WO2023078538A1

WO2023078538A1 - Method and apparatus for transmitting audio

Info

Publication number: WO2023078538A1
Application number: PCT/EP2021/080462
Authority: WO
Inventors: Barbaros Kirisken
Original assignee: Vestel Elektronik Sanayi Ve Ticaret A.S.
Priority date: 2021-11-03
Filing date: 2021-11-03
Publication date: 2023-05-11

Abstract

A source device (10) carries out a DRM-learning process with one or more detected sink devices (20) to learn whether or not the respective sink device (20) is capable of receiving and decoding digital-rights-management, DRM, protected audio. If any of the sink devices (20) is not capable of receiving and decoding DRM -protected audio, the source device (10) transmits the audio on at least two channels for receipt by the sink devices (20), in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device (20) that is not capable of receiving and decoding DRM-protected audio, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device (20) that is capable of receiving and decoding DRM-protected audio.

Description

METHOD AND APPARATUS FOR TRANSMITTING AUDIO

Technical Field

The present disclosure relates to a method of and apparatus for transmitting audio from a source device to one or more sink devices.

Background

It is known to transmit audio from a source device to sink devices. For example, an audio playback device may wirelessly transmit audio to plural loudspeakers for playing out the audio. Problems can arise if the sink devices are not DRM (digital rights management) enabled (for example, some older devices that are unable to playback audio that is DRM-protected), or if some of the sink devices are DRM-enabled and some of the sink devices are not DRM-enabled.

Summary

According to a first aspect disclosed herein, there is provided a method of transmitting audio from a source device to one or more sink devices, the method comprising: the source device carrying out a discovery process to detect the presence of one or more sink devices; the source device carrying out a DRM-learning process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding digital-rights- management, DRM, protected audio; wherein if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the source device transmits the audio on at least two channels for receipt by the detected sink devices, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device that is not capable of receiving and decoding DRM-protected audio, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device that is capable of receiving and decoding DRM-protected audio. In an example, if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is mono audio.

In an example, the audio that is transmitted on the two channels is the same mono audio.

In an example, if all of the detected sink devices are capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is stereo audio with DRM enabled, with left stereo being transmitted on a first of the channels and right stereo being transmitted on a second of the channels.

In an example, the source device is connected wirelessly to the sink devices and transmits the audio wirelessly for receipt by the detected sink devices.

In an example, audio that is transmitted with DRM enabled is transmitted with a higher sound quality than audio that is transmitted with no DRM enabled.

In an example, the source device has at least two radios, and audio that is transmitted with DRM enabled and audio that is transmitted with no DRM enabled are transmitted in stereo by the respective radios.

According to a second aspect disclosed herein, there is provided a source device for transmitting audio to one or more sink devices, the source device being constructed and arranged to: carry out a discovery process to detect the presence of one or more sink devices; carry out a DRM-leaming process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding digital-rights-management, DRM, protected audio; wherein if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the source device is arranged to transmit the audio on at least two channels for receipt by the detected sink devices, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device that is not capable of receiving and decoding DRM-protected audio, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device that is capable of receiving and decoding DRM-protected audio.

In an example, the source device is arranged such that if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is mono audio.

In an example, the source device is arranged such that if all of the detected sink devices are capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is stereo audio with DRM enabled, with left stereo being transmitted on a first of the channels and right stereo being transmitted on a second of the channels.

In an example, the source device is arranged such that audio that is transmitted with DRM enabled is transmitted with a higher sound quality than audio that is transmitted with no DRM enabled.

In an example, the source device has at least two radios, and is arranged such that audio that is transmitted with DRM enabled and audio that is transmitted with no DRM enabled are transmitted in stereo by the respective radios.

According to a third aspect disclosed herein, there is provided a method of transmitting audio for play back by plural sink devices, the method comprising: at least one of the sink devices carrying out a discovery process to detect the presence of one or more other sink devices, said at least one of the sink devices being capable of receiving and decoding digital-rights-management, DRM, protected audio; the at least one of the sink devices carrying out a DRM-learning process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding DRM-protected audio; a source device transmitting audio with DRM enabled; said at least one of the sink devices receiving and decoding the audio with DRM enabled; and said at least one of the sink devices transmitting the audio with no DRM for receipt and play back by a detected sink device that is not capable of receiving and decoding DRM-protected audio.

