Standard Telephones & Cables Limited

Standard Telephones and Cables Public Limited Company

Gary A Sarson

John R Bungard

David M Davidson

Michael P McKenzie

In a small business-type telephone exchange, calls are set up in TDM manner using PCM to convey speech and data if the exchange serves any data terminals. The exchange has subscriber line ports (SLC) each serving two lines and a smaller number of trunk line ports (TLC) each serving a trunk to a local exchange. Each port has a local processor (SIP) with analogue-digital and digital to analogue conversion circuitry (CODEC-FILTER-TSAC). The exchange also has a central processor (CCP) with its associated memories (ROM, RAM, EAROM). The ports and the central processor are interconnected by an intelligence bus (PCM BUS) and a signalling bus (SIG BUS). The central processor (CCP) and the port processors (SIP) co-operate in call setting with all communications between the processors over the signalling bus. During operation the central processor (CCP) polls the ports via the signalling bus (SIG BUS) for ports needing the services of the central processor and also to pass call control information to those ports. To set up a call between the lines, or a line and a trunk, two time slots in the TDM cycle are allocated to the call by the central processor, one for each direction of transmission. The intelligence bus (PCM bus) is thus used only to convey intelligence, i.e. speech and/or data and tones.

Telecommunication exchange

A telephone exchange conference circuit for three-party conferences, receives the PCM bytes from the conferees in a FIFO store (4), and checks the bytes in a processor (1) to find the one for the loudest outgoing speech. This is then sent to another FIFO store (5) for transmission to the other conferees. This loudest speech sample is sent to all conferees other than the one which originated that sample: he gets a silent code. The system has two time slots per conferee, one for 'talk' and for 'listen'.

Telephone exchange conference circuit

In a digital PCM-TDM telephone exchange, supervisory tones needed are centrally generated by a digital synthesizer. This includes a PROM (1) in which PCM codes for the tones needed are stored in successive memory locations. The tones used are the DR tone, which is a mixture of 350 Hz and 440 Hz, and the BN tone, which is 400 Hz. These are read out under clock pulse control (5-6-7) such that the DR tone bytes are read out at time slot 0 and the BN tone bytes are read out at time slot 1. They are then sent over a common intelligence bus (4) to all line and trunk interfaces. At each such interface the tone bytes are converted to analogue form so as to be continuously available. The tone (or tones) to be used is (or are) selected in response to instructions from the central control each of which specifies a program in the interface's processor. Such a program controls both the selection of the tone (or tones) needed, and the cadence (continuous, or interrupted in a defined manner).

Growing community of inventors

Ottawa, Canada

Gary A Sarson