WO2010080374A2

WO2010080374A2 - Method and system for reducing howling in a half-duplex communication system

Info

Publication number: WO2010080374A2
Application number: PCT/US2009/067957
Authority: WO
Inventors: Nissim Gean; Itzhak Avayu; Avishai A. Yacobi
Original assignee: Motorola, Inc.
Priority date: 2008-12-18
Filing date: 2009-12-15
Publication date: 2010-07-15
Also published as: GB0823150D0; WO2010080374A3; GB2466454B; GB2466454A

Abstract

A method for reducing howling in a half-duplex communication system containing radios is presented. Prior to receiving an audio signal transmitted from a sending radio to receiving radios in the half-duplex communication system, the sending radio emits a talk-permit tone from a speaker. If any receiving radios are sufficiently close to the sending radio, they receive the talk-permit tone on their microphones. In that event, those radios, detect whether the talk-permit tone exceeds a predetermined threshold, and if that result occurs, the volume of each detecting radio speaker is automatically limited to a maximum allowed volume.

Description

METHOD AND SYSTEM FOR REDUCING HOWLING IN A HALF-DUPLEX COMMUNICATION

SYSTEM

FIELD OF THE DISCLOSURE

[0001] The present disclosure relates generally to wireless communication systems and more particularly to methods and systems for reducing howling in such communication systems.

BACKGROUND

[0002] Most problems in a communications system arise from highly technical issues, but an equally serious problem can arise from the mundane circumstance of a receiver's speaker becoming acoustically coupled to a transmitter's microphone. When that occurs, a positive feedback loop may be formed if the loop gain is larger than OdB at a frequency where the loop phase is a multiple of 2π. Under these conditions, the electro-acoustic circuit may become unstable and produce a condition known as howling, owing to the loud, screeching sound produced. Howling not only disrupts communication, but it may also overload and damage power amplifiers and speakers,

[0003] Half-duplex communication systems inherently minimize this problem because each system device opens only one channel (receive or transmit) at a time, but howling can still occur when two devices are sufficiently close to produce acoustic coupling.

[0004] Some communication systems minimize howling by using signal processing techniques to detect and minimize the effect. Filters can be inserted into the signal path to decrease loop gain at critical frequencies where howling may occur. Such systems, however, require considerable processing, adding cost and overhead. In any event, these systems only detect the phenomenon after it has started. Also, the desired signal is often distorted by the filters.

[0005] Accordingly, there remains a need for a method that reduces howling in half- duplex communication systems, in an efficient and cost-effective manner. BRIEF DESCRIPTION OF THE FIGURES

[0006] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments. [0007] FIG. 1 is a block diagram of an exemplary environment in which embodiments of the claimed invention may function.

[0008] FIG. 2 is a block diagram illustrating the origin of howling in a half-duplex communication system.

[0009] FIG. 3 is a block diagram of an embodiment of a system for preventing howling in a half-duplex communication system.

[0010] FIG. 4 is a flowchart of an embodiment of a method for preventing howling in a half-duplex communication system.

[0011] FIG. 5 is a flowchart of an alternate embodiment of a method for preventing howling in a half-duplex communication system

[0012] FIG. 6 sets out a signal diagram illustrating an embodiment of a method for preventing howling in a half-duplex communication system employing the Terrestrial Trunked Radio (TETRA) protocol.

[0013] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the claimed invention.

[0014] The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the claimed invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

DETAILED DESCRIPTION

[0015] The following terms are used throughout this document and are defined here for clarity and convenience. All of the following items are described in connection with a half-duplex communication system.

[0016] Request- for-permission-to-transmit signal: A signal sent by a system device to a central control unit requesting permission to transmit over the communication system.

[0017] Permission-to-transmit signal: A signal sent by a central control unit to a system device in response to a request-for-permission-to-transmit signal granting permission to transmit.

[0018] Prepare-to-receive signal: A signal sent by a central control unit to receiving units indicating that the communication system is ready to transmit messages. [0019] Talk-permit tone: An audio signal generated at a system unit in response to receiving a permission-to-transmit signal; the tone alerts the user that permission to transmit has been granted.

