Patent GB-1133480-A (GB)
Title: Phonograph apparatus
IPC: G11B7/12
Filed: 1966-11-15
Abstract: 1,133,480. Optical system for sound reproduction from grooved records. P. B. DAHLEN. 15 Nov., 1966 [8 Dec., 1965], No. 51223/66. Heading G5R. [Also in Division G2] An optical system for reproducing sound from a grooved gramophone record comprises a first system for focusing a narrow beam of light on the surface of a record and a second system which conveys the reflected optical signal to an electronic circuit which supplies a loud-speaker and controls the position of the tone arm. The optical system, Fig. 2, includes a light source 13 and a collimator 14 which sends a narrow, substantially parallel beam of light through prisms 15 and 16, and the objective lens 17 to the surface of a record 11. The prism 21 is situated partly in the path of the beam of light incident on the objective lens so that the left hand side of the latter is in the shadow of the prism 21. The light reflected from fine groove records has three components, one reflected from the flat surface of the disc, a second component being multi-reflected polarized light and a third component which does not pass back through the objective lens and is lost from the system. The first two components pass through the lens, through the prisms 21 and 22, a filter 23 which removes the polarized light, through prisms 24, 25, 26 and microscope 27, through prism 28 to eyepiece 29 which focuses the light on a light mask 35 after passing it through a prism 32. The mask 35 comprises the ends of two fibre optic rods 37, 38 separated by a micro-fine junction 36, the other ends of the fibre rods being connected to light sensitive devices 41, 42 which convert the optic signals into electric signals. In the image of the record, Fig. 5, the dark areas represent the grooves and the light areas the flat surface of the record. The image of the groove passes under the optical mask 35 and the different amounts of light falling on the ends of the two optic rods cause different magnitudes of current to flow through the phototransistors 51, 52, Fig. 8 (corresponding respectively to light sensitive elements 41, 42). The outputs of the phototransistors supply the amplifiers 57, 58, which themselves supply lamps 61 and 62. The light from lamps 61, 62 is picked up by photo-cells 63 and 64 respectively, these forming two arms of a Wheatstone bridge, the element 75 of which controls the position of the tone arm so as to tend to make the arm follow the sound track in the record. In a monophonic system the output of one of the amplifiers is fed through a capacitor 59 or 60 to a loudspeaker. In a stereophonic system the optical signals in the fibre rods are different, each representing one of the sound tracks as depicted in Fig. 6 (not shown) so both amplifier outputs are used. In wide groove records a light signal 12a, Fig. 7, is reflected from the bottom of the groove and the signal is fed through a third fibre optic rod 39, through a filter 44 to a phototransistor 45 to give a signal for sound reproduction, the lower two optic rods 37, 38 being used solely for tone arm position control. The light filter 44 is of varying opacity so that if the line 12a falls on one side of the end of the fibre rod 39 the image passed to the phototransistor 45 will be strong but if the signal falls on the other side it will be weakened by the filter, this difference in the intensity of the signal being converted to an electrical signal for sound reproduction.