Brief Description of the Drawings

To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made by way of example to the accompanying drawings in which:

Figure 1 shows schematically an example of a source device in communication with plural sink devices;

Figure 2 shows schematically an example of the source device and a sink device exchanging information;

Figure 3 shows schematically an example of a packet structure for distribution of audio; and

Figure 4 shows schematically an example of sink devices accepting and rejecting audio.

Detailed Description

As noted, it is known to transmit audio from a source device to one or more sink devices. For example, an audio playback device can be used to wirelessly transmit audio to plural loudspeakers for playing out the audio. Problems can arise if the sink devices are not DRM (digital rights management) enabled (i.e. are unable to playback audio that is DRM-protected), or if some of the sink devices are DRM- enabled and some of the sink devices are not DRM-enabled. As a particular example in which embodiments described herein may be used, the source device is an audio playback device that uses wireless transmissions, using for example short-range wireless communication technology (SRWT), to distribute audio to different sink devices, which may for example be or include wireless speakers which play out the audio. One or more of the sink devices are DRM-enabled such that they can receive and decode DRM-protected audio that is transmitted by the source device, such that the sink devices can play back DRM-protected audio. On the other hand, there may be sink devices that are not DRM-enabled and which therefore cannot play DRM-protected content. Such non DRM-enabled sink devices may for example be older, “legacy”, devices. Such a situation may occur in practice if the user wishes to distribute the audio for playback in several locations, such as a number of different rooms in a house or other dwelling, or in a house and in a garage or outhouse, etc.

In such a scenario, in some cases, if the user desires to distribute the audio to both the DRM-enabled sink devices and to the non-DRM-enabled sink devices, the source device can only distribute low-quality audio. This may occur if for example the SRWT enforces a DRM scheme like the Serial Copy Management System (SCMS) in which certain sink devices cannot be connected for high quality audio. This includes for example some A2DP sink devices, where A2DP is the “Advanced Audio Distribution Profile” which defines how multimedia audio can be streamed from one device to another over a Bluetooth connection as an example of SRWT. The only other alternative to being restricted to low quality audio, even on the DRM- enabled sink devices, is to only use the DRM-enabled sink devices, but that means that the user cannot use the non-DRM-enabled sink devices.

Referring to Figure 1, this shows schematically an example of a source device 10 in communication with plural sink devices 20. Whilst in this example the source device 10 is in wireless communication with each of the plural sink devices 20 (as indicated by the double-headed arrows in the drawing), in other examples the connection between one or more of the sink devices 20 and the source device 10 may be a wired connection. The source device 10 may be for example an audio playback device 10, which in general may be any device that can output audio signals to be passed ultimately to one or more speaker devices to output audio. The audio playback device 10 may be a mobile device. The audio playback device 10 may be for example a cellular phone (including a so-called “smart phone”), an MP3 player, a tablet or notebook computer, some other audio playback device such as a radio, a CD player, etc. The source device 10 includes one or more processors, radio transmitters, receivers, etc. to enable communication with the sink devices 20 and to enable the required processing of signals for transmission to and received from the sink devices 20. The source device 10 may have the audio stored locally at some local data storage of the source device 10, or may access the audio locally from local data storage to which the source device 10 is connected, and/or may stream the audio from a remote audio source, for example over the Internet.

The sink devices 20 may be or include loudspeakers (or, more simply, “speakers”) or may be connected to loudspeakers. Herein, the term “loudspeaker” is in general used to describe an electroacoustic transducer which converts an electrical audio signal into a corresponding sound. The loudspeaker may be of the “dynamic cone speaker” type having a diaphragm or cone which is driven to move by a voice coil. In other examples, the loudspeaker may be of another type, including for example a magnetostatic speaker, an electrostatic speaker, etc. The loudspeaker may be a floor mounted or shelf mounted speaker, etc., or may be incorporated into headphones, or ear buds or the like. The sink devices 20 include one or more of processors, radio transmitters, receivers, etc. to enable communication with the source device 10 and to enable the required processing of the signals received from the source device 10 and, at least in some cases, to send signals to the source device 10.