[0020] Generally speaking, the following discussion sets out a number of embodiments of a method for reducing howling in a half-duplex communication system. Broadly, prior to receiving an audio signal of the user transmitted from the sending radio at receiving radios in the half-duplex communication system, the sending radio emits a talk-permit tone from a speaker; detecting radios receive the talk-permit tone using microphones; and the detecting radios detect whether the talk- permit tone exceeds a predetermined threshold. As used in this description, the term "receiving radios" includes all radios in the system capable of receiving the transmitted signal from the radio transmitting that signal (the "sending radio"), and "detecting radios" denotes those receiving radios that detect the talk-permit tone from the sending radio. If a detecting radio does determine that the detected talk-permit tone exceeds the predetermined threshold, the volume of each detecting radio speaker is automatically limited to a maximum allowed volume. [0021] FIG. 1 depicts a half-duplex communication system, or network, that provides an exemplary environment 100 in which embodiments of the claimed method can function. The half-duplex communication system 100 includes a central control unit 102 and communication devices 104, 106, 108, 110, 112. The central control unit 102 controls communication between the communication devices 104, 106, 108, 110, 112. The exemplary environment 100 is shown in simplified form and it would be clear to a person of ordinary skill in art that the half-duplex communication system 100 may include many more communication devices and central control units than are illustrated here.

[0022] The communication devices 104, 106, 108 are wireless devices that communicate with the central control unit 102 using wireless communication channels. The communication devices 104, 106, 108 may be capable of communicating with each other directly using wireless communication channels without employing the central control unit 102. The wireless communication may employ any of suitable technology solutions such as Wireless Fidelity (Wi-Fi), Worldwide Interoperability for Microwave Access (WiMAX), Global System for Mobile Communications (GSM), Personal Communications Service (PCS) and Digital Advanced Mobile Phone Service (D-AMPS). It should be noted that the technology solution will be chosen by those in the art based on specific needs of communications application. Additionally, it is anticipated that technological development will bring other solutions into use in the future, and that those solutions may be deployed in connection with the claimed invention, which is not limited in any way by the network technology.

[0023] In the illustrated embodiment, the half-duplex communication system 100 employs the Terrestrial Trunked Radio (TETRA) protocol, an open digital standard (TETRA Release 2) defined by the European Telecommunications Standard Institute (ETSI). In this embodiment, each of the communication devices 104, 106 and 108 is a handheld TETRA radio, operated by a member of a network such as a public safety organization. Each TETRA radio is a transceiver, including a speaker and a microphone, as well as an on-board processor that performs various control functions. Individual units employ Push-to-Talk (PTT) technology. The details of the control functions are set out in the TETRA Standard and will not be discussed further here. The central control unit can include automatically-operating modules that maintain ongoing communications under the TETRA protocol, or the system can employ a dispatcher (not shown) who can provide human control and direction to the network.

[0024] As noted, the communication system 100 operates in half-duplex mode. Thus, each device is operable in only one direction at a time (sending or receiving)

[0025] Further, the central control unit 102 includes a gateway 116, providing interconnectivity to other communications networks. One link could be provided to Public Switched Telephone Network (PSTN) 114, which in turn provides links to landline telephone systems, culminating at devices such as telephone 112, or to computer 110 through a modem. The computer 110 may be a standalone device, or it can represent a computer network such as the Internet, Local Area Network (LAN), and Metropolitan Area Network (MAN). It will be understood that the illustrated environment can be altered substantially, either by expansion of the base network or by the addition of further linked networks.

[0026] FIG. 2 illustrates the origin of howling in the half-duplex communication system 100. As shown, the communication devices 104 and 106 are very close together, with the communication device 108 farther from both of those units. The communication device 104 is shown sending a message to the central control unit 102, which then re-transmits the message to the communication devices 106 and 108. At the sending radio 104, a microphone is activated to receive audio input from the user, and the speaker is deactivated. The opposite actions are performed at the receiving radios 106 and 108, where the microphones are deactivated and the speakers are activated to emit the audio content of the received message.

