GB2123193A

GB2123193A - Public address system

Info

Publication number: GB2123193A
Application number: GB08316762A
Authority: GB
Inventors: James Ernest Gleave
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-06-19
Filing date: 1983-06-20
Publication date: 1984-01-25
Also published as: GB2123193B; GB8316762D0

Abstract

A central station (10) is linked to a plurality of loudspeaker stations (11) by a common digital and audio highway (12). Each loudspeaker station has a unique address (22) and digital signals are addressed to the loudspeaker stations to turn the loudspeakers (13) on and off and establish the loudness levels by way of variable gain amplifiers (14). Each loudspeaker station (11) includes digital circuitry centered on a microprocessor (17) for responding to address and control words from the central station (10) and controlling the amplifier (14). To test whether a loudspeaker is speaking, an adjacent microphone (24) can return an answering signal to the digital highway by way of an amplifier (26) and analog input port (25). In a modified embodiment the answering signal is sent on the audio highway and the amplifier (14) is energised for test purposes by a tone generator controlled by the digital circuitry. <IMAGE>

Description

SPECIFICATION Public address system The present invention relates to a public address (PA) system comprising a central station connected by cable to a plurality of loudspeakers and having the ability to make announcements selectively from individual loudspeakers or groups of loudspeakers.

Such PA systems are widely used in industrial premises and on board ship to name just two examples.

The practice has been to make up PA systems from power amplifiers and loudspeakers. The loudspeakers are electro-magnet or electro-static devices in which incoming messages, in the form of audio frequency signals from the power amplifiers, are applied to a diaphram which vibrates to announce the message. The power amplifiers are generally located in the central station position and connected by respective cables to the remotely positioned loudspeakers. Each central amplifier drives one or more loudspeakers, but all loudspeakers connected to one amplifier receive and announce the same message.

Such known systems obviously provide a degree of selectivity in that only the desired amplifiers may be enabled for a given message. Moreover the gains of the amplifiers can be adjusted individually to establish satisfactory loudness levels. However it is not possible to be selective within a group of loudspeakers served by the same amplifier and the adjustment of relative loudness levels of the loudspeakers within a group can only be effected by changing transformer tappings or adjusting pad resistors at the loudspeakers themselves. Moreover each amplifier has to have its own cabling for sending the audio signals to its group of loudspeakers, The object of the present invention is to overcome these disadvantages of the prior art and, in particular, to provide a system which can be completely selective as to which loudspeakers give out any particular message.

According to the present invention, there is provided a public address system comprising a central station connected to a plurality of loudspeakers by a single cable highway capable of carrying both audio signals and digital signals, means at the central station for sending out digital signals identifying the loudspeakers individually, and control devices associated with the loudspeakers respectively and each including a switching device for switching the associated loudspeaker on or off and digital means responsive to digital signals addressed to the control device in question to operate the switching device.

The switching devices are preferably variable gain devices which can be operated under control of the digital means to establish the loudness level of the associated loudspeaker.

The cable highway preferably also distributes power to the control devices. In this case the most straightforward implementation uses a six-wire cable with one pair for power, one pair for audio and one pairfor digital data. A four-wire cable may be used with power and data on one pair and audio on the other. It is even possible to put power, audio and data on a single pair. Regardless of the implementation the highway may be regarded as comprising a data highway and an audio highway. The configuration of the highway, whether star, ring or mixed is immaterial.

It is advantageous to provide a test facility whereby it is possible to determine from the central station whether each loudspeaker is actually speaking. To this end a microphone is mounted adjacent each loudspeaker and a signal is returned to the central station when the microphone is picking up a signal.

Various possibilities exist in this respect, as follows: (1) The microphone provides a signal to an analog port of the associated digital means and an answering digital signal indicating "message picked up" is generated for return to the central station. The test may then be effected by turning on only one loudspeaker and seeing whether the answering signal is received. Alternatively, the loudspeakers may be all turned on and the answering signals stored in the respective digital means which are then polled in turn to see which provide the answering digital signal.

