Patent GB-1409101-A (GB)
Title: Demultiplexing
IPC: H04J1/08
Filed: 1972-09-26
Abstract: 1409101 Demultiplexing WESTERN ELECTRIC CO Inc 26 Sept 1972 [27 Dec 1971] 44361/72 Heading H4R  In a demultiplexing arrangement for a wide band frequency multiplexed signal of general form F(t) which may be of double side bond or single side bond form, the incoming multiplexed signal is sampled at 102 and each sample is digitized to form a sequence of digital signals F(m) which are stored in a buffer memory 106. Sequences of N such signals are read out in turn and applied to circuitry performing computational operations in either the time domain (Figs. 1, 1c) or the frequency domain (Figs. 1A, 1B, not shown), and sequences of N output signals are fed out on a line 113 to a memory 250 having associated therewith memory access circuitry 260 which supplies predetermined output signals on the basis of known characteristics of the input signals to corresponding output lines 280-0 to 280-(L-1), where L is the number of multiplexed channels, to give on each line a corresponding demultiplexed and demodulated channel signal. The Specification contains a detailed mathematical discussion of the arrangements and only the briefest outline will be given. In the time domain arrangement shown sequences F k (l), l = 0... N-1, are read out from the buffer memory 106 and are weighted by corresponding weighting coefficients H(l) in multiplier 110 to give the product F k (l)H(l). The coefficients H(l) are said to supply a shaping function often referred to as a "time window". The products F k (l)H(l) are then supplied to a fast fourier processor 112, which may be of the type disclosed in Specification 1,211,859, to produce an output S(k, u) where where u is an integer such that u = 0,1..., N- 1, k is an arbitrary constant such that F k (l) is a segment of F(t) lying between t = k-N+ 1 and t = k. For a particular k the outputs generated will be S(k, 0), S[k, 1) ... S(k, N-1). By use of the memory 250 and access circuitry 260 the S(k, u) coefficients are chosen for which u = 0, 2b, 4b &c. where b is the positive frequency bandwidth of each component channel and these signals are applied to the respective channel output leads 280-0 to 280-(L-1). The operation of an arrangement in the frequency domain is even more complex. In this case the output of buffer memory 106 is fed to a fast fourier processor which fourier transforms an input sequence of N signals. The fourier transformed signals are then weighted according to frequency window function values W(l), and for achieving the weighting a convolution circuit is used, which may take the form of a programmed digital computer. The output signals from the convolution circuit are then supplied to output lines as described for the time domain embodiment. It is stated that any of the functions described in relation to the two embodiments may be performed by a programmed special purpose digital computer. Moreover it is suggested that the "fast fourier transform" processing may be replaced by equivalent "discrete fourier transform" processing. The frequency multiplexed signals may be transmitted by microwave or cable systems and relate to long-haul telephony.