In a particular example, which is in wide use currently, the source device 10 distributes audio to the sink devices 20 using A2DP. As noted, A2DP is the “Advanced Audio Distribution Profile” which defines how multimedia audio can be streamed from one device to another over a Bluetooth connection. A2DP uses the SBC (low-complexity subband codec), though for example MP3 or some other compression may be used. Some type of DRM may also be applied to the audio that is transmitted from the source device 10 to the sink devices 20. An example DRM is the Serial Copy Management System (SCMS), as mentioned, though other types of DRM are known. As an alternative to A2DP, AptX may be used.

Referring now to Figure 2, this shows schematically an example of the source device 10 and one of the sink devices 20 exchanging information. Initially, the source device 10 carries out a discovery process to detect the presence of any sink devices 20 that are nearby. This may for example occur automatically when the source device 10 and/or the sink devices 20 are connected to a power source or powered up, may occur from time to time on a “polling” basis, may be instigated manually by a user, etc.

For example, the source device 10 may advertise its presence by broadcasting signals for receipt by nearby sink devices 20, which can then respond by connecting to the source device 10. Alternatively or additionally, the source device 10 may detect signals that are broadcast by the sink devices 20 to advertise their presence, and can then respond by connecting to the relevant sink device(s) 20. This is indicated schematically in Figure 2 by way of example with the source device 10 sending a pairing request 30 to a sink device 20, and the sink device 20 replying with a pairing response 32. The pairing request 30 and the pairing response 32 enable the source device 10 and the sink device 20 to wirelessly connect with each other in accordance with the relevant wireless protocol. The wireless connection between the source device 10 and the sink device 20 may be carried out using a security layer “handshake” 34 between the devices 10, 20, using passwords and/or digital keys, etc., in a manner known per se.

Once connected, the source device 10 and the sink device 20 exchange information so that inter alia the source device 10 can learn whether or not the sink device 20 is DRM-enabled, that is, whether or not the sink device 20 is able to play back audio that is DRM-protected. As shown schematically at 36, the sink device 20 may for example send one or more of bits, licence keys, product keys, etc. to the source device 10 by which the source device 10 can learn that the sink device 20 is DRM-enabled. The absence of the appropriate bits, licence keys, product keys, etc., from the sink device 20, or indeed the absence of any response to a request for such bits, licence keys, product keys, etc. from sink device 20, may be taken by the source device 10 as an indication that the sink device 20 is not DRM-enabled.

This is repeated for each of the sink devices 20i, . . . , 20_n which have been detected.

The source device 10 therefore learns whether each sink device 20 is or is not DRM-enabled.

If all of the detected sink devices 20 are DRM-enabled, that is all of the detected sink devices 20 are capable of receiving and decoding DRM-protected audio, then the source device 20 can send DRM enabled audio to each of the connected sink devices 20. As an example, the audio may be transmitted on two radio channels as stereo audio with DRM enabled, with left stereo being transmitted on a first of the channels for play back by the or each left-side sink device 20 and right stereo being transmitted on a second of the channels for play back by the or each right-side sink device 20. The audio may be of a relatively high sound quality, such as using CD quality (which is sampled at 44.1 kHz and using 16 bits, such that every second, 44,100 16-bit samples are provided for the left side and 44,100 16-bit samples are provided for the right side), or at 345 kilobits per second at 48 kilohertz, 256 kbit/s, etc.

On the other hand, if there is at least one detected sink device 20 that is not DRM-enabled, then in an example the source device 10 transmits the audio on at least two channels for receipt by the detected sink devices 20, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device 20 that is not DRM-enabled, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device 20 that is DRM-enabled. The two channels that are used respectively for the non-DRM-protected audio and the DRM-protected audio may be the left and right (or vice versa) stereo channels. In such a case, the audio signal, if originally stored or received in stereo, is converted to mono prior to transmission by the source device 10, so that each sink device 20, whether DRM-enabled or not, still receives the same “complete” audio signal, albeit in mono rather than stereo.

