Patent GB-1433961-A (GB)
Title: Loudspeaking telephone arrangement
IPC: H04M9/10
Filed: 1974-02-14
Abstract: 1433961 Controlling by sound INTERNATIONAL STANDARD ELECTRIC CORP 14 Feb 1974 [16 Feb 1973] 6757/74 Heading G4F [Also in Division H4] A loudspeaking telephone arrangement includes a plurality of speakers and at least one microphone in the ceiling of a room. Preferably the speakers and microphone(s) are mounted in a false ceiling together with compression and gain amplifiers, some circuitry of conventional loudspeaking telephone arrangements (e.g. a voice operated relay) and a tape recorder. The system is switched by a hand held audio control unit having key tone generating facilities. Access to the false ceiling is through a service door.  The portable control unit 11 (Fig. 1B) has a key tone pad 2 for generating particular audio tones on operation of a key, and a speaker 4 which transduces the amplified tones generated by the pad 2. The unit 11 is powered by a battery 5 and solar charger 6. The generated tones emitted by speaker 4 are picked up by one or more microphones 7 (Fig. 1C) M (Fig. 2C) located in a false ceiling. The microphone outputs are fed via mixers 8 (Fig. 1C) to an amplifier 9 and compressor 10 and thence to a control circuit 11 (Fig. 1A) and to a voice operated relay 12. Relay 12 is part of a conventional loudspeaking telephone arrangement. An output from relay 12 and derived from the system control circuit 11 is applied to a gain amplifier 13, a compressor 14 and a power amplifier 15 and then to loudspeakers 16 (Fig. 1C) S (Fig. 2C) in the false ceiling. The control circuit 11 (Fig. 1A) includes a tone receiving and converting arrangement 17 coupled to the output of compressor 10, and a gate 20 coupled to a selector 21. Arrangement 17 receives the audio tones from the hand held unit 1 via the microphones 7 and converts them to D.C. digital signals (see Fig. 3, not shown). Key * on hand held unit 1 operates gate 20 to connect the selector 21. Other keys function to select one of two main lines 1, 2 or an intercommunication line 3, hold one or other of the two main lines 1, 2, release the main lines 1, 2, select the record, playback or rewind facility of a tape recorder 22, and release all sections at one time. After selection of a main line 1, 2, dial tone is received and the keys of hand held unit 1 may be used to dial calls through central office equipment. A condition lamp display 23 is included. Flashing lamps on a display panel match each selection. The speakers S may be in fibreglass sound insulated chambers (see Fig. 28, not shown), sound being emitted through ceiling ports (30B) in the formation of a + sign. In a small room there may be only four speakers arranged at the corners of a square with a single microphone at the centre (Fig. 2A, not shown). In calibrating, an audio oscillator drives a loudspeaker four feet under each microphone M and the mixers 8 are adjusted for each microphone M to give a -2DBM level output. Compression amplifier 10 is adjusted to maintain a constant level output of -2DBM with inputs of various levels. Persons sitting and standing in the room are clearly heard by a remote listener. The speaker outputs are balanced by use of a 1000 Hz test frequency and setting the compressor 14 and amplifier 15 to give 60 DB sound level, measured with a sound level meter four feet above the floor under each speaker.