[0027] Normally, communications devices in a system such as communication system 100 are sufficiently separated to avoid audio coupling between the receiving radio speaker and the sending radio microphone. Here, however, the sending radio 104 and the receiving radio 106 are located in close proximity, so that the audio emitted by receiving radio 106 is received by the microphone at the sending radio 104. Conditions of loop gain, frequency and phase are usually such that acoustic coupling immediately sets up a positive feedback loop, producing howling, in the form of a piercing, shrill sound from the receiving radio speaker.

[0028] Situations in which communications devices operate in close proximity can occur frequently. For example, police officers may be co-located in a number of situations, such as a crowd control operation. If one of the officers sends a radio message to the network, she could be standing close enough to another network member so that acoustic coupling is established, setting up howling.

[0029] FIG. 3 diagrams a system 300 for reducing howling in the half-duplex communication system 100. The system 300 includes the central control unit 102, the sending radio 104 and the receiving radio 106, and it may include multiple additional receiving radios as well.

[0030] The sending radio 104 includes a transmitter 302, a receiver 304, a speaker 306 and a microphone 308. It will be noted by those in the art that the depictions of FIG. 3 include only those components that are directly involved in the operation of the illustrated embodiments, for purposes of simplicity . Those in the art will understand that these radios include unmentioned features generally known in the art, such as PTT buttons, manual controls, and the like. The speaker 306 and the microphone 308 can be automatically activated and deactivated. The sending radio 104 may further include a processor 310 and a memory 312.

[0031] The communication system 100 operates employing a number of control signals that orchestrate the communications sequence, based on the protocol used by the communication system 100. The control signals used in the TETRA protocol are discussed in detail in conjunction with Fig. 6 below. Here, the sending radio initiates a message when its user presses PTT button or switch and the transmitter 302 sends a request-for-permission-to-transmit signal to the central control unit 102. The request- for-permission-to-transmit signal may contain additional information, for example, priority of the sending radio 104, priority of the signal to be transmitted, time for which the transmission will be carried out, buffers available at the sending radio 104 and channels that will be used for communication. The central control unit 102 responds with a permission-to-transmit signal to the sending radio 104 and a prepare - to-receive signal to the receiving radio 106 and any other receiving radios in the system 300. The receiver 304 receives the permission-to-transmit signal from the central control unit 102, and in response, the speaker 306 emits the talk-permit tone at a predetermined volume informing the user that she is cleared to transmit. The permission-to-transmit signal may include additional information, for example, channels to be used for communication, buffers available at the central control unit 102 and the time for which permission is granted.

[0032] The receiving radio 106 includes a microphone 314, a speaker 316, and a receiver 318. It will be apparent to persons of ordinary skill in art that the receiving radio 106 may include more than one of each of speaker, microphone and receiver. The microphone 314 and the speaker 316 can be automatically activated and deactivated and upon receiving the prepare -to-receive signal at the receiver 318, the microphone 314 is activated and the speaker 316 is deactivated for a listening period. Typically, the listening period will be short duration of time, typically 50 ms - 1 s, so that user of the sending radio 104 is not required to wait for long periods of time. The listening period includes time required to emit the talk-permit tone at the sending radio 104; time required to receive the talk-permit tone at the receiving radio 106; and some guard time to perform processing.

[0033] Due to physical proximity of the sending radio 104 to the receiving radio 106, the talk-permit tone emitted by the speaker 306 is audible at the receiving radio, causing the microphone 314 to receive the tone. Thereafter, the receiving radio 106 detects whether the talk-permit tone exceeds a predetermined threshold. The receiving radio 106 may further include a processor 320 which detects whether the talk-permit tone exceeds the predetermined threshold. The processor 320 can detect whether a received sound is actually a talk-permit tone, employing any of a number of methods. For example, the processor 320 can calculate a cross-correlation between the talk-permit tone stored in the receiving radio 106 and the received talk-permit tone to detect whether the received talk-permit tone exceeds the predetermined threshold. The receiving radio 106 may further include a memory 322 to store the talk-permit tone which is cross-correlated with the received talk-permit tone to detect whether the received talk-permit tone exceeds the predetermined threshold. If the talk-permit tone exceeds the predetermined threshold, the processor 320 limits the volume of the speaker 316 to a maximum allowed volume. Further, if the talk-permit tone exceeds the maximum threshold (which is higher than the predetermined threshold), the processor 320 may mute the speaker 316. The maximum allowed volume is set at a level that reduce the loop gain to be smaller than OdB to minimize the possibility for howling, or alternately, the receiving radio 106 can mute the speaker 306, if the detected talk-permit tone exceeds a maximum threshold. The maximum threshold corresponds to a threshold level that meets operating conditions at which the occurrence of howling is essentially unavoidable (e.g., the radios are next to each other).

