Patent Grant
9681236
Filed of the Invention
The invention relates to a system and a method for providing sound to at least one user, wherein audio signals from an audio signal source, such as a microphone for capturing a speaker's voice, are transmitted via a wireless link to a receiver unit, such as an audio receiver for a hearing aid, from where the audio signals are supplied to means for stimulating the hearing of the user, such as a hearing aid loudspeaker.
Description of Related Art
Typically, wireless microphones are used by teachers teaching hearing impaired persons in a classroom (wherein the audio signals captured by the wireless microphone of the teacher are transmitted to a plurality of receiver units worn by the hearing impaired persons listening to the teacher) or in cases where several persons are speaking to a hearing impaired person (for example, in a professional meeting, wherein each speaker is provided with a wireless microphone and with the receiver units of the hearing impaired person receiving audio signals from all wireless microphones). Another example is audio tour guiding, wherein the guide uses a wireless microphone.
Another typical application of wireless audio systems is the case in which the transmission unit is designed as an assistive listening device. In this case, the transmission unit may include a wireless microphone for capturing ambient sound, in particular from a speaker close to the user, and/or a gateway to an external audio device, such as a mobile phone; here the transmission unit usually only serves to supply wireless audio signals to the receiver unit(s) worn by the user.
Typically, the wireless audio link is an FM (frequency modulation) radio link operating in the 200 MHz frequency band. Examples of analog wireless FM systems, particularly suited for school applications, are described in European Patent Application EP 1 864 320 A1 and corresponding U.S. Pat. No. 7,648,919 B2 and in International Patent Application Publication WO 2008/138365 A1 and corresponding U.S. Pat. No. 8,345,900 B2.
In recent systems, analog FM transmission technology has been replaced by technology employing digital modulation techniques for audio signal transmission, most of them working on other frequency bands than the former 200 MHz band.
U.S. Pat. No. 8,019,386 B2 relates to a hearing assistance system comprised of a plurality of wireless microphones worn by different speakers and a receiver unit worn at a loop around a listener's neck, with the sound being generated by a headphone connected to the receiver unit, wherein the audio signals are transmitted from the microphones to the receiver unit by using a spread spectrum digital signals. The receiver unit controls the transmission of data, and it also controls the pre-amplification gain level applied in each transmission unit by sending respective control signals via the wireless link.
International Patent Application Publication WO 2008/098590 A1 and corresponding U.S. Patent Application Publication 2010/019836 A1 relate to a hearing assistance system comprising a transmission unit having at least two spaced apart microphones, wherein a separate audio signal channel is dedicated to each microphone, and wherein at least one of the two receiver units worn by the user at the two ears is able to receive both channels and to perform audio signal processing at ear level, such as acoustic beam forming, by taking into account both channels.
In wireless digital sound transmission systems, not only audio data is to be transmitted but also control data, for example, for controlling the volume of playback of audio signals, for configuring the operation mode of the devices, for querying the battery status of the devices, etc. The transmission of such control data causes, compared to audio data transmission alone, overhead to the system in current consumption and/or delay which should be minimized.
There are certain known methods for concurrent transmission of audio data and control data. A schematic overview concerning the basic types of such concurrent transmission is shown in
In general, transmission of control data can be made either “out-of-band” or “in-band”. In this context “out-of-band” means that different logical communication channels are used for audio data transmission and control data transmission, i.e., audio and control data are transmitted in separate digital streams. Such technique is used, for example, in mobile and fixed telephony networks. “In-band” means that control data is somehow combined with the audio data for transmission. In digital transmission of audio signals, usually the audio data as provided by the analog-to-digital converter is compressed prior to transmission by using an appropriate audio-codec. The resulting compressed audio data stream can be either transmitted sample-by-sample, i.e., as an essentially continuous stream, or in packets of samples.
Another known example of in-band control data transmission for sample-by-sample audio transmission is shown in
A similar principle of in-band control data transmission for a packet-based audio data transmission is shown in
In
Any such combined audio and control data transmission method either introduces a large delay in the transmission of the control commands or introduces a large overhead in terms of bit rate reserved for control traffic, which translates into a power consumption overhead.
