Tone generation circuit for automatic PCM-TDM telecommunication exchange

An automatic telecommunications exchange in which speech and other intelligence and conveyed in TDM manner using PCM, comprising a plurality of terminal interfaces interconnected via a common intelligence bus and a common signalling bus, each terminal interface serving subscribers' lines served by the exchange and each said terminal interface including local processing means connected to the common signalling bus;

• centrally-located tone generation means for generating tones and for supplying the tones to the terminal interfaces via the common intelligence bus with each said tone being conveyed in a predetermined time slot of a TDM cycle;
• a central control processor for producing control signals which are sent via the signalling bus to the local processing means of a terminal interface serving a line requiring a supervisory tone, the supervisory tone comprising one or more continuous or varying ones of the tones, which control signals represent both the selection of the tone or tones carried on the intelligence bus and the cadences of the selected tone or tones required for said supervisory tone, and in which in response to such control signals the local processing means selects from the intelligence bus the required tone or tones to be applied to the line and controls the cadence of the selected tone or tones as called for by the control signal.