Patent GB-1341916-A (GB)
Title: Digital voice and data telephone subscriber station
IPC: H04M11/06
Filed: 1971-04-19
Abstract: 1341916 Telephone substations with data facilities, digital transmission systems INTERNATIONAL STANDARD ELECTRIC CORP 19 April 1971 [2 Feb 1970] 20690/71 Headings H4K and H4P [Also in Divisions G4-G6] A telephone substation provides for reception and transmission of digitally coded speech and digital data and uses the same modulation equipment for both types of signal. The substation is provided with a handset or headset and is powered by a local battery charged from the exchange by way of a local D.C.-to-D.C. converter. Speech is delta-modulated and digitally encoded for transmission. as 3-bits of the 8-bit words employed. by the system, outgoing bits being di-phase modulated to reduce errors arising from phase variation and signal fading. Received signals are converted to a D.C. bit stream and, if speech, are decoded to analogue form. Speech may be secured by encryptation provided as a selectable option in the substation. Special encryptation to provide greater security can be provided with the use of plug-in units and code wheels. Digits and supervisory signals are sent between substation and exchange as 8-bit code words, digits being spaced by an interdigit code and supervisory signals being repeated 32 times for security. Reception is judged according to a majority vote. Data is sent using the 8-bit code for supervisory signals and a choice of transmission rate from 2400 to 75 bits/sec. Data may comprise signals for facsimile transmissions or, with the use of a data interface, may originate from such data equipment as a computer or teletypewriter. The substation may be optionally mobile with a radio link to the exchange or work over a 4-wire exchange line. A number of substations may be connected to share the same exchange link. Such substations set up connections in the voice mode and agree on any other mode of operation required, manual controls being provided to switch to the various data modes. Replacement of the hook switch restores the controls to normal standby in the voice mode. Delta modulation is effected by band limiting the voice signal, after compression, and differencing samples in an amplifier, the difference being coded as one sign bit and two bits to represent magnitude of difference. Received 3-bit delta-modulation codes control the flow of adjusting pulses, positive or negative and in number suitable to reconstitute the analogue signal in an integrator. Substation off-hook condition automatically results in sending a seize code to which the exchange responds with a dial tone code to connect up local 600 cycle tone. The substation keyboard sets up the wanted numbers as 2-outof-7 codes in the 8-bit words which are read out serially with an interdigit code between digits. When sufficient digits have been received, the exchange sends a talk enable code and the keyboard is disconnected. If the called substation is being called the exchange reverts ring-tone code to connect up interrupted local 1600 cycle tone. On reply by the distant substation the ring-tone code is removed and conversation proceeds. Received codes condition substations to clear or encrypted voice modes. The exchange calls a substation by sending a period of reset code to ensure standby conditions if this has been upset by an aborted call despite clear down by on-hook condition. Reset code is followed by a test code to see if the substation is idle and free from concurrent attempts at seizure. If substation returns an acknowledgement of free state the exchange sends a seize code and marks the substation busy. Ringing code from the exchange causes 1600 and 600 cycle tones to be connected for periods of one second with two second intervals. Ringing at the substation is confirmed by a code to the exchange, reply is signalled to the exchange which reverts a code to set-up the speech path.  Data sending may require the use of a data adapting subset which provides an interface between the data source and the substation-toexchange link and also provides signalling and error control functions, Figs. 7 and 8, not shown. The data subset comprises a keyboard having buttons to set the data rate to any one of 6 values, a set of format buttons, a set of alphabet buttons, and a set of command and function control buttons. The class of data to be sent is signalled between adaptor subsets by sending codes indicating the identity of operated buttons. Redundancy and majority vote techniques are used to obtain security of transmission. Data control signals are received at a low bit rate while status signals are received at the higher rate of transmission over the substation to exchange link, separate detectors and lamp indicator systems being provided for each type of signal. Encryption of the data stream may be effected in the substation apparatus.