It is an object of the invention to provide for a digital sound transmission method and system, wherein control data transmission is achieved in such a manner that both power consumption overhead and delay in control data transmission is minimized.
According to the invention, this object is achieved by a method and a system as described herein.
The invention is beneficial in that, by replacing part of the audio data by control data blocks, with each control data block including a marker for being recognized by the receiver unit(s) as a control data block and a command for being used for control of the receiver unit, delay in the command transmission can be kept very small (as compared to, for example, the interleaved control data transmission shown in
Hereinafter, examples of the invention will be illustrated by reference to the accompanying drawings.
In
First, the method of
The control channel overhead to the system is given by the relationship:
The overhead caused by the control channel will be evaluated as the ratio between control bit rate and audio bit rate O1=DC/DA.
A control message is a packet starting with a start frame delimiter (of, e.g., a one byte size), followed by the command data (of, e.g., a 2 bytes size at minimum) and terminated with a CRC (of a 16 bits size at a minimum). This gives a control frame of size 5 bytes. The delay to get such a message through the control channel is:
The overhead versus delay curve for this method 1 is shown in
Next, the method of
The resulting total bit rate is
where TA=4 ms is the interval between audio packets.
The overhead is computed as the ratio between the number of bits reserved for control divided by the number of audio and base overhead bits:
A control frame size of 5 bytes is considered, including, as for method 1, one byte start frame delimiter, 2 bytes command and 2 bytes CRC. The delay is computed as the number of 4 ms periods required to transmit the 5 bytes control frame:
T2=TA·┌40/NC┐
When the number of control bits NC is equal to the size of a control message, the delay becomes minimum with T2=TA.
The overhead versus delay curve for this method is shown in
If the G.722 standard is used in mode 2 and if the interval between audio packet is kept at 4 ms, the number of audio bits becomes NA=224. If the radio packets are limited to 256 bits, this leaves hence 32 bits for control information. The delay in this case would be 4 ms, as 2 bytes command and 2 bytes CRC can be transmitted in a single radio packet. There is no need of start frame delimiter since, in this case, control frames are not segmented over several radio packets. The overhead in this case is:
This operating point is shown as a circle in
Finally, the method of
The (maximum) delay with this method is the interval between beacon reception:
T3−TC
The overhead versus delay curve for this method is shown in
The present invention relates to a system for providing hearing assistance to at least one user, wherein audio signals are transmitted, by using a transmission unit comprising a digital transmitter, from an audio signal source via a wireless digital link to at least one receiver unit, from where the audio signals are supplied to means for stimulating the hearing of the user, typically a loudspeaker, wherein control data is to be transmitted via the digital link in a manner that the trade-off between delay in the transmission of the control commands and introduction of a large power consumption overhead involved in the prior art methods of
As shown in
The system may include a plurality of devices on the transmission side and a plurality of devices on the receiver side, for implementing a network architecture, usually in a master-slave topology.
The transmission unit typically comprises or is connected to a microphone for capturing audio signals, which is typically worn by a user, with the voice of the user being transmitted via the wireless audio link to the receiver unit.
The receiver unit typically is connected to a hearing aid via an audio shoe or is integrated within a hearing aid.
In addition to the audio signals, control data is transmitted bi-directionally between the transmission unit and the receiver unit. Such control data may include, for example, volume control or a query regarding the status of the receiver unit or the device connected to the receiver unit (for example, battery state and parameter settings).
In
Another typical use case is shown in
A modification of the use case of
According to a variant of the embodiments shown in
The transmission units 10, 110 may comprise an audio input for a connection to an audio device, such as a mobile phone, a FM radio, a music player, a telephone or a TV device, as an external audio signal source.
