Patent GB-1336931-A (GB)
Title: Communication systems
IPC: H04N7/00
Filed: 1971-04-19
Abstract: 1336931 Automatic exchange systems WESTERN ELECTRIC CO Inc 19 April 1971 [28 Jan 1970] 20496/71 Heading H4K A switching office is connected to a plurality of substations by corresponding plurality of audio links and can selectively connect a broadband line common to the substations in parallel with any audio link. As described the invention is incorporated into a key telephone system. The figure shows two substations each equipped with a video set 2, 4 and a key telephone set 1, 3. In a manner not detailed each substation can connect to any other substation for a video/audio call using the extension keys such as ext. 10. Each substation also has a plurality of keys T1, T2 associated with corresponding wideband video links 30 and 34 to the switching office. Associated with each video link is an audio only link 31 and 37 for each substation e.g. the keys T1, T2 of subset 1 are connected to audio links 31 and 35 exclusive to that subset and individually associated with the video links 30 and 34 respectively. Each of these audio links has a line circuit such as 17 and 7 at each end. To initiate an outgoing video call from station A the subscriber selects an idle wideband link 30 or 34, the busy idle/calling states being indicated by lamps associated with the corresponding keys T 1 and T2, and selects that link by operation of a key. If for example key T1 is operated to select link 30 line circuit 17 responds and line circuits such as 18 and 19 associated with that link and other substations are marked busy causing busy condition to be indicated at keys T1 of other subsets. The line circuit 7 at the other end of the selected audio link responds and in response thereto the switching office marks line circuits 8, 9 busy to prevent an incoming call ringing the video link 30 and connects a register to the audio link to receive the wanted subscriber designation. When the video channel is connected at the distant end the local office returns a signal over the video channel 30 to a block 16 which includes a cable equalizer and a loopback circuit which at this stage provides electrical continuity between the transmit and receive pairs of the video link to allow a continuity test of the video link to be performed. The loopback interconnections are broken when the calling station video set is connected to the link 30. For an incoming call to the substation A the switching office effects a hunt over the line circuits 7 and 13 associated with that substation. When an idle line circuit is found this implies that the corresponding video link is idle. The connection is completed in a manner generally similar to that for an outgoing call except that if the continuity test on the video link proves negative a further connection attempt via a different video link may be made, and that when the line circuit such as 7 is taken into use, the associated line circuit such as 17 responds and all subsets receive a busy condition. When the line circuit 17 receives ringing signals it causes the corresponding subscriber key lamp to flash and prepares a circuit which connects the video link to the subscriber when he answers.