[0034] The talk-permit tone stored in the sending radio 104 may be any audio signal, with a volume set sufficiently loud to be audible to a user under expected operating conditions. Further, the talk-permit tone should be easily detectable (for example, the talk-permit tone should be chosen such that the tone is insensitive to sampling points as well as to acoustic channel distortion) by the microphone of a detecting radio so that the probability of a wrong decision (false positive and/or negative) at the receiving radios is minimized. The designer can accomplish that goal in a number of ways within the scope of the skill in the art, as by considering the frequency sensitivities of the speaker and microphones, and optimizing the ability of a system to emit and to respond to a given tone. Other methods of accomplishing that result will be apparent to those in the art. Each of the radios may include a memory pre-loaded with the talk-permit tone. Examples of tones that may be used are tones that have been used in TETRA and IDEN radios or tones or signals previously used in radios with PTT ability.

[0035] In an alternative embodiment, in addition to automatically limiting volume of the speaker 316 at the receiving radio 106, a visual or tactile signal can be provided by the receiving radio 106 to the user, indicating that the radio is likely to cause howling. The user may take action to prevent howling in future communication, by moving farther from the sending radio 104 for example.

[0036] When the audio signal is no longer being received, the processor 320 restores the volume of the speaker 316 to the original volume. [0037] FIG. 4 is a flowchart of a method 400 for reducing howling in the half-duplex communication system 100. Here, the sending radio 104 sends a request- for- permission-to-transmit signal to the central control unit 102 to initiate communication, when the user presses the sending radio PTT button. The central control unit 102 responds with a permission-to-transmit signal to the sending radio 104 and a prepare - to-receive signal to the receiving radio 106, when resources are available to carry out the communication.

[0038] In response to receiving the prepare-to-receive signal at the receiving radio 106, the microphone 314 is activated and the speaker 316 is de-activated for the listening period at step 402. Further in response receiving the permission-to-transmit signal at the sending radio 104, the speaker 306 emits the talk-permit tone stored in the sending radio 104 at the predetermined volume (step 404) to alert the user of the sending radio 104 that she may begin talking. A plurality of talk-permit tones may be stored in each radio in the half-duplex communication system 100, one which is selected and emitted.

[0039] Due to physical proximity of the receiving radio 106 to the sending radio 104, the microphone 314 receives the talk-permit tone at step 406. As noted above, detection of the talk-permit tone makes the receiving radio 106 a detecting radio 106, which proceeds, at step 408, further to detect whether the talk-permit tone exceeds a predetermined threshold.

[0040] If the talk-permit tone does exceed the predetermined threshold, the volume of the speaker 316 is automatically limited to the maximum allowed volume, at step 410. In one embodiment, the speaker is not muted, however, unless the talk-permit tone exceeds a maximum threshold (a value higher than the predetermined threshold). If the talk-permit tone does not exceed the predetermined loudness threshold, the method 400 jumps to step 412.

[0041] Alternatively, the detecting radio could limit the speaker's maximum volume (step 410) based on the difference between the detected volume or any other measure(s) (such as energy, correlation, frequency, etc.) of the talk-permit tone and the predetermined threshold. Further, in the various embodiments described herein, the maximum allowed volume is directly proportional to the difference between the detected measure of the talk-permit tone and the predetermined threshold.

[0042] It should be noted that the actions set out in steps 402-410 all occur before the audio signal from the sending radio is received at the detecting radio. Finally, at step 412, the microphone 314 is de-activated and the speaker 316 is activated for a message transmission duration during which the detecting radio 106 receives the audio signal from the sending radio 104. During the message transmission duration, the volume of the speaker 306 is maintained at the maximum allowed volume. When the audio signal is no longer being received by the detecting radio 106 (because the sending radio stops transmitting), in one embodiment, the volume of the speaker 306 is restored to the original volume (the volume set manually by the user prior to the automatically-set limit).