(2) The microphones are all connected to the highway and each control device includes a test tone generator for energising its loudspeaker alone when commanded from the central station via the associated digital means. A single loudspeaker is accordingly energised and the central station checks to see if an answering audio signal appears on the highway.

(3) The features of (1 ) and (2) are combined so that the tests involve only digital signals passing from the central station and returning thereto from the control devices.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure lisa block diagram of a system embodying the invention, Figure 2 is a partial block diagram illustrating a modified embodiment, Figure 3 is a block diagram of the central station of Figure 1, and Figures 4 and 5 illustrate alternative realisations of a loudspeaker station.

Referring to Figure 1,the system comprises a central station 10 linked to a plurality of loudspeaker stations 11 by a highway 12 which comprises an audio highway 12a and a digital highway 12d in accordance with one of the various implementations thereof referred to above. For simplicity, Figure 1 shows only two loudspeaker stations 11 and the details of only one such station are illustrated. The loudspeaker stations are identical and their number is limited only by such practical considerations as the power of the amplifier at the central station 10 and the attenuation introduced by the audio highway 12a.

Turning to the construction of a loudspeaker station 11, the audio highway 12a is connected to a loudspeaker 13 through a variable gain amplifier 14 whose gain may be varied from zero up to a predetermined maximum under control of a signal on a line 15. The amplifier 14 thus acts as a switching device which can be used to turn the loudspeaker 13 on and off, corresponding to appreciable amplifier gain and zero gain respectively.

The data highway 12d is connected to digital circuitry comprising a plurality of devices communicating by way of an internal bus 16. The devices on the bus comprise a microprocessor 17, a master timing circuit 18, a program memory (ROM) 19, a working memory (RAM) 20, a data input/output port 21 which provides the means of communication with the data bus 12d, and a patch device 22 which is set up to provide a unique code identifying the loudspeaker station in question. Moreover, a control unit 23 is an analog output port which provides the gain control signal on the line 15.

The scheme of communication between the central station 10 and each loudspeaker station 11 may be elaborated to any extent desired, essentially by providing appropriate software in the program memory 19 at each loudspeaker station and a program memory in the central station. However, it suffices to consider a simple, basic pattern of communication according to which the central station transmits on the data highway 12d an address word followed by a control word. These words may be distinguished by the value of the most significant bit, for example. The address words match the identifying codes of the patch units 22 and, when a loudspeaker station 11 receives its own address code, it responds to the following control word. All other loudspeaker stations 11 ignore the control word.The control words need merely correspond to a plurality of preselected gain settings for the amplifier 14, including zero gain. Thus, to turn any given loudspeaker 13 off, the central station 10 merely has to send the address word corresponding to the loudspeaker station 11 in question, followed by the zero gain control word. The processor 17, acting under control of the program in its ROM 19 causes the control unit 23 to place the zero gain signal on the line 15. Similarly, to turn a loudspeaker 13 on, the address word is sent followed by a word establishing the desired gain for that loudspeaker.

As a refinement on this simple scheme, the program may be arranged to store the current "on" gain for the amplifier 14 in the RAM 20. The central station 10 then merely sends "on" and "off" control words and an "on" control word causes the gain to be set to the value stored in the working memory 20. When this gain is found to be unsuitable, the central station 10 can send control words which adjust the stored gain value in the RAM 20 to a greater or smaller value, as desired.

Figure 1 also illustrates one way of implementing a test facility for each loudspeaker 13. Adjacent to the loudspeaker 13 is a microphone 24 which feeds an analog input port 25 by way of an amplifier 26.

The port 25 is connected to the internal bus 16. To test any given any loudspeaker station 11, a tone is sent on the audio highway 1 2a with the relevant amplifier 14 turned up to the required gain. Provided the amplifier 14 and loudspeaker 13 are working satisfactorily, the microphone 24 will pick up a corresponding signal and the digital circuits are programmed to store a signal from the port 25 which can be referred to as an answering signal. This signal can be sent automatically to the central station on the data highway 12d or it can be sent in response to a polling request from the central station 10. In any event, it is possible to relay the answering signal back to the central station to show that the loudspeaker 13 is providing an acoustic output.