This is shown schematically in Figure 3, which shows an example of a packet structure for distribution of the audio. At 40, the audio data is in its original form, whether stored locally at the source device 10 or received from a remote source, such as over the Internet or some other network. At 42, if the original audio data 40 was saved in stereo format, then it is converted to mono format. This can be done by a simple summing of the left and right stereo signals (and dividing the amplitude of the sum by 2 to retain the original amplitude).

The mono signal is then sent to the two channels for transmission to the sink devices 20. One version of the mono signal 44 has no DRM, and is intended for the non-DRM-enabled sink devices 20. This is therefore sent to one of the channels 46 (shown here as the left channel for illustrative purposes) and transmitted to the sink devices 20. The other version of the mono signal 48 has DRM, and is intended for the DRM-enabled sink devices 20. This is therefore sent to the other of the channels 50 (shown here as the right channel for illustrative purposes) and transmitted to the sink devices 20.

As shown, the data stream of the audio data 44 that has no DRM has a number of packets, including particularly a security code packet 60, a header 62, and a payload 64 containing the audio data itself. The audio data in the payload 64 has no DRM applied. The data stream of the audio data 48 that has DRM similarly has a security code packet 70, a header 72, and a payload 74 containing the audio data. In this case, the audio data in the payload 74 has DRM applied.

Referring to Figure 4, this shows the source device 10 transmitting mono audio with no DRM on a first channel, indicated by dashed lines; and the source device 10 transmitting mono audio with DRM on a first channel, indicated by dotted lines. A first sink device 20A is not DRM-enabled, and so cannot decode DRM-protected audio. The first sink device 20A therefore rejects the DRM-protected audio which is being transmitted on the second channel and only accepts the non-DRM-protected audio which is being transmitted on the first channel. A second sink device 20B is DRM-enabled, and so can decode DRM-protected audio. The second sink device 20B therefore the non-DRM-protected audio which is being transmitted on the first channel and only accepts the DRM-protected audio which is being transmitted on the second channel.

In short, examples described herein enable a source device 10 to deliver audio which can be played back by sink devices 20 whether the sink devices 20 are DRM- enabled or not. The source device 10 learns whether each connected sink device 20 is DRM-enabled or not. If any connected sink device 20 is not DRM-enabled, that is cannot play back DRM-protected audio, then the source device 10 transmits audio with no DRM on a first channel for receipt and play back by the or each non-DRM- enabled sink device 20; and the source device 10 transmits audio with DRM on a second channel for receipt and play back by the or each DRM-enabled sink device 20. The two channels may be the channels that are conventionally used by the source device 10 to transmit left and right stereo signals, with any stereo audio being converted to mono so that each DRM-enabled sink device 20 and non-DRM-enabled sink device 20 receives a “complete” audio signal, albeit in mono. In a specific example, the source device 10 and some or all of the sink devices 20 may use a modified version of the “Advanced Audio Distribution Profile” (A2DP) for the wireless distribution of the audio to enable this mode of operation.

As an option, the audio that is transmitted with no DRM may have a lower audio quality than the audio that is transmitted with DRM. For example, the audio that is transmitted with no DRM may have a lower bit rate and/or sample rate than the audio that is transmitted with DRM.

As another option, instead of converting a stereo audio source to mono and transmitting the mono audio with and without DRM over the usual left and right stereo channels, the source device 10 may transmit left and right stereo audio with DRM over left and right channels using a first radio for use by DRM-enabled sink devices 20, and may transmit left and right stereo audio with no DRM over left and right channels using a second radio for use by non-DRM-enabled sink devices 20. This clearly requires two radios at the source device 10, but does provide full stereo to both DRM-enabled sink devices and non-DRM-enabled sink devices 20.

