Abstract:
Example techniques relate to a playback device that facilitates queue identification. In an example implementation, a playback device stores an association between a playback queue and a first application identifier. After storing the association between the playback queue and the first application identifier, the playback device receives, from a first computing device, a first message comprising a second application identifier. The playback device determines that the second application identifier corresponds to the first application identifier, and responsive to determining that the second application identifier corresponds to the first application identifier, transmits, to the first computing device, a queue identifier associated with the playback queue.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §120 to, and is a continuation of, U.S. patent application Ser. No. 14/330,856, filed on Jul. 14, 2014, entitled “Queue Identification,” the contents of which are fully incorporated by reference herein. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The disclosure is related to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to media playback or some aspect thereof. 
       BACKGROUND 
       [0003]    Options for accessing and listening to digital audio in an out-loud setting were limited until in 2003, when SONOS, Inc. filed for one of its first patent applications, entitled “Method for Synchronizing Audio Playback between Multiple Networked Devices,” and began offering a media playback system for sale in 2005. The Sonos Wireless HiFi System enables people to experience music from many sources via one or more networked playback devices. Through a software control application installed on a smartphone, tablet, or computer, one can play what he or she wants in any room that has a networked playback device. Additionally, using the controller, for example, different songs can be streamed to each room with a playback device, rooms can be grouped together for synchronous playback, or the same song can be heard in all rooms synchronously. 
         [0004]    Given the ever growing interest in digital media, there continues to be a need to develop consumer-accessible technologies to further enhance the listening experience. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Features, aspects, and advantages of the presently disclosed technology may be better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0006]      FIG. 1  shows an example media playback system configuration in which certain embodiments may be practiced; 
           [0007]      FIG. 2  shows a functional block diagram of an example playback device; 
           [0008]      FIG. 3  shows a functional block diagram of an example control device; 
           [0009]      FIG. 4  shows an example controller interface; 
           [0010]      FIG. 5  shows an example flow diagram for an example method; 
           [0011]      FIG. 6  shows example events and example devices; 
           [0012]      FIG. 7  shows example events and example devices; 
           [0013]      FIG. 8  shows example events and example devices; 
           [0014]      FIG. 9A  shows example events and example devices; 
           [0015]      FIG. 9B  shows example events and example devices; 
           [0016]      FIG. 10  shows example events and example devices; 
           [0017]      FIG. 11  shows example events and example devices; 
           [0018]      FIG. 12A  shows example events and example devices; 
           [0019]      FIG. 12B  shows example events and example devices; 
           [0020]      FIG. 13A  shows example events and example devices; 
           [0021]      FIG. 13B  shows an example device; 
           [0022]      FIG. 13C  shows example events and example devices; and 
           [0023]      FIG. 14  shows an example flow diagram for another example method. 
       
    
    
       [0024]    The drawings are for the purpose of illustrating example embodiments, but it is understood that the inventions are not limited to the arrangements and instrumentality shown in the drawings. 
       DETAILED DESCRIPTION 
     I. OVERVIEW 
       [0025]    In some situations, a control device of a media playback system may control a playback device of the media playback system by detecting inputs and executing a control application associated with the media playback system. For example, the control device (e.g., a computer, a smartphone, and the like) may detect inputs. Based on the detected inputs, the control device may send a message to the playback device that causes the playback device to access and play back media. Other examples exist. 
         [0026]    For instance, a first control application on a first control device may cause the playback device to populate media items of a first playlist into a playback queue accessible to the playback device. Afterward, a second control application on a second control device (or on the first control device) may cause the playback device to overwrite contents of the playback queue with media items of a second playlist. Then, the first control application on the first control device may send a command to the playback device that attempts to manipulate the playback queue as populated by the first control application. However, the media items that the first control application caused to be populated into the playback queue may no longer be populated in the playback queue. 
