Patent GB-1531533-A (GB)
Title: Data transmission system
IPC: H03H17/00
Filed: 1976-01-30
Abstract: 1531533 Multiplex transmission OKI ELECTRIC INDUSTRY CO Ltd 30 Jan 1976 [5 Feb 1975] 03821/76 Heading H4M A multichannel multiplex orthogonal VSB transmission system for a PAM signal is designed such that the available bandwidth of a narrowband line accommodates the maximum number of channels theoretically possible, and is such that as far as possible the effects of inter-symbol and inter-channel interference are eliminated. The modulating arrangement to which the input channels are applied at the transmitting end of the system incorporates at least one two channel modulator in which signals on two input channels, e.g. 1 1 , 1 2  are applied to an adder (e.g. 2 1 ) and a subtractor (2 2 ). The outputs from the adder and subtractor are respectively applied to roll-off filters 3 1 , 3 2  which have the same amplitude roll-off transfer characteristics with a factor less than 50%, the same predetermined fixed delay and phase transfer characteristics of + #/4 and - #/4 respectively in the amplitude roll-off region. The roll-off filter outputs are applied respectively to multipliers 4 1 , 4 2 . A carrier wave e.g. Cl is applied to the multiplier 4 2  directly and via #/2 fixed phase shifter 8 1  to multiplier 4 1 . The multiplier outputs are supplied to an adder (e.g. 5 1 ) which provides the two channel modulator output. The system normally includes a number of such two channel modulators, in order that more than two input channels can be multiplexed, and incorporates arrangements 10-15 to supply a plurality of carriers e.g. C 1 , C 2  (one for each two channel modulator) at the frequency interval fc, where fc is the pulse repetition frequency in each channel. The outputs of the two-channel modulators are applied to a summing arrangement 6 to provide the overall modulating arrangement output. In such a system, phase shifters 9 1 , 9 2  enable the phase of the carrier to each two channel modulator to be adjusted to eliminate interchannel interference. The demodulator at the receiver (Fig. 6, not shown) incorporates arrangements for regenerating a receiver carrier and arrangements complementary to those at the transmitter for multiplying input signals with demodulation carrier waves derived from the receiver carrier. Demodulator operation must be synchronized with the phase angle of the carrier wave in order to control the phase of the demodulating carrier. Connected to each output of a demodulated channel at the receiver is a sampling circuit (Figs. 8 and 9, not shown) so that the transmitted signals may be reproduced. In response to a channel signal a phase adjustable sampling pulse is generated at a frequency fc/ 2  to sample the channel by a gate E. The sampled signal is delayed (45 1 -45 4 ) and the signal at the centre tap of the display is multiplied with the sum of the other delays (in circuit 48). The multiplied output controls a phase shifter (43) to shift the phase of the sampling pulse. The circuit includes attenuators (46 1 -46 4 ) in the signal paths of various delayed signals. These are adjusted to avoid dispersion of intersymbol interference in the sampled pulse output. In order to reduce disturbances due to interference in the actual transmission lines, an automatic equalizer of transversal filter type is used (Fig. 10, not shown). The input channels (44 1 -44 4 ) from the sampling circuit outputs are applied to transversal filter portions (61 1 -61 10 ) and combined via adders (62) and decision circuits (63) to channel outputs (67). The transversal filters themselves (Fig. 11, not shown) appear to be of conventional type, with the gains of multipliers in various sections controlled by an error signal input. An alternative form of automatic equalizer, in which the number of transversal filters needed is reduced is explained (Fig. 12, not shown), interchannel interference being eliminated by carrier synchronization and timing.