Patent GB-491812-A (GB)
Title: Sound recording and reproducing apparatus
IPC: H04R17/04
Filed: 1937-03-12
Abstract: 491,812. Phonographic recorders and reproducers. PROCTOR, B. A. March 12, 1937, No. 7331. Convention date, March 26, 1936. [Class 40 (iv)] [Also in Group XL] In a piezo-electric sound-recording or reproducing device in which a crystal unit is torsionally operated by electrical currents corresponding to the sound to be recorded, or to produce electrical variations corresponding to the vibrations recorded on a disc, the unit is supported at one end in a fixed position and is connected at the opposite end to a pivot rotatably supported in line with the torsional axis of the unit. The supporting and clamping means at the fixed end are constructed and arranged so that when the crystal has been clamped into position, it is in the condition of no strain when electric potential is not applied to it and is able to respond to an equal degree in each direction when operating potentials are applied. The crystal unit is supported in a holder H having an upwardly extending lug 11 attached to an arm (not shown) which carries the recorder or reproducer. The holder H has a block 14 supporting the clamping means engaging one end of the crystal unit which comprises slabs c<1>, c<11> of Rochelle salt. The crystal unit is supported at that end by a bracket 16 attached to the block by screws 17 and having an inwardly turned portion 18 provided with a pin 19 engaging holes in the elements 21 of a leaf spring 22, attached to an elongated top plate 23 having the shape of an inverted-U which abuts against the lower face of the crystal assembly. After the end of the crystal unit carrying the stylus has been clamped into position, the right-hand end takes up a position corresponding to that of no strain and screws 24 mounted in the block are rotated until they just touch the upper surface of the crystal unit. A screw 100 mounted in the block and engaging an inwardly-projecting portion of the bracket 16 is adjusted so that the clamping means is caused to compress the element against the screws 24, the desired pressure to give the maximum response being determined by an electrical measuring apparatus. A pad 9 of felt or other material is interposed between the ends of the crystal elements and the bracket 16. Terminals 28, 29 mounted on an insulating shell 10, are connected to metal foils c located between and on the outside surfaces of the slabs c<1>, c<11>, by strips 26, 27, the former being provided with fingers 30, 31 and the latter having a finger 32 engaging the foil c between the crystal elements c, ell. The end of the crystal unit carrying the stylus S is mounted in a T-shaped clamping member 33 which is divided in a horizontal direction at its outer portions, as indicated at 38, and through which pass clamping screws 39. The clamping member 33 carries a downwardly extending portion 12 provided with two sockets 45, 103 by which the inclination of the stylus can be varied according to the material of the disc on which the sound is to be recorded. The stylus is clamped by a set screw 46 passing through the front wall h of the holder H. The clamp 33 is provided with a ring 40 to which is secured a pivot P extending through the front wall h and engaged by a plunger 41 which is adjustably held against the pivot P by a spring 43 and screw 44. Plugs 49 of damping material inserted in sockets 50 in the block 14 are adjustable relative to the member 33 carrying the stylus by screws 51. A spark gap, indicated at 101, bridges the terminals 28, 29 to protect the crystal from the effects of accidental surges of high potential. The crystal elements are enclosed in a blanket B of sound-deadening material, such as rubber. Electrical variations of a frequency corresponding to the mechanical resonance characteristics of the device are damped or suppressed by a filter circuit comprising a shunted choke in series and a tuned circuit in parallel with the output of the amplifier, as indicated in Fig. 6 (not shown). The Specification as open to inspection under Sect. 91, describes an arrangement for hill-and-dale recording in which a crystal unit is secured at three of its corners and the stylus-holder is mounted at the free corner of the unit. This subject-matter does not appear in the Specification as accepted.