         [0027]    In some cases, interactions of one or more control devices executing one or more control applications to access, alter, or populate a playback queue of the playback device may be enabled by way of application identifiers and queue identifiers. For example, the first control device may send a message to the playback device that (i) identifies the first playlist associated with the first control application and (ii) includes a first application identifier associated with the first control application. The message may cause the playback device to populate the playback queue with media items corresponding to the first playlist and send to the first control device a first queue identifier associated with the populated playback queue. 
         [0028]    The second control device may cause the playback device to repopulate the playback queue by sending a message to the playback device that identifies a second playlist associated with the second control application (or alternatively the first control application) and includes a second application identifier (or alternatively the first application identifier). Then, the first control device may send a message to the playback device that includes an application identifier and if the application identifier provided by the first control device is currently associated with the playback queue, the playback device may send to the first control device a queue identifier associated with the playback queue. 
         [0029]    Alternatively, if the first control device sends to the playback device an application identifier not associated with the playback queue, then the playback device may send to the first control device an indication that the application identifier is not associated with the playback queue. In this way, if a second control device executing a second control application has taken control of the playback queue or otherwise deleted the originally populated playback queue, the first control device may receive an indication to that effect. 
         [0030]    Some examples described herein may involve a playback device receiving one or more messages from a control device. The one or more messages may identify (i) a playlist and (ii) an application identifier associated with an application in use by the control device. Based on receiving the one or more messages, the playback device may populate a playback queue of the playback device with one or more media items associated with the playlist. The playback device may also associate the received application identifier with the playback queue. Additionally, the playback device may send, to the control device, a queue identifier that represents the playback queue. 
         [0031]    In one aspect, a method is provided. The method may involve receiving, by a playback device, one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0032]    In yet another aspect, a non-transitory computer readable medium is provided. The non-transitory computer readable medium has stored thereon instructions executable by a playback device to cause the playback device to perform functions. The functions include receiving, one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0033]    In another aspect, a playback device is provided. The playback device includes a processor and a computer readable medium storing instructions that when executed by the playback device cause the playback device to perform functions comprising: receiving one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0034]    In yet another aspect, a method is provided. The method includes sending, by a control device, one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0035]    In yet another aspect, a non-transitory computer readable medium is provided. The non-transitory computer readable medium has stored thereon instructions executable by a control device to cause the control device to perform functions. The functions include sending one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0036]    In another aspect, a control device is provided. The control device includes a processor and a computer readable medium storing instructions that when executed by the control device cause the control device to perform functions comprising: sending one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0037]    It will be understood by one of ordinary skill in the art that this disclosure includes numerous other embodiments. While some examples described herein may refer to functions performed by given actors such as “users” and/or other entities, it should be understood that this is for purposes of explanation only. The claims should not be interpreted to require action by any such example actor unless explicitly required by the language of the claims themselves. 
       II. EXAMPLE OPERATING ENVIRONMENT 
       [0038]      FIG. 1  shows an example configuration of a media playback system  100  in which one or more embodiments disclosed herein may be practiced or implemented. The media playback system  100  as shown is associated with an example home environment having several rooms and spaces, such as for example, a master bedroom, an office, a dining room, and a living room. As shown in the example of  FIG. 1 , the media playback system  100  includes playback devices  102 - 124 , control devices  126  and  128 , and a wired or wireless network router  130 . 
         [0039]    Further discussions relating to the different components of the example media playback system  100  and how the different components may interact to provide a user with a media experience may be found in the following sections. While discussions herein may generally refer to the example media playback system  100 , technologies described herein are not limited to applications within, among other things, the home environment as shown in  FIG. 1 . For instance, the technologies described herein may be useful in environments where multi-zone audio may be desired, such as, for example, a commercial setting like a restaurant, mall or airport, a vehicle like a sports utility vehicle (SUV), bus or car, a ship or boat, an airplane, and so on. 
