Patent GB-1315927-A (GB)
Title: Telephone systems
IPC: H04Q11/04
Filed: 1972-03-01
Abstract: 1315927 Automatic exchange systems; multiplex pulse code signalling SIEMENS AG 1 March 1972 [2 March 1971] 9459/72 Headings H4K and H4L In a delta modulation telephone system including a plurality of telephone subscriber stations each of which possess a delta modulator and demodulator for speech signal compression, and also a signalling means for the production of signalling binary words, each signalling binary word is combined with a fixed given preliminary signalling binary word to form a signalling binary word combination which is transmitted a plurality of times consecutively, both delta modulation speech signals and signalling binary word combinations comprising first and second binary state signals and being transmitted in the same manner; there is also provided a means of limiting the amplitude of the speech signals which occur in the circuit part of the telephone subscriber stations operating in the voice frequency range and the signalling binary combinations are chosen such that in a sequence formed by a signalling binary word combination transmitted a plurality of times consecutively, in a period of time provided for the receiving- end analysis thereof, there occurs a larger number of first binary signals than can occur in an equal length sequence of individual delta modulated speech signals, taking into account the limitation in the amplitude of the speech signals. The amplitude of the speech frequency signals may be limited by virtue of the characteristics of the subscriber equipment, e.g. by the characteristics of a microphone, or a special limiter circuit may be used.  In a preferred embodiment the preliminary signalling binary word comprises at least 7 binary signals and contains at least 7 of the first binary signals, e.g. at least 7 binary 1's. The signalling binary word is chosen to contain 6 binary signals of which at least 4 are first binary signals, e.g. at least 4 binary 1's. Hence if the signal is repeated n times it will contain at least 11n binary 1's and so the speech amplitude is limited such that an equal length sequence of delta modulated speech signals will always contain a smaller number of binary 1's, therefore enabling the receiving apparatus to distinguish between speech and signalling information. In order to distinguish between switching signalling information and calling signalling information as produced by a subscriber keyboard, a two bit signal of 01 or 10 is inserted in the signalling word combination between the preliminary signalling binary word and the signalling binary word depending on which of the two types of signalling information is involved. It is stated that the invention can be carried into effect using principles which are known, e.g. as disclosed in German Specification 1,946,064. It is further indicated that the signalling binary words can be produced with the aid of contacts actuated by a keyboard provided in the subscriber station whilst the preliminary signalling word can be produced by fixed wiring. The binary word comination thus firstly produced in parallel representation can then be input into a so-called multiplexer from which it is then output in serial fashion.