[0043] In an alternate embodiment, shown in FIG. 5, determination of the likelihood of producing howling proceeds by calculating a cross-correlation between the stored talk-permit tone and the received talk-permit tone at the detecting radio 106. The cross-correlation may be transformed to any other measure(s) (such as volume, energy, frequency, etc.). For simplicity, the method set out here addresses the audio coupling established between a transmitter (corresponding to the sending radio 104) and a receiver (corresponding to the detecting radio 106).

[0044] In common with the method of FIG. 4, the transmitter emits an audible signal, such as the talk-permit tone, at step 502. That tone is then detected by a receiver in step 504. The signal is then processed by cross-correlating it with a stored signal, to determine first whether the detected audio signal is the talk-permit tone or simply random noise. Then, the cross-correlation further compares the volume or other measure of the detected signal with a threshold, also stored at the receiver. The threshold is predetermined at a level to separate scenarios likely to cause howling from those too faint to pose a problem. The cross-correlation is calculated at step 506, and then step 508 compares the result to the threshold. The cross-correlation may be calculated by the processor 320 or by an external device in communication with the receiver. [0045] Here, the receiver speaker volume is limited by an amount based on the relationship between the calculated cross-correlation and the threshold amount. That difference is calculated at step 510, and the speaker volume is automatically limited in step 512

Table 1

Table 1 illustrates an example of a set of calculated cross-correlations (their percentages representation in this example) and their corresponding maximum allowed speaker volume levels. To use this table, the receiver determines a peak value of the detected audible signal by calculating the cross-correlation of the stored talk-permit tone with itself (also known as auto-correlation).

[0046] As seen in Table 1, when the calculated cross-correlation is 50 - 100%, indicating that the receiver and transmitter are very close to each other, then the receiver speaker is muted to prevent howling. When the calculated cross-correlation lies in the range of 25% to 50% of the peak value, however, indicating that the receiver and transmitter are farther apart, then the system may prevent howling by limiting the maximum speaker volume to 25% of its maximum value, if the volume is set higher than that. Further, a calculated cross-correlation of 0 to 25% indicates an even wider separation between receiver and transmitter, permitting a maximum speaker volume of 50% of its maximum value to prevent howling. Finally, if the calculated cross-correlation equals zero, the receiver and transmitter are so far apart that howling will not occur, and the speaker volume may stay at its initial volume. [0047] It will be understood by those in the art that the values shown in Table 1 are illustrative, and that other values can be implemented to suit particular conditions. Further, a data structure as shown in Table 1 can be implemented in a number of ways known in the art, such as a lookup table. Further alternatives will be apparent to those in the art.

[0048] In a further alternative, signal intensity can be estimated from the calculated cross-correlation and volume can be limited based on estimated intensity levels. Moreover, distance between the receiver and transmitter can be estimated from the calculated cross-correlation, and speaker volume can be limited based on that distance.

[0049] Further, it should be noted that the limiting the speaker volume in either of the illustrated methods of FIGS. 4 and 5 can be performed in a number of ways, depending on the system configuration. In either embodiment, however, the volume of the speaker 316 is limited such that the output of the speaker 316 reaching the microphone 308 does not cause howling.

[0050] The methods of FIGS. 4 and 5 may be repeated each time a radio in the half- duplex communication system initiates communication in response to PTT button activation. Alternatively, the emitting, receiving, detecting, and automatically limiting steps may be repeated only in specified instances. For example, if message communication has occurred among radios during a given time period with no occurrence of howling, the system may omit further detection steps for a selected time period. The opposite result could be selected if howling is determined to be likely; that is, if proximity between two or more radios is found, those radios could automatically limit speaker volume for a period of time. Further, when the likelihood of howling is determined, the system could take note of the sending radio identity, and if a previously detecting radio requests permission to transmit within the period of time, the previous sending radio could be instructed to automatically limit volume without going through the detecting / calculating / limiting process, thereby saving time, processing, and battery usage.