In another example to deal with the problem that one or more of the sink devices 20 are DRM-enabled such that they can receive and decode DRM-protected audio that is transmitted by the source device 10 whereas one or more of the sink devices 20 are not DRM-enabled and therefore cannot play DRM-protected content, the source device 10 only transmits DRM-protected audio. That DRM-protected audio is only accepted by DRM-enabled sink devices 20 and is rejected by any sink devices 20 that are not DRM-enabled. In this example however, the various sink devices 20 communicate with each other, using a security layer “handshake”, etc., between the sink devices 20, so that one or more of the DRM-enabled sink devices 20 can learn if there are any non-DRM-enabled sink devices 20 present. If so, then in this example, at least one of the DRM-enabled sink devices 20 redistributes audio that it has received from the source device 10, but with no DRM. That is, the DRM- enabled sink device 20 will decode the DRM-protected audio and transmit the DRM- free audio to the non-DRM-enabled sink devices 20 (if for example permitted by the content provider, at least for that in instance).

Referring to Figure 5, this shows schematically a signal diagram of an example of a discovery process carried out by a source device and a sink device. It will be understood that one sink device is shown for the signalling diagram of Figure 5 and that a similar process may be carried out with plural sink devices. In the example shown, the source device is a host audio source 100 and the sink device is a slave speaker 102, which in use plays out audio received from the host audio source 100.

At 500, each of the host audio source 100 (or other source device) and the slave speaker 102 (or other sink device) is placed in a discovery mode. This may for example occur automatically when the device is connected to a power source or powered up, may occur from time to time on a “polling” basis, may be instigated manually by a user, etc. At 502, the host audio source 100 sends a pairing request or enquiry to the slave speaker 102. At 504, the slave speaker 102 replies to the host audio source 100 with a pairing response. As part of this pairing process, the host audio source 100 and the slave speaker 102 may exchange one or more secret digital keys at 506. This enables the host audio source 100 and the slave speaker 102to wirelessly connect with each other in accordance with the relevant wireless protocol.

At 508, the slave speaker 102 sends device information to the host audio source 100. This enables the host audio source 100 to learn whether or not the slave speaker 102 is DRM-enabled, that is, whether or not the slave speaker 102 is able to play back audio that is DRM-protected.

This is repeated for each slave speaker 102 that has been detected. The host audio source 100 therefore learns whether each slave speaker 102 is or is not DRM- enabled.

As 510, the host audio source 100 can then send appropriate data to the or each slave speaker 102. For example, as discussed in more detail above, if all of the detected slave speakers 102 are DRM-enabled, then the host audio source 100 can send DRM enabled audio to each of the connected slave speakers 102. On the other hand, if there is at least one detected slave speaker 102 that is not DRM-enabled, then in an example the host audio source 100 transmits the audio on at least two channels for receipt by the slave speakers 102, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each slave speaker 102 that is not DRM-enabled, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each slave speaker 102 that is DRM-enabled.

It will be understood that the processor or processing system or circuitry referred to herein may in practice be provided by a single chip or integrated circuit or plural chips or integrated circuits, optionally provided as a chipset, an applicationspecific integrated circuit (ASIC), field-programmable gate array (FPGA), digital signal processor (DSP), graphics processing units (GPUs), etc. The chip or chips may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor or processors, a digital signal processor or processors, baseband circuitry and radio frequency circuitry, which are configurable so as to operate in accordance with the exemplary embodiments. In this regard, the exemplary embodiments may be implemented at least in part by computer software stored in (non-transitory) memory and executable by the processor, or by hardware, or by a combination of tangibly stored software and hardware (and tangibly stored firmware).

Reference is made herein to data storage for storing data. This may be provided by a single device or by plural devices. Suitable devices include for example a hard disk and non-volatile semiconductor memory (including for example a solid- state drive or SSD).

Although at least some aspects of the embodiments described herein with reference to the drawings comprise computer processes performed in processing systems or processors, the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of non-transitory source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other non-transitory form suitable for use in the implementation of processes according to the invention. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a solid- state drive (SSD) or other semiconductor-based RAM; a ROM, for example a CD ROM or a semiconductor ROM; a magnetic recording medium, for example a floppy disk or hard disk; optical memory devices in general; etc.

