Patent GB-875021-A (GB)
Title: Improvements relating to signal receivers for pulse signalling systems
Inventor: STALLWORTHY FRANK ALLEN
IPC: H04Q1/453
Filed: 1958-05-27
Abstract: 875,021. Voice-frequency receivers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 5, 1959 [May 27, 1958], No. 16877/58. Class 40 (4). A receiver for signals comprising a plurality of frequencies comprises an acceptor circuit responsive to one of the frequencies, and a rejector circuit responsive to any other frequency and operative to disable the acceptor circuit, the whole being preceded by a band-stop filter which prevents the passage of the other frequencies of the plurality. Receiver circuit, Fig. 2.-As described, digital signals and two other signals indicating the beginning and the end of a set of voice-frequency digital signals are sent as combinations of two-out of six frequencies. The signals are fed to a plurality of hand-pass filters FC, each of which in turn feeds a plurality of detectors D, of which there is one for each frequency combination. Thus if the detector corresponds to the digit 1 which is signalled by the combination of frequencies F 1 , F 2 , filter FC attenuates frequency F 2  but F 1  passes to the input valve V 1  of detector D. This valve feeds an acceptor circuit A.C. tuned to frequency F 1 , but provided with negative feedback (over winding tt 1 ) to reduce the rise time of the circuit. The valve also feeds a broadly tuned rejector circuit R.C. The outputs of these circuits are fed to voltagedoubler circuits whose outputs are connected in opposition and so dimensioned that the output of the rejector circuit overcomes that of the acceptor circuit, except when pure frequency F 1  is received. In the latter case the output of the acceptor circuit predominates to back-bias V 2  and release R. In the event of F 1  being received in combination with a frequency other than F 2  (which may be either a signal frequency or speech) this other frequency passes the filter FC to prevent the detector from responding. The Figure also shows a broad-band detector D13 responsive to the reception of any of the signalling frequencies, the accepter circuit A.C. being broadly tuned and the rejector circuit being heavily damped to provide a lower guard ratio. This ensures that operation of this detector always straddles that of any of the individual frequency detectors. The reservoir capacitors of the acceptor circuits are also omitted, and a smoothing network substituted to enable response to take place in spite of beating between neighbouring signalling frequencies. Error detection.-In Figs. 3, 4 contacts 1r . . . 12r are normally operated but released on operation of a correspondingly numbered twofrequency detector and contacts 13r operate similarly for the broad-band detector. An alarm is required to be sounded if none or more than one of the two-frequency detectors operates when the broad-band detector operates. When contacts 13r release energizing current is provided for the right-hand winding of ED which is balanced when two and only two of leads P 1  . . . P 4  are energized over contacts released by the individual detectors. These are arranged so that operation of more than two detectors or of no detector cause the energization of more than or less than two of these leads so that ED operates to give an alarm. By providing a connection y the alarm relay ED may also be caused to operate on the release of any one (alone) of the detector contacts or on the release of all these contacts if these are received without the correct preliminary signal having been sent. Relay ED may, however, be caused to sound an alarm only if an error is detected without this preliminary signal being sent the register being arranged to send a " re-order " signal if the error occurs after the preliminary signal has been sent. The register may then be arranged to pulse relay RA each time it is released and if a re-order signal has not been given the release pulse is accompanied by earth on lead L 6 , which tends to discharge condenser CC. If a re-order signal has been given this earth is absent and CC tends to charge up. In the event of an unwarrantably high proportion of re-order signals CC charges sufficiently to cut off valve V which releases RB to give an alarm. Transmission of digits to a register.-Transmission of a voice frequency set of digits is preceded by a two-frequency combination causing the appropriate detector to release contacts 11r to bring up CR. This prepares the register circuits for registration of digits received in V.F. form. On cessation of this signal CS also comes up to enable the error detector. This circuit also comprises means for transmitting the received digits to the register as potentials on one or two of leads L 1  . . . L 4 . At the end of the transmission a further combination of frequencies causes the appropriate detector to release contacts 12r to operate CT which releases CS. Relay CT is held until a signal from it to the register initiates the retransmission of the received digits whereupon the register disconnects lead x. Specification 782,293 is referred to.