In each of such use cases, the transmission unit 10 usually comprises an audio signal processing unit (not shown in
An example of a transmission unit 10 is shown in
The VAD 24 uses the audio signals from the microphone arrangement 17 as an input in order to determine the times when the person 11 using the respective transmission unit 10 is speaking. The VAD 24 may provide a corresponding control output signal to the microcontroller 26 in order to have, for example, the transmitter 28 sleep during times when no voice is detected and to wake up the transmitter 28 during times when voice activity is detected. In addition, a control command corresponding to the output signal of the VAD 24 may be generated and transmitted via the wireless link 12 in order to mute the receiver units 14 or saving power when the user 11 of the transmission unit 10 does not speak. To this end, a unit 32 is provided which serves to generate a digital signal comprising the audio signals from the processing unit 20 and the control data generated by the VAD 24, which digital signal is supplied to the transmitter 28. The unit 32 acts to replace audio data by control data blocks, as will be explained in more detail below with regard to
According to one embodiment, the transmission units 10 may be adapted to be worn by the respective speaker 11 below the speaker's neck, for example, as a lapel microphone or as a shirt collar microphone.
An example of a digital receiver unit 14 is shown in
Rather than supplying the audio signals amplified by the variable gain amplifier 62 to the audio input of a hearing aid 64, the receiver unit 14 may include a power amplifier 78 which may be controlled by a manual volume control 80 and which supplies power amplified audio signals to a loudspeaker 82 which may be an ear-worn element integrated within or connected to the receiver unit 14. Volume control also could be done remotely from the transmission unit 10 by transmitting corresponding control commands to the receiver unit 14.
Another alternative implementation of the receiver maybe a neck-worn device having a transmitter 84 for transmitting the received signals via with an magnetic induction link 86 (analog or digital) to the hearing aid 64 (as indicated by dotted lines in
In general, the role of the microcontroller 24 could also be taken over by the DSP 22. Also, signal transmission could be limited to a pure audio signal, without adding control and command data.
Details of the protocol of the digital link 12 will be discussed by reference to
The preferred codec used for encoding the audio data is sub-band ADPCM (Adaptive Differential Pulse-Code Modulation).
In addition, packet loss concealment (PLC) may be used in the receiver unit. PLC is a technique which is used to mitigate the impact of lost audio packets in a communication system, wherein typically the previously decoded samples are used to reconstruct the missing signal using techniques such as wave form extrapolation, pitch synchronous period repetition and adaptive muting.
Preferably, data transmission occurs in the form of TDMA (Time Division Multiple Access) frames comprising a plurality (for example, 10) of time slots, wherein in each slot one data packet may be transmitted. In
Preferably, a slow frequency hopping scheme is used, wherein each slot is transmitted at a different frequency according to a frequency hopping sequence calculated by a given algorithm in the same manner by the transmitter unit 10 and the receiver units 14, wherein the frequency sequence is a pseudo-random sequence depending on the number of the present TDMA frame (sequence number), a constant odd number defining the hopping sequence (hopping sequence ID) and the frequency of the last slot of the previous frame.
The first slot of each TDMA frame (slot 0 in
The second slot (slot 1 in
Rather than allocating separate slots to the beacon packet and the response of the slaves, the beacon packet and the response data may be multiplexed on the same slot, for example, slot 0.
The audio data is compressed in the transmission unit 10 prior to being transmitted.
Usually, in a synchronized state, each slave listens only to specific beacon packets (the beacon packets are needed primarily for synchronization), namely those beacon packets for which the sequence number and the ID address of the respective slave device fulfills a certain condition, whereby power can be saved. When the master device wishes to send a message to a specific one of the slave devices, the message is put into the beacon packet of a frame having a sequence number for which the beacon listening condition is fulfilled for the respective slave device. This is illustrated in
Periodically, all slave devices listen at the same time to the beacon packet, for example, to every tenth beacon packet (not shown in
Slaves whose ID is not know to the network master will listen to the beacon satisfying the condition with an ID equal to 0.
Each audio data packet comprises a start frame delimiter (SFD), audio data and a frame check sequence, such as CRC (Cyclic Redundancy Check) bits. Preferably, the start frame delimiter is a 5 bytes code built from the 4 byte unique ID of the network master. This 5 byte code is called the network address, being unique for each network.
In order to save power, the receivers 61 in the receiver unit 14 are operated in a duty cycling mode, wherein each receiver wakes up shortly before the expected arrival of an audio packet. If the receiver is able to verify (by using the CRC at the end of the data packet), the receiver goes to sleep until shortly before the expected arrival of a new audio data packet (the receiver sleeps during the repetitions of the same audio data packet), which, in the example of
In order to further reduce power consumption of the receiver, the receiver goes to sleep already shortly after the expected end of the SFD, if the receiver determines, from the missing SFD, that the packet is missing or has been lost. The receiver then will wake up again shortly before the expected arrival of the next audio data packet (i.e., the copy/repetition of the missing packet).