In an alternative embodiment (Figure 2) the microphone amplifier 26 is connected directly to the audio highway 12a. The amplifier 14forthe loudspeaker 13 has two mutually isolated inputs; for example a differential amplifier may be employed.

The control unit 23 also controls a test tone generator 27 which is connected to the second input of the amplifier 14. In order to test a given loudsDeaker 13, a digital signal is now sent from the central station 10 to cause the control unit 23 to enable the test tone generator 27. Provided the amplifier 14 and loudspeaker 13 are working properly, a signal is picked up by the microphone 24 and fed back to the audio highway 12a through the amplifier 26. This answering audio signal can be sensed at the central station.

It will be seen that, in the case of Figure 1, testing is effected by sending out an audio test signal and receiving a digital answering signal. In Figure 2, a digital test signal is sent out and an audio answering signal is received. If both the analog input port 25 of Figure 1 and the test tone generator 27 of Figure 2 are employed, it is possible to send out a digital test signal and receive a digital answering signal.

Figure 3 shows one form of the central station 10.

A microphone 30 is employed in making announcements and the audio highway 1 2a is driven by way of a preamplifier 31 and a power amplifier 32. The digital circuitry at the central station is in essence the same as that at each loudspeaker station 11. An internal bus 33 provides communication between a microprocessor 34, timing circuits 35, a program memory 36, a working memory 37 and a data input/output port 38 which provides the link with the data highway 12d. Naturally the program in the ROM 36 is different from the program in each of the ROMs 19 of the loudspeaker stations 11. In addition, it is necessary for the operator at the central station to be able to provide the commands which are necessary to turn loudspeakers on and off, adjust gains at loudspeaker stations and carry out test sequences.

To this end a dedicated front panel may be provided but it is preferred to provide a keyboard 39 for keying in the required commands. For the embodiment of Figure 2, an analog input port 40 may be provided to sense the answering audio signal on the audio highway 12a. In either of the embodiments of Figures 1 and 2, it may be arranged that an answering loudspeaker station also sends back its own identifying address code as further confirmation that the addressed station is the one giving the answer that it is functioning correctly.

The detailed digital implementation need not be described since it will follow well known microp rocessortech niques and essentially involves writing the correct operating programs to control the sending of address and control words from the central station 10 in accordance with the commands entered at the keyboard 39.

Figure 4 shows one form of construction for a loudspeaker station. The cable highway 12 has branches for feeding the loudspeaker stations and one station is shown with a loudspeaker 43 having terminals 44 inside a rear housing 45. Within this housing there is provided a self-contained unit 46 connected between the cable highway branch 42 and the terminals 44. The unit 46 contains all of the digital and audio circuitry of the loudspeaker station and the pick up microphone 24.

In an alternative construction (Figure 5) the unit 46 is placed within a separate enclosure 47. This may be advantageous when embodying the invention in an existing PA system. It may also be desirable when complying with flameproofing requirements in hazardous locations.

As described above, control words are addressed to individual loudspeaker stations. However, it is possible, using known techniques, to address control words to groups of stations, to enable groups of loudspeakers to be simultaneously turned on and off.

Claims

1. A public address system comprising a central station connected to a plurality of loudspeakers by a single cable highway capable of carrying both audio signals and digital signals, means at the central station for sending out digital signals identifying the loudspeakers individually, and control devices associated with the loudspeakers respectively and each including a switching device for switching the associated loudspeaker on or off and digital means responsive to digital signals addressed to the control device in question to operate the switching device.

2. A system according to claim 1, wherein the switching devices are variable gain devices operable under control of the digital means to establish the loudness level of the associated loudspeaker.

3. A system according to claim 1 or 2, wherein each loudspeaker has an associated microphone and the digital means are operative in a test mode to enable an answering signal to be returned to the central station from the microphone in response to a test signal applied to the loudspeaker.

4. A system according to claim 3, wherein the digital means control a test signal generator connected to the loudspeaker.

5. A system according to claim 3 or 4, wherein the digital means include an analog input port connected to receive a signal from the microphone.

6. A public address system substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 1 taken in conjunction with any of Figures 2 to 5 of the accompanying drawings.