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US3488445 (A)
Abstract of corresponding document:
GB1205598 (A)
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1,205,598. Multiplex systems. WESTERN ELECTRIC CO. Inc. 26 Oct., 1967 [14 Nov., 1966], No. 48661/67. Headings H4L and H4R. In a multiplex data transmission signal each channel signal is shaped by a shaping filter which has an amplitude frequency response having even symmetry about the centre frequency, and odd symmetry of the square of the response about frequencies midway between the centre frequencies of adjacent channels, with zero response beyond the centre frequency of the adjacent channel, and the phase/frequency characteristic differs by 90 degrees between adjacent channels. Data is fed to the channels at a transmission rate equal to twice the spacing between adjacent channels centre frequencies. The system is such that adjacent channels overlap in the frequency spectrum. The transmitted signals may be binary digits or of multilevel form derived from analogue samples. The received signals are detected by correlation techniques using matched filters. It is shown that an amplitude frequency characteristic that is a cosine function, Fig. 2(a) (not shown), or a positive half cycle of a cosine function, Fig. 2(c) (not shown), or that given by an over-coupled double-tuned circuit, Fig. 3(a) (not shown), fulfil the requirements for the shaping filters amplitude frequency response while the phase characteristic, if all the filters are to have identical characteristics, should be linear with ripple terms having odd symmetry about the mid-frequency points between adjacent channels. The filters may be adjusted to the transmission frequency of each channel or they may, as shown in Fig. 4, be identical and the outputs from the filters 44 transformed to the appropriate frequency in modulators 45 before the outputs are summed, in 47, and filtered to remove the unwanted sidebands produced in the modulation step. A synchronizing signal fs is added to the combined channel signals in summing amplifier 47 before the signal is filtered and frequency changed to the appropriate frequency band for transmission. At the receiver, Fig. 6, the incoming signal has the synchronizing signal picked off at 70 and the channels are transformed to the appropriate base-band position by modulators 74, and then filtered by 76 to produce only the wanted channel signal and the overlapping side bands of the adjacent channels. The information contained in the wanted channel is obtained by applying the output of filter 76 to a matched filter 78, which comprises a transversal equalizer adapted to have an impulse response which is the inverse of the waveform to which it is matched, and then sampling the output, at the original sampling frequency 2f8, in sampler 79.
