Patent GB-1511009-A (GB)
Title: Sound recording on colour film by hue modulation
IPC: G11B7/24085
Filed: 1975-11-06
Abstract: 1511009 Sound recording on colour film by colour hue modulation ASSOCIATION OF MOTION PICTURE & TELEVISION PRODUCERS Inc 6 Nov 1975 [8 Nov 1974] 46048/75 Heading G5R A motion picture colour film has a sound track of constant width and including a plurality of light selective media which absorb light of different colour, each medium being uniformly distributed across the track, and the optical densities of two of the media are modulated longitudinally of the track in mutually opposite phase relation in accordance with a common sound signal, the corresponding mutually opposite density variations of said two media being superposed throughout the area of the track. The sound is thus represented as a modulation of the effective hue transmitted by the film. Three light selective media may be provided, in one layer or in respective layers, responsive to red, green and blue respectively. Single channel recording can be effected by passing a microphone 40 signal, Fig. 2, via amplifier 42 to inverter 44 giving signals on lines 46, 47 oscillating about the same reference voltage but of opposite polarity. Scanner 52 causes cathode ray tube 50 to produce a bright line which shifts from a position 54 to position 55 under control of a voltage source 57 and a switch 56 which switch passes from the output of noise reducing circuit 49 the signal on line 46 or 47 in accordance with whether the CRT line is at 54 or 55, the selected signal modulating the intensity of the CRT beam. The CRT screen may have green and red phosphors at 54, 55 respectively or colour filter 63 may be provided so that intensity modulated green and red scanning lines pass from mirrors 61, 62 via lens 60 on to colour negative 64 to give a sound track thereon hue modulated along a red/green axis. Playback of a colour positive 70, Fig. 3, corresponding to negative 64 may be effected by splitting light transmitted via film 70 into red and green components using a reflective or transmissive diffraction grating 80, photosensors 85, 86 responding to respective components. The difference signal of sensors 85, 86 formed by transformer T1 passes via amplifier 92 to loudspeaker 94, the sun signal of sensors 85, 86 being extracted by transformer T2 and passed via rectifier 90 to form a gain control signal for amplifier 92 to effect reduction of noise due to dirt and scratches. The blue layer of the film may be modulated in a conventional manner to give a second sound channel or a control signal channel. Alternatively STEREO recording may be effected by recording the left signal or a red/green axis hue modulation, and the right signal as a yellow/ blue axis hue modulation, the blue layer being modulated as one component and the green and red layer being modulated in common to give the yellow component. Alternatively red/green axis modulation may correspond to (L + R) and yellow/blue to (L-R). Noise reduction may be improved in single channel recording by dividing the sound recording area into two parallel tracks which are hue modulated along the red/cyan axis, for example, with the same sound signal but opposite polarities, the signals recovered from the two tracks being subtracted on replay, Figs. 8, 9, (not shown). Left, centre and right signals may be recorded in respective ones of three side-by-side tracks as modulations along the red/green axis in each track. To replay such a film, the single photocell of a conventional sound film reproducer can be replaced by an assembly, Figs. 10, 11 (not shown) and Fig. 13, in which the light transmitted by the film is split by a prism (164) into different colour components focused by a lens (168) on to corresponding regions of a photosensor having red 196, green 194 and blue 192 responsive sensors for each of the left, centre and right tracks. In a playback circuit, Fig. 14, differential amplifiers A1 give the difference signals of the red and green sensors of each track, such signals passing to amplifier A3 the gain of which is controlled to effect volume expansion by a signal on line 204 developed by differential amplifier A4 from the centre track blue sensor, amplifier A4 receiving via variable resistor R1 an adjustable portion of any variation in the red and green centre track sum signal for eliminating noise due to dust and scratches. The gain of each amplifier A1 is similarly controlled to eliminate noise by processing the summed red and green signal of the corresponding channel. Distribution of the left channel output between left stage and left wall speakers and of the right channel output between right stage and right wall speakers in a cinema is effected by variable gain amplifiers A5, A6 and inverters A8 receiving control signals from the left and right track blue sensors. The photosensor assembly, Fig. 13, includes an infra-red responsive sensor 198 extending across all three tracks for replay of conventional variable area or variable density mono sound via centre stage speakers when switch 51, Fig. 14, is in its upper position.