Patent GB-648710-A (GB)
Title: Communication channel identification in telephone or like systems
Inventor: BELLAIRS GUY FFARINGTON
IPC: H04M7/16
Filed: 1947-03-05
Abstract: 648,710. Automatic exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. March 5, 1947, No. 1097/48. Divided out of 648,674. [Class 40 (iv)] In a high frequency telephone system, each substation is allotted a characteristic carrier frequency built up from frequencies representative of the digits of the station number (as described in Specification 648,674), and for metering purposes the carrier frequency is broken down again at the exchange by a heterodyne and filtering process to identify the calling station. The whole range of frequencies covered by 500 carriers is divided amongst five filters IF1 to IF5 (Fig. 2), only one of which accepts the frequency to be analysed. Operation of a detector D determines the hundreds digit. The modulators M fed with different heterodyne frequencies Fr1 to Fr5 translate the frequencies accepted by IF1 to IF5 to a common secondary frequency range accepted by the filters F. The process is repeated a second and a third time by filters 2F0 to 2F9 and 3F0 to 3F9, the second translation being carried out by modulators M1 and filters F1. The second and third digits are determined by detectors D1, D2. In a modification, Fig. 3, the frequency to be analysed is fed from a terminal I, and is modulated successively at M1 with frequencies 00-09 from a switch S1. When the resultant intermediate frequency falls within the pass range of filter F1, the switch S1 is stopped by the operation of a detector D1 and a relay R1. Switch S2 is started up at the same time to determine the second digit using frequencies 000 to 009. The process is repeated in switch S3 for the third digit using frequencies 0000 to 0009. The switches S1, S2, S3 may be mechanical switches or may comprise a series of gas-discharge triodes successively triggered. Metering outgoing calls at exchange. The frequency-finder FF1, Fig. 5, is substantially the same as that described in Specification 648,674 and 648,708, and serves, when a calling frequency on CC1B is found and when relay A operates to produce from a variable oscillator V0 a return frequency characteristic of the calling station. This frequency is applied to the return cable CC1A. On reply of the called party, relay D (not shown), operates and over contact D1 applies the characteristic return frequency to terminal MP of a second frequency finder FF2. The finder FF2 finds the frequency and operates relay A<SP>1</SP> which connects the frequency over contact a<SP>1</SP>2 to frequency identifying metering equipment in MME. The frequency from VO may be modulated at M6 by a fixed marker frequency MO applied at the wanted line selector WLS in accordance with the distance fee for the call. After demodulation at DET<SP>1</SP>, the motor frequency operates a relay R1, R2 or R3 according to the distance fee. The marker oscillators MO may be distinguished by pulses instead of by frequency. The metering frequency may be applied to MP at three minute intervals so that fees may be computed in accordance with the duration of the call. A relay Z (not shown) operates when the frequency has been identified and the charges assessed. A fixed oscillator FO produces a frequency of 2000 cycles which on operation of Z is mixed at M3 with the calling party's return frequency to form a metering acknowledgement signal. The 2000 cycles is separated again at M4 and operates a relay K to disconnect the finder FF1 from the metering point MP.