Patent GB-605128-A (GB)
Title: Multiple signal transmission systems
IPC: H04J3/10
Filed: 1945-12-14
Abstract: 605,128. Telephone transmission systems. PATELHOLD PATENTVERWERTUNGS- & ELEKTRO-HOLDING AKT.-GES. Dec. 14, 1945, No. 33956. Convention date, Dec. 23, 1944. [Class 40 (iv)] [Also in Group XL (c)] For reducing cross-talk in a distributer type multiplex system, couplings are provided from each channel to at least the next channel, the coupling of each channel to the succeeding channel being greater than the coupling in the reverse direction. The couplings may be automatically controlled by the output of a modulator fed with a control tone received on one channel and the output of the same tone present in another channel. In one embodiment, the channel outputs ae ... fe, Fig. 4, from the receiving distributer are fed to individual amplifiers V1 ... V6 and the output from each amplifier is fed to the two succeeding amplifier inputs with adjustable amplitudes through potential dividers R1 ... R6 each having two taps. Undesirable couplings introduced thus between two channels, for example between channels ae, be, through R6 are avoided by bridge connections or resistances which are large compared with the line impedance. Amplifiers may be inserted in the input leads to the potential dividers to prevent undesired couplings and the amplifiers V1 ... V6 can then be omitted or replaced by resistances. In a second embodiment, Fig. 5, the correcting means is located in the common transmission channel K between the transmitting and receiving distributers S, E respectively. It comprises a delay line L with tapping points from which signals delayed by an integral number of inter-channel periods are obtained and fed through adjustable potential dividers R1, R2, R3 and individual coupling transformers back into the line. The elements R1, R2 R3 may be replaced by adjustable amplifiers or a common amplifier inserted at the input of the delay line, the sign and amplitude of the correcting voltages thus being adjustable to eliminate cross-talk. Several of these correcting circuits may be included along the line and either correction circuit may be used at the transmitting and/or receiving end. The amount of cross-talk may be indicated, so that the correction may be adjusted by sending a control tone over one channel and measuring the' amplitude of the tone, separated by a filter, derived from other channels. The adjustment may be made automatically by deriving a D.C. control voltage from a product-forming modulator circuit fed by inputs of control tone derived from the two channels affected, Fig. 7 (not shown). The modulator may be replaced by a wattmeter movement providing a mechanical displacement for automatic correction purposes. An alternative method of correction comprises a modification of the distributer at the transmitting or receiving end. When the receiving distributer is a multi-target cathoderay tube, extra targets may be provided outside each main channel target and connected through suitable attenuators to supply a fraction of the channel signal to succeeding channels. In a modification, synchronously operated distributers at the receiving or the transmitting end, Figs. 8 and 9 (not shown), are operated one or two &c. channels out of step, to provide correcting signals at the right instants. It is shown that the compensation can be made practically constant for slight phase deviations of the transmitter and receiver distributers if the retardation of the correcting signal is initially adjusted so that their maximtun occurs slightly before they are applied to the channel being corrected, Fig. 10 (not shown).