Patent GB-300913-A (GB)
Title: Improvements in systems for reproducing sounds from sound records
IPC: H03G3/20
Filed: 1928-11-19
Abstract: 300,913. British Acoustic Films, Ltd., (Assignees of PouLsett, A., and Petersen, A. C. G.). Nov. 19, 1927, [Convention date]. Optical apparatus. - Relates to a method of optically recording and reproducing sounds, of the kind in which, for economical and efficient working, an intentional distortion is introduced in recording, by diminishing the amplitudes of the low frequency vibrations on the record, relatively to the amplitudes of the higher frequency vibrations,. compensation for this distortion being effected by emphasizing the lower frequencies during the reproduction of sound from the record. According to the invention, the necessary compensation is effected by using in the reproducing apparatus a salenium cell connected in the grid circuit of an amplifying valve, the cell having in circuit therewith, in addition to a current source, an impedance which is small relatively to the ohmic resistance of the selenium cell, and which comprises an inductance and an ohmio resistance of the selenium cell, and which comprises an inductance and an ohmic resistance of suitably chosen valves. For producing the photographic record having the distortion referred to above, a condenser microphone C, Fig. 1 (or any other suitable microphone having a condenser in series therewith) is inserted together with a battery b in the grid circuit of a valve t, and a resistance R is connected across tha grid circuit as shown. The valve operates in known manner any suitable device m such as an oscillograph, which records the sound on a film f. Fig. 2 shows an arrangement in accordance with the invention for reproducing from the record, comprising a selenium cell s on to which light falls after passing through the film f, the cell being inserted in the grid circuit of an amplifying valve T in series with a battery B and a blocking condenser k. An inductance L and an ohmic resistance r in series therewith are connected as shown across the grid circuit of the valve for the purpose of compensating for the distortion introduced in the recording. It is shown in the Specification that the relationship between the values of the resistance r, the inductance L, the resistance R and the capacity C used in the reproducing and in the recording apparatus L should be determined by the formula r = --. R.C Specification 221,817, [Class 40 (iv) Telephones &c.], is referred to. The Specification as open to inspection under Sect. 91 (3) (a) comprises also the, following matter. To prevent distortion arising through. capacity effects &c. due to the use of long leads in the reproducing apparatus, the arrangements. shown in Fig. 3 (Cancelled) may be adopted. The selenium cell s is inserted in the primary circuit of a step-down transformer O, which is connected by leads 1, 2 to a step-up transformer O<1>. The cell s is in this case connected through the battery B to earth but in a modification (Fig. 4 (Cancelled), not shown) the selenium cell is connected direct to earth, and the transformer O<1> is connected through the resistance r and the battery B to earth. The cell and the transformer O may then be enclosed in a conductive case, preferably in a double-walled case of copper and iron respectively. This subject-matter does not appear in the Specification as accepted.