Abstract: Conference call provisions are incorporated in a Time Division Multiplex (TDM) signalling exchange wherein the signals are summed energywise and transmitted to a receiving subscriber set without subtracting the signal from that subscriber set and any tendency toward regeneration is obviated by filtered signal reconstruction in conjunction with sample-and-hold signal reconstruction of the TDM signalling system. Simple low-pass, high-pass and band-shaping filters are adequate for the purpose.

Abstract: Calling signals for tone-frequency telephone calling and like communications systems are generated in digital form. For the eight tone frequencies used in American telephone systems, a group of about 50 bits (average) for each tone is stored in an electronic store in delta modulation data form. Circuitry according to the invention is arranged for repetitively reproducing the contents of the store to produce a substantially continuous waveform. Bits for one high frequency band tone and one low frequency band tone are selectively reproduced and applied to a delta demodulation circuit for producing the desired tone-frequency analog signal. In a basic embodiment, start-stop operation is used for producing a calling signal for a single switching operation. A continuously operating tone frequency generating circuit embodiment is arranged for producing a multiple of continuously available tone signalling outputs.