Patent GB-1302057-A (GB)
IPC: H04J3/02
Filed: 1970-09-24
Abstract: 1302057 Time-division-multiplex switching networks, active filters INTERNATIONAL STANDARD ELECTRIC CORP 24 Sept 1970 [30 Sept 1969] 45505/70 Heading H4K and H3U To avoid the use of low pass filters of high quality for each subscriber's line of a multiplex switching network, speech is multiplexed direct from the lines at a high rate (32 kcs.), intermediate highways of one stage of the network each being served by a digital filter controlled at frequencies related to the multiplexing rate in order to bridge the transition to and from further highways of a lower multiplexing rate (8 kcs.). As shown in Fig. 1 the network uses 4- wire switching, the pulse amplitude modulated output of digital filters SF on the GO intermediate highways being fed to PCM encoders C working at the multiplex frequency f1 (8 kcs.), the pulse amplitude modulations of the PCM decoders D on the RETURN highways being put through digital filters SF to provide a frequency spectrum which, at the higher multiplex frequency f2 (32 kcs.) of the switching network, allows the subscribers' lines to receive the demodulated pulse trains without need of high frequency cut off filters beyond what is provided by the impedance of the lines themselves. The GO channel filter action is shown in Figs. 2a to 2e. Sampling of speech on lines and switching on the network is effected at 32 kcs. to produce the overlapping sidebands of Fig. 2a, speech frequencies being shown hatched. Pass bands of a first stage of digital filtering are shown in Fig. 2b and are such as to produce the interleaved harmonies, of Fig. 2c, at 16 and 48 kcs. A second stage of filtering with pass bands as shown in Fig. 2d interleaves further harmonics to produce the full spectrum that would have existed if sampling were at 8 kcs. originally. Such a spectrum is suited to conversion in the PCM coder C. The RETURN channel filter action is designed, inversely, to reduce the full spectrum of Fig. 2e to the spectrum of Fig. 2a, the impedance of individual subscriber's lines cutting off the sidebands existing about the switching frequency of 32 kcs. RETURN channel filter action is described with reference to Figs. 3a to 3e, not shown. Filtering to provide pass bands as shown in Figs. 2b, 3b, 2d, and 3d, can be achieved simply by blocking alternate pulses of each amplitude modulated channel pulse train, or by combining alternate pulses for forwarding at half the original rate. To provide good cross-talk attenuation the delay circuits employed in the digital filters are commutated delay circuits, a storage capacitor being provided for each channel of the associated channel multiplex, a distributor recording the successive channel pulses on successive capacitors which are scanned later to remultiplex the channels, Figs. 5 and 6, not shown. Reading out of and writing into each such storage capacitor may be effected in the first and second halves, respectively, of each channel slot. Writing in of channel modulations in the second half of channel slots may be made regenerative by switching in a feedback loop, Fig. 7 not shown. The storage capacitors may be in two groups, one being used for reading while the other is used for writing and vice versa in alternate cycles. Storage capacitors may be worked in resonance with their input and output circuits or amplifiers may be used for regeneration. Integrated circuits may be employed but it is pointed out that such circuits cannot be produced with the precision needed for resonant transfer.