[0051] FIG. 6 illustrates signal relationships in an embodiment of method for reducing howling in a half-duplex communication system employing the TETRA protocol. In the example half-duplex communication system, a Switching and Management Infrastructure (SwMI) 602 acts as a central control unit, a TETRA Mobile Station (MS) 604 acts as a sending radio and a TETRA MS 606 acts as receiving radio.

[0052] A user A operating the TETRA MS 604, presses PTT button on the TETRA MS 604, switching that unit to sending mode and transmitting a U-SETUP signal (request- for-permission-to-transmit) to the SwMI 602 at time Tl. At time T2, the SwMI 602 receives the U-SETUP signal, and if resources are available the SwMI 602 sends two signals at time T3 : a D-CONNECT signal (permission-to-transmit) to the TETRA MS 604 and a D-SETUP signal (prepare-to-receive) to the TETRA MS 606.

[0053] In response to receiving the D-SETUP signal at time T4, the TETRA MS 606 activates its microphone for a listening period. Also, in response to receiving the D- CONNECT signal at time T5, the TETRA MS 604 emits a talk-permit tone from its speaker. As the TETRA MS 604 is located in close proximity to the TETRA MS 606, the microphone at the TETRA MS 606 receives the talk-permit tone emitted by the speaker at the TETRA MS 604 at time T6. The listening period duration is sufficient to emit the talk-permit tone at the TETRA MS 604 and receive the talk-permit tone at the TETRA MS 606. As the TETRA MS 606 detects the talk-permit tone, that unit automatically limiting the volume of its speaker to prevent howling. After the listening period the TETRA MS 606 deactivates its microphone and activates its speaker to prepare to hear the audio message.

[0054] User A starts speaking into the TETRA MS 604 microphone at time T7 in response to hearing the talk-permit tone. The audio signal is sent to the TETRA MS 606 via the SwMI 602, and user B at the TETRA MS 606 starts hearing the audio signal at time T8. User A releases the PTT button when all messages have been sent, at time T9, prompting the TETRA MS 604 to send a U-TX CEASED signal (end transmission) to the SwMI 602. At TlO the SwMI 602 receives the U-TX CEASED signal at time TlO and in turn sends D-TX CEASED signals to the TETRA MS 604 and the TETRA MS 606 at time TI l indicating the transmission is over. Upon receiving the D-TX CEASED signal at time T 12 and T 13, respectively, the TETRA MS 606 restores its speaker volume to a normal level, and the TETRA MS 604 switches to waiting mode. [0055] In various embodiments of the present invention, the disclosed methods may be implemented as a computer program product for use with a computer system. Such implementations may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk). The medium may be a tangible medium (e.g., optical or analog communications lines). The series of computer instructions embodies all or part of the functionality previously described herein with respect to the system. Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory. It is expected that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink wrapped software) or preloaded with a computer system (e.g., on system ROM or fixed disk). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention are implemented as entirely hardware, or entirely software (e.g., a computer program product).

[0056] In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. The individual features of the various embodiments shown in the figures and described in the text may be incorporated in different manners such that some of the features of one or more embodiments are provided in a different embodiment.

[0057] The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

Claims

1. A method for reducing howling in a half-duplex communication system containing radios, the method comprising: prior to receiving an audio signal transmitted from a sending radio at receiving radios in the half-duplex communication system: emitting a talk-permit tone from a speaker of the sending radio, the talk- permit tone stored in the sending radio, receiving the talk-permit tone at detecting radios using microphones of the detecting radios, the receiving radios containing the detecting radios, and detecting whether the talk-permit tone exceeds a predetermined threshold at the detecting radios; and automatically limiting a volume of a speaker of each detecting radio to a maximum allowed volume when the audio signal is reproduced at the detecting radio, if the detected talk-permit tone exceeds the predetermined threshold at the detecting radio.

2. The method of claim 1 further comprising: prior to receiving the audio signal, activating a microphone and deactivating a speaker of each receiving radio for a listening period in response to receiving a prepare-to-receive signal from a central control unit of the half duplex communications system, wherein the emitting step occurs in response to a permission-to-transmit signal from the central control unit, received at the sending radio.

3. The method of claim 1 , wherein the automatically limiting step further comprises muting the speaker of each detecting radio.

4. The method of claim 1, wherein the automatically limiting step further comprises muting the speaker of each detecting radio if the detected talk-permit tone exceeds a maximum threshold that is higher than the predetermined threshold.