The examples described herein are to be understood as illustrative examples of embodiments of the invention. Further embodiments and examples are envisaged. Any feature described in relation to any one example or embodiment may be used alone or in combination with other features. In addition, any feature described in relation to any one example or embodiment may also be used in combination with one or more features of any other of the examples or embodiments, or any combination of any other of the examples or embodiments. Furthermore, equivalents and modifications not described herein may also be employed within the scope of the invention, which is defined in the claims.

Claims

1. A method of transmitting audio from a source device to one or more sink devices, the method comprising: the source device carrying out a discovery process to detect the presence of one or more sink devices; the source device carrying out a DRM-learning process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding digital-rights- management, DRM, protected audio; wherein if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the source device transmits the audio on at least two channels for receipt by the detected sink devices, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device that is not capable of receiving and decoding DRM-protected audio, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device that is capable of receiving and decoding DRM-protected audio.

2. A method according to claim 1, wherein if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is mono audio.

3. A method according to claim 2, wherein the audio that is transmitted on the two channels is the same mono audio.

4. A method according to any of claims 1 to 3, wherein if all of the detected sink devices are capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is stereo audio with DRM enabled, with left stereo being transmitted on a first of the channels and right stereo being transmitted on a second of the channels.

5. A method according to any of claims 1 to 4, wherein the source device is connected wirelessly to the sink devices and transmits the audio wirelessly for receipt by the detected sink devices.

6. A method according to any of claims 1 to 5, wherein audio that is transmitted with DRM enabled is transmitted with a higher sound quality than audio that is transmitted with no DRM enabled.

7. A method according to any of claims 1 to 6, wherein the source device has at least two radios, and audio that is transmitted with DRM enabled and audio that is transmitted with no DRM enabled are transmitted in stereo by the respective radios.

8. A source device for transmitting audio to one or more sink devices, the source device being constructed and arranged to: carry out a discovery process to detect the presence of one or more sink devices; carry out a DRM-leaming process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding digital-rights-management, DRM, protected audio; wherein if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the source device is arranged to transmit the audio on at least two channels for receipt by the detected sink devices, in which the audio on a first of the channels is transmitted with no DRM for receipt and playback by the or each sink device that is not capable of receiving and decoding DRM-protected audio, and in which the audio on a second of the channels is transmitted with DRM enabled for receipt and playback by the or each sink device that is capable of receiving and decoding DRM-protected audio.

9. A source device according to claim 8, arranged such that if any of the detected sink devices is not capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is mono audio. 17

10. A source device according to claim 8 or claim 9, arranged such that if all of the detected sink devices are capable of receiving and decoding DRM-protected audio, the audio that is transmitted on the two channels is stereo audio with DRM enabled, with left stereo being transmitted on a first of the channels and right stereo being transmitted on a second of the channels.

11. A source device according to any of claims 8 to 10, arranged such that audio that is transmitted with DRM enabled is transmitted with a higher sound quality than audio that is transmitted with no DRM enabled.

12. A source device according to any of claims 8 to 11, wherein the source device has at least two radios, and arranged such that audio that is transmitted with DRM enabled and audio that is transmitted with no DRM enabled are transmitted in stereo by the respective radios.

13. A method of transmitting audio for play back by plural sink devices, the method comprising: at least one of the sink devices carrying out a discovery process to detect the presence of one or more other sink devices, said at least one of the sink devices being capable of receiving and decoding digital-rights-management, DRM, protected audio; the at least one of the sink devices carrying out a DRM-learning process or a handshaking or a pairing process with each of the detected sink devices to learn whether or not the respective detected sink device is capable of receiving and decoding DRM-protected audio; a source device transmitting audio with DRM enabled; said at least one of the sink devices receiving and decoding the audio with DRM enabled; and said at least one of the sink devices transmitting the audio with no DRM for receipt and play back by a detected sink device that is not capable of receiving and decoding DRM-protected audio.