GB655696A - Systems of transmission of intelligence by carrier currents - Google Patents

Abstract

655,696. Carrier transmission. STANDARD TELEPHONES & CABLES, Ltd. Sept. 10, 1948, No. 23837. Convention date, Oct. 17, 1947. [Class 40 (iv)] In a wide-band carrier transmission system in which a block of channels is dropped off at an intermediate station and replaced by a similar block of other channels, attenuation of through-channels adjacent either side of the block of channels rejected by filtering is neutralized by combining the outgoing channels with inversely attenuated adjacent channels which are derived by separately phase-inverting the incoming signal and combining it with the filtered through-channels, followed by selective filtering. As shown, the band of channels to be dropped off is rejected from the throughchannels, together with an idle frequency band on either side, by a band-stop filter 2. The incoming channels are also applied through phase changers 7 and 8 and then combined in antiphase with the filtered through-channels. The resulting frequency band, which consists of the elimination zone and the adjacent attenuated frequency bands due to the filter 2, is applied to filters 17 and 18 which select respectively the frequency bands on either side of the elimination zone. These bands are finally filtered at a lower frequency range by sharp cut-off filters 25, 26 after passing through phase and amplitude adjusting devices 19, 20, the frequencies being reduced and then restored to their original range, after filtering, by frequency changers 21, 27 and 22, 28, each controlled by a common oscillator 23, 24. The frequency band to be dropped off is selected by a band-pass filter 32 and reduced to a lower frequency range at a frequency changer 33. The replacement group of channels is stepped up to the frequency range of the dropped off group at a frequency changer 35, which shares a common oscillator 34 with the frequency changer 33, and is then combined with the outputs from frequency changers 27, 28. The attenuated frequency bands and replacement channels are combined with the through channels after passing through a filter 29, the through channels being applied through a phasecorrector 4. Amplifiers 3, 5, 30 and 9 to 16 are provided in the circuit, amplifiers 9 to 16 being buffer amplifiers used to prevent interaction between circuits.