5. The method of claim 1 , wherein the maximum allowed volume is based on the difference between a detected volume of the talk-permit tone and the predetermined threshold.

6. The method of claim 5, wherein the maximum allowed volume is a non-zero volume.

7. The method of claim 1, wherein the detecting step comprises calculating a cross-correlation between the stored talk-permit tone and the received talk-permit tone at each detecting radio.

8. The method of claim 1, further comprising: storing a plurality of talk-permit tones in each radio; and selecting one of the stored talk-permit tones to be emitted by the speaker of the sending radio.

9. The method of claim 1, further comprising de-activating a microphone and activating a speaker of each receiving radio for a message transmission period in which the audio signal is reproduced at the receiving radio.

10. The method of claim 1 further comprising repeating the emitting, receiving, detecting, and automatically limiting steps each time one of the radios initiates communication in response to a Push-To-Talk (PTT) button activation.

11. The method of claim 1 further comprising restoring the volume of the speaker to an original volume at each detecting radio when the audio signal is no longer being received by the detecting radio.

12. The method of claim 1, wherein the half-duplex communication system employs Terrestrial Trunked Radio (TETRA) protocol.

13. A system for reducing howling in a half-duplex communication system, the system comprising: a sending radio having a speaker and being configured to emit a talk-permit tone from the speaker, wherein: the talk-permit tone is stored in the sending radio, and the emitting occurs at a time prior to sending an audio signal over the half-duplex communication system; one or more receiving radios, each having a microphone and a speaker, and each receiving radio configured to: receive the talk-permit tone using the microphone; detect whether the talk-permit tone exceeds a predetermined threshold; and automatically limit the volume of the speaker to a maximum allowed volume during reproduction of the audio signal, if the detected talk-permit tone exceeds the predetermined threshold.

14. The system of claim 13, further comprising a central control unit configured to transmit a prepare-to-receive signal to the receiving radios and a permission-to- transmit signal to the sending radio, both transmissions occurring in response to receiving a request-for-permission-to-transmit signal from the sending radio.

15. The system of claim 13, further comprising a central control unit, wherein: the sending radio further comprises a transmitter and a receiver, the transmitter being configured to send a request-for-permission-to-transmit signal to the central control unit, the receiver being configured to receive a permission-to-transmit signal from the central control unit, and the speaker being further configured to emit the talk-permit tone in response to receiving the permission-to-transmit signal, each receiving radio further comprises a receiver, the receiver being configured to receive a prepare-to-receive signal from the central control unit, the microphone being activated and the speaker being deactivated for listening period in response to receiving the prepare-to-receive signal, and the central control unit is configured to transmit the prepare-to-receive signal to the receiving radios and the permission-to-transmit signal to the sending radio, both transmissions occurring in response to receiving the request- for-permission-to- transmit signal from the sending radio.

16. The system of claim 13, wherein each receiving radio further comprises a processor, the processor being configured to calculate a cross-correlation between the talk-permit tones stored in the receiving radio and the received talk-permit tone.

17. The system of claim 13, wherein each receiving radio is configured to mute the speaker if the detected talk-permit tone exceeds a maximum threshold at the receiving radio, the maximum threshold being higher than the predetermined threshold.

18. The system of claim 13, wherein each receiving radio is further configured to restore the volume of the speaker to an original volume when the audio signal is no longer being received.

19. A communication method comprising: preventing howling between a transmitter and a receiver during transmission from the transmitter to the receiver of an audio signal received from a user of the transmitter by a microphone of the transmitter by: prior to the receiver receiving the audio signal: the transmitter emitting an audible signal stored therein at a predetermined volume, the receiver detecting the emitted audible signal, cross-correlating the detected audible signal with a stored signal, determining whether the cross-correlation exceeds a predetermined threshold, and if the cross-correlation has exceeded the predetermined threshold, calculating by how much the cross-correlation exceeds the predetermined threshold, and automatically limiting a volume of a speaker of the receiver based on the calculation when the audio signal is reproduced by the receiver.

20. The method of claim 19, wherein the audible signal is a tone selected from a plurality of tones stored at the transmitter.