Patent GB-389116-A (GB)
Title: Improvements in or relating to voice-operated relay circuits
IPC: H04B3/20
Filed: 1931-09-10
Abstract: 389,116. Two - way transmission. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Bjornson, B. G. ; 57, Horatio Street, New York, U.S.A.) Sept. 10, 1931, No. 25373. Convention date, Sept. 12, 1930. Addition to 348,704. [Class 40 (iv).] In the voice-operated relay circuit described in the parent Specification wherein a relay is controlled through tandem detectors, the second of which is selectively responsive to syllable frequency current, the relay is rendered operative in response to excessive or sustained voice frequency currents of high-energy level by means associated with the first detector. Moreover, the second detector comprises two units of different sensitivity and provision is made for rendering the relay inoperative independently of the first detector when the relay is employed in a two-way system. As shown in the Figure, whenever speech arriving over line L opens a path for itself at contact 20 but noise current is ineffective, part of the voice currents are shunted through the tuned circuit 25, amplifier 26, and detector 28 to a filter 30 adapted to pass only currents of pitch frequency (75-315 cycles). The detector 31, which may be of the type described in the parent Specification, provides a cut-off at about two cycles and the filter 33 passes currents below 25 cycles only so that currents of syllable frequency are impressed on the detector 35, 36. The detectors 35, 36 are of different sensitivity, the arrangement of grid-biasing being such that detector 35 operates quickly to avoid initial clipping of the speech syllables and the detector 36 is less sensitive and operates only during the decay of a syllable. Thus the relay 37 is operated in response to voice currents to effect the operation of relays 21, 23 which respectively open the speech path 12 and disable the receiving branch 13. A relay 60 operated by detector 28 is adjusted to operate when the current level through the detector is high to effect the removal of grid bias from detector 35 and maintain relay 37 operated in response to excessive or sustained voice currents of high energy level. Speech currents arriving over the receiving branch 13 operate relay 45 which effects energization of winding 50 of relay 51. Relay 51 shortcircuits transformer 34 and prevents operation of relay 37 bv noise or voice currents over path 12. At the cessation of speech, relay 45 releases and condenser 55, which has previously discharged through resistance 59, is charged from battery 56 over winding 57 to maintain relay 51 operated until charging is complete and thereby prevent voice or noise currents over path 12 from disabling the branch 13 upon the cessation of a syllable. The provision of short-circuiting arrangements for transformer 34 may allow the use of a delay network 19 having short delay characteristics or the omission of the network. In a modification, a series of bandpass filters is connected in parallel to the output of the speech detector 28. The filters are designed to pass four equal bands of the pitch frequencies. Each filter operates through a syllabic filter and detector an individual relay, the contacts of the individual relays being connected in series to form a normal short-circuit across the battery of the relays 21, 23. The circuit controlled by the relay 60 is associated with any one of the filter circuits and the contact of the relay 51 is connected in series with the supply battery of the relays 21, 23. In a further modification, the detector 28 is in the form of a grid circuit detector which allows the use of a filter 33, having a single condenser connected to the mid-point of two inductances. Moreover the amplifier 16 is in the form of a push-pull amplifier which is normally inactive thus rendering the short-circuiting relay 21 unnecessary. The push-pull amplifier is controlled by a valve operated by the output of the filter 33. In operation part of the syllabic currents causes reduction in the plate current of the control valve which in turn reduces the negative bias of the push-pull amplifier thereby rendering it inoperative. A relay operated by the syllabic detectors opens the plate circuit of the control valve to maintain the amplifier in operation during speech.