Patent GB-646501-A (GB)
Title: Improvements in or relating to public address systems
IPC: H04R29/00
Filed: 1947-04-18
Abstract: 646,501. Public address systems. STANDARD TELEPHONES & CABLES, Ltd., and OUTRAM, L. J. April 18, 1947, No. 10408. [Class 40 (iv)] A public address system is automatically tested when the system is switched on, and an indication given as to whether the system is functioning correctly. As shown in Fig. 1, a microphone 1 is connected through an amplifier 2 to a selector switch 3, an attenuation filter and limiter network 17 being connected across the amplifier to generate, by backcoupling, a testing oscillation whose frequency is outside the audio frequency band. A power amplifier 4 and a group selector switch 5 are connected to each terminal of the selector switch 3, the group selector switch being ganged to a similar selector switch 6. A loudspeaker group 12 is connected by a transmission line 9 and coupling transformers 7, 10 to each terminal of the group selector switch, the windings of the transformers connected to the transmission line being split and linked by condensers 8. The testing oscillation is continuously transmitted to the loudspeaker group, where it is rectified by an attenuation and rectifier network 18 and circulated in the closed loop formed by the loudspeaker line, the split transformer windings, and one coil of a doublewound relay A having an adjustable D.C. bias applied to its other winding. The relay remains operated, and completes a circuit at contact a1 for a common fault lamp 15 and a loudspeaker group fault lamp 16, unless the rectified test current is at the correct level. Separate wires, a phantom circuit, or an earth return system, may be used as alternatives to the split transformer windings to connect the rectifier network to the relay A. In a second embodiment the circuit is tested before use by an audio-frequency oscillation within the loudspeaker range. A separate source of signals is connected in parallel with the microphone by an additional contact a2 of the relay A, a correct level signal neutralizing the bias on relay A to open the contacts a1, a2. An additional circuit is necessary to prevent the relay re-operating at this stage. In a modification, Fig. 2, the separate source of signals is controlled by the contacts cc1 of a slow release relay CC connected, through contacts c1, in parallel with a slow-to-operate relay C and the contacts a1. Completion of the circuit by the selector switch 6 operates the relay CC which connects the source of signals to the circuit until the operation of relay C opens the contacts c1. A holding circuit for the relay A to keep the contacts a1 open when the test signal is disconnected comprises further contacts a3, Fig. 3, in series with the relay and an additional bank of contacts on the selector switch 6. Separate relay circuits are required for each loudspeaker group. The relays may also be located at the loudspeakers and may pass back operating currents for the fault indicators over the loudspeaker lines in phantom or over a pilot line. The Provisional Specification describes a relay system of a similar type to the second embodiment, with the exception that the line circuit is switched from the loudspeakers to a signal receiver by additional contacts on the relay controlling the signal source, without the use of line transformers.