Patent GB-535147-A (GB)
Title: Improvements in or relating to electric signalling over power lines
IPC: H04B3/54
Filed: 1940-01-23
Abstract: 535,147. Multiplied transmission over power lines. GENERAL ELECTRIC CO., Ltd., WESTELL, E. P. L., FARNSWORTH, J., and OLLERENSHAW, C. Jan. 23, 1940, No. 1378. [Class 40 (iv)] In a carrier transmission system over a number of power lines all radiating from a central station, the available frequency band is divided into groups each comprising two or more adjacent channels of width approximating to that of the original transmission band, and to each line is allocated individually one such group or two or more non-adjacent groups. For example, a group 16 kc. wide may provide four speech channels about 3 kc. wide, preferably two channels for each direction of transmission with the adjacent pairs on either side of the mid-frequency transmitting in the same direction.Protective signals may be sent at the mid-frequency, and calling signals may be sent at single frequencies between the adjacent pairs of channels aforesaid. If however a two way channel is used, it is preferably located between two corresponding oneway channels. The terminal apparatus at the central station for a group of channels according to the former arrangements is shown in Fig. 2. Speech arriving over the line T(l) passes to a frequency-changer FC1 fed with a 52 kc. intermediate carrier, and the upper side-band selected by a filter BF1 passes to a second frequency-changer FC3 fed with a 180 kc. carrier. The required side-band is selected by a filter BF2 and after amplification passes through a hybrid network H1 and coupling network to the power line. Speech arriving over the line T(2) passes first to a frequency-changer FC5 fed with a 4 kc. carrier, and the upper side-band selected by a filter HP1 enters the frequencychanger FC1. The receiving paths contain similar apparatus, but the filter BF4 selects the lower side-band and the frequency-changer FC2 is fed with a 68 kc. intermediate carrier. The calling signals accompany the speech bands, while the protective signals are applied to and taken from the hybrid network H1. All the carriers required may be generated by two multi-vibrators under the control of a single oscillator, Fig 3 (not shown). Preferably the band-pass filters are of the balanced transformer type, Fig. 4 (not shown), the frequency changers are of the ring type disclosed in Specification 456,574, [Group XL], and the amplifiers employ negative feed-back. When two groups of channels are imposed on the same power line, the double-band coupling network disclosed in Specification 380,539, [Group XL], may be used. The apparatus at the corresponding terminal station is similar, but the parts FC1, 2, BF1, 4 are connected by the dotted instead of by the full lines. If the power line is a ring main with several stations, however, both sets of connections would be provided at all stations together with voice-operated switches for establishing one set or the other. An alternative method is to use a single channel for both directions of transmission, and the terminal apparatus is then arranged as shown in Fig. 5, the transmitting and receiving paths being connected to the low-frequency line through a hybrid network H4, and a voice-operated switch VOS being provided to render operative the transmitting path and disable the receiving path by withholding the intermediate carrier from the frequency-changer FC2. In addition, two oneway telegraph channels of narrow width are provided, the lines T(2), R(2) being connected through separate frequency-changers to the transmitting and receiving paths by way of hybrid networks H2, H3.