording, where the intensity of light from a lamp 35 illuminating film 39 through a slit plate 36 is modulated by amplified sound from a microphone 31. A filter 37 prevents blue light from reaching the film, so that information is recorded on the red and green layers only. The filter 37 may be replaced by a red or green filter, so that information is recorded on either the red or the green layer only. In a modification of Fig. 2, two slits are provided in the slit plate 36, the filter below the first slit is omitted, and above the second slit a second lamp is provided which either emits blue light or is associated with a blue-passing filter. The second lamp is connected to a separate voltage source and is of much higher intensity than lamp 35. Also, it acts on the film after the information has been recorded and serves to over-expose the blue layer and thus eliminate the signal from that layer. Fig. 4 (not shown), illustrates variable-width sound recording analogous to Fig. 2. Fig. 5 illustrates a modification of Fig. 4 where, after information-recording by apparatus 31 to 53, the blue layer of film 39 is over-exposed by light from a blue lamp 43 to eliminate the signal from that layer. For playback, the film may pass beneath two spaced illuminated slits; the light passing through the film from the first slit is transmitted through a blue-passing filter beneath the film before reception at a photodetector; the light from the second slit passes through the film to a second photodetector. The output from the first photodetector is passed through a timedelay circuit to one input of a differential amplifier, while the output from the second photodetector is passed directly to the other input of the differential amplifier. The output of the differential amplifier provides the information signal. For playback according to Fig. 7, light beam 87 passes through film 39 and impinges on a beam splitter 89, providing two beams 90 and 92; of these, beam 92 passes through a bluepassing filter 69 on its way to photodetector 67. Thus, photodetector 65 is modulated by noise and information signals, while photodetector 67 is modulated by noise only. The circuitry illustrated acts as a noise subtractor, so that the information signal appears at output terminal 97. The invention may be applied to the recording of sound on film which includes picture images, but if a "sound-only" record is required, only a single interior layer (green or red) is needed, and the full width of the film may be used. The photosensitive material may be in the shape of a disc instead of the shape shown.