Patent GB-1418384-A (GB)
Title: Transmission system
IPC: H03C1/60
Filed: 1973-06-12
Abstract: 1418384 Frequency multiplex systems TELECOMMUNICATIONS RADIOELECTRIQUES ET TELEPHONIQUES 12 June 1973 [15 June 1972] 27829/73 Heading H4R In a multiplex system the analogue signals are converted to digital form and fed through a cascade arrangement of a Fourier transform circuit and a digital filter, in which the digital filter coefficients characterize the transfer function of a low pass filter having a pass band which is equal to half the bandwidth of the individual channel signals, and the transform circuit is fed with a number of coefficients characterizing respective carrier frequencies, each of which is an even multiple of the low pass filter cut off frequency. Fig. 1 shows a block diagram of a system for receiving a frequency division multiplex signal in the band 312 to 552 kHz comprising 60 4 kHz telephony channels. The signal is transposed by a 558 kHz carrier F 4  - #f/2, and filtered to provide a signal in the band 6 to 246 kHz which is sampled and digitized in the A to D converter 4 at a sampling frequency of 512 kHz with each sample comprising 12 bits. The coded samples are fed to a serial to parallel converter 5 which provides on its output 2N, where N is the number of input channels, interleaved series of samples, which are applied to the 2N registers r 0  to r 2N-1  each having a storage capacity of one code word. The contents of the registers are simultaneously applied, at a frequency of 4kHz, to the digital filters A 0  to A 2N-1  to which filter coefficients corresponding to a low pass filter of bandwidth 2 kHz are applied from a store 6. The outputs from the digital filters are applied to the fast fourier transform system 7 to which carrier signals, in phase and in quadrature, are applied from store 6a, which effectively changes the low pass filter characteristic to band pass of total width 4 kHz and centred on the particular carrier frequency applied. To obtain the corresponding base band signals the outputs of the transform circuit are applied to the quadrature demodulators do to d N-1 , the in phase and quadrature components for each carrier are applied respectively to a delay 8 #T and a digital low pass filter 9 which interpolates between adjacent input samples and provides a corresponding delay #T. The resulting signals are applied to circuits 10, 11, which invert alternate samples and is the digital equivalent of quadrature modulation with a carrier of 2 kHz. The two signals are summed to provide at each demodulator output samples of the corresponding channel at a repetition rate of 8 kHz. In the frequency division multiplexer Fig. 7 the analogue channels, after conversion to digital form in A to D converters E 0  to E N-1  are applied via respective quadrature modulators M 0  to M N-1 , and the fast Fourier transform circuit 30 to the digital low pass filters H 0  to H 2N-1 . The filter outputs are supplied to registers r 0  to r 2N-1  to allow them to be gated serially to the common output line 32 from which the multiplexed signal can be obtained via d to a converter 33, filter 34 and frequency transposing modulator 35. The individual units of the multiplexer and demultiplexer are identical. The particular arrangement of operating the filters 2a or 32a as low pass filters allows the majority of the system to operate at a low sampling rate. The coefficients for the filters 2a or 32a and 9 or 29, and for the transform circuit 7 or 30 may all be contained in a single store since many coefficients are common to more than one operation. The system may be used for converting from frequency to time division multiplex, or vice versa, Fig. 9 (not shown). An embodiment of the low pass filter calculator which multiplies and sums a range of signal samples with a range of response functions is described with respect to Fig. 5 (not shown).