An example of duty cycling operation of the receiver is shown in
According to the invention, control data may be transmitted instead of audio data, thereby avoiding any overhead in the system while minimizing delay of control data transmission. This is indicated in
In
For enabling such masking action, the receiver unit 14 is adapted to detect the replacement of compressed audio data by a control data block 50.
Preferably, the control data block 50 starts with a predefined flag which allows the receiver unit 14 to distinguish control data from audio data, thereby acting as a marker. The flag is followed by the command and then by a CRC word. For example, the flag may comprise 32 bits, and also the CRC word may comprise 32 bits. With a 32 bits flag, the probability to find the flag in a random bit stream is ½32. Such an event will happen, on average, every 232/64,000=18 hours with a 64 kbps compressed audio bit rate having a random 0/1 distribution. The flag should be selected in such a manner that it is unlikely to be found in a typical compressed audio stream.
If a flag is found in noise, it is very likely (probability: 1½32) that the CRC will be wrong and hence the command will not be applied.
The total size of the control data block 50, for example, may be 8 bytes (consisting of a 4 bytes flag, a 2 byte command and a 2 byte CRC). This corresponds to 16 samples in the G.722 standard or 1 ms with 16 kHz sampling.
As already mentioned above, the control data is supplied, together with audio data to the DSP 74, where it is used for control of the receiver unit 14.
During the time window 57, the receiver unit 14 may take a masking action for masking the temporary absence of received audio data, such as applying a packet loss concealment extrapolation algorithm, generating a masking output audio signal, such as a beep signal, or muting of the audio signal output of the receiver unit 14. The packet loss concealment algorithm, for example, could be G.722 appendix IV, and it could be applied in such a manner that no delay is added, via pre-computation of the concealment frame before it is known if this concealment frame will be required or not. Generating a beep signal would make sense of a beep is required anyway as a feedback to the user for the reception of the transmitted command. However, as some commands may not require a beep, the option of applying a packet loss concealment algorithm may be preferred. Muting of the output signal is the most basic way to minimize the effect of the missing audio information, while packet loss concealment extrapolation is preferred.
As in the example of
232×TA=232×4×10−3=198 days.
In addition, a CRC word at the end of the packet will protect against false detections.
Alternatively, the control data marker could be realized as a signaling bit in the header of the audio data packet. Such marker enables the receiver unit 14 to detect that audio data has been replaced by control data in a packet. Since the data transmission in the example of
In the example of
In the examples of
In
As also indicated in
The content of the received redundant audio data block copy 51B may be used for “masking” the loss of audio data caused by replacement of the first copy of the audio data packets 51B by the control data packet 50 (in fact, in case that one of the two remaining copies of the audio data packets 51B is received by the receiver unit 14, there is no loss in audio data caused by replacement of the first audio data packet 51B by the control data packet 50). Thus, the decompressed audio data stream 54 remains uninterrupted even during that frame when the control data packet 50 is transmitted, since then the second copy of the audio data packet 51B is received and decompressed, see
The embodiment of
It is noted that the invention may be combined with one of the prior art transmission schemes. For example, the method shown in
One example for a control command for which low delay is desirable is a “mute” command wherein ear level receiver units 14 are set in a “mute” state when the microphone arrangement 17 of the transmission unit 10 detects that the speaker using the microphone arrangement 17 is silent. Transmitting the mute command via the beacon would take much time, since the beacon, in the above system, is received by ear level receiver units every 128 ms, for example. When applying replacement of audio data by control data packets according to the invention, in the above example, a maximum delay of 4 ms is reached for the transmission of such “mute” command.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/054901 | 3/30/2011 | WO | 00 | 11/8/2013 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2012/130297 | 10/4/2012 | WO | A |
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| Number | Date | Country |
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| 1 241 664 | Sep 2002 | EP |
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| Number | Date | Country | |
|---|---|---|---|
| 20140056451 A1 | Feb 2014 | US |
