Call signal generator with variable pulse characteristics - has cyclically addressed RAM storage for starting and ending ringing tones

摘要

Purely electronic switching circuits are for the selection of a telephone ringing signal at a called subscriber station from a number of alternative characteristics. It is considered desirable that a called station recognises whether a call is local or long distance, urgent business or private, etc. This is realised by the combination of a circuit common to a group of subscriber stations and a further supplementary circuit individual to each station. The former (TGr) is connected to a peripheral exchange control plant (PS) from which station selector signals (AT1 to ATn) are applied to a code converter (COD) which supplies address inputs, cyclically repeated, to a data storage circuit (RAM) which when combined with a call command (B4) stores the desired call mode in the form of a selection code as parallel data entry bits (D1 to D3). The RAM output lines (Q1 to Q3) transfer this selector code to an analogue switching array (RSEL) which switches one of several tone signal input combinations (CO, C2). The desired pulsing combination is passed to the cited second circuit group individual to each called station to produce the audio call signal selected. When the subscriber answers, a voltage appears on the corresponding individual feedback line (TM1 to TMn) for renewed data sampling. The data now being zero, the call entry is deleted.