         [0000]    a. Example Playback Devices 
         [0040]      FIG. 2  shows a functional block diagram of an example playback device  200  that may be configured to be one or more of the playback devices  102 - 124  of the media playback system  100  of  FIG. 1 . The playback device  200  may include a processor  202 , software components  204 , memory  206 , audio processing components  208 , audio amplifier(s)  210 , speaker(s)  212 , and a network interface  214  including wireless interface(s)  216  and wired interface(s)  218 . In one case, the playback device  200  may not include the speaker(s)  212 , but rather a speaker interface for connecting the playback device  200  to external speakers. In another case, the playback device  200  may include neither the speaker(s)  212  nor the audio amplifier(s)  210 , but rather an audio interface for connecting the playback device  200  to an external audio amplifier or audio-visual receiver. 
         [0041]    In one example, the processor  202  may be a clock-driven computing component configured to process input data according to instructions stored in the memory  206 . The memory  206  may be a tangible computer-readable medium configured to store instructions executable by the processor  202 . For instance, the memory  206  may be data storage that can be loaded with one or more of the software components  204  executable by the processor  202  to achieve certain functions. In one example, the functions may involve the playback device  200  retrieving audio data from an audio source or another playback device. In another example, the functions may involve the playback device  200  sending audio data to another device or playback device on a network. In yet another example, the functions may involve pairing of the playback device  200  with one or more playback devices to create a multi-channel audio environment. 
         [0042]    Certain functions may involve the playback device  200  synchronizing playback of audio content with one or more other playback devices. During synchronous playback, a listener will preferably not be able to perceive time-delay differences between playback of the audio content by the playback device  200  and the one or more other playback devices. U.S. Pat. No. 8,234,395 entitled, “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is hereby incorporated by reference, provides in more detail some examples for audio playback synchronization among playback devices. 
         [0043]    The memory  206  may further be configured to store data associated with the playback device  200 , such as one or more zones and/or zone groups the playback device  200  is a part of, audio sources accessible by the playback device  200 , or a playback queue that the playback device  200  (or some other playback device) may be associated with. The data may be stored as one or more state variables that are periodically updated and used to describe the state of the playback device  200 . The memory  206  may also include the data associated with the state of the other devices of the media system, and shared from time to time among the devices so that one or more of the devices have the most recent data associated with the system. Other embodiments are also possible. 
         [0044]    The audio processing components  208  may include one or more digital-to-analog converters (DAC), an audio preprocessing component, an audio enhancement component or a digital signal processor (DSP), and so on. In one embodiment, one or more of the audio processing components  208  may be a subcomponent of the processor  202 . In one example, audio content may be processed and/or intentionally altered by the audio processing components  208  to produce audio signals. The produced audio signals may then be provided to the audio amplifier(s)  210  for amplification and playback through speaker(s)  212 . Particularly, the audio amplifier(s)  210  may include devices configured to amplify audio signals to a level for driving one or more of the speakers  212 . The speaker(s)  212  may include an individual transducer (e.g., a “driver”) or a complete speaker system involving an enclosure with one or more drivers. A particular driver of the speaker(s)  212  may include, for example, a subwoofer (e.g., for low frequencies), a mid-range driver (e.g., for middle frequencies), and/or a tweeter (e.g., for high frequencies). In some cases, each transducer in the one or more speakers  212  may be driven by an individual corresponding audio amplifier of the audio amplifier(s)  210 . In addition to producing analog signals for playback by the playback device  200 , the audio processing components  208  may be configured to process audio content to be sent to one or more other playback devices for playback. 
         [0045]    Audio content to be processed and/or played back by the playback device  200  may be received from an external source, such as via an audio line-in input connection (e.g., an auto-detecting 3.5 mm audio line-in connection) or the network interface  214 . 
         [0046]    The network interface  214  may be configured to facilitate a data flow between the playback device  200  and one or more other devices on a data network. As such, the playback device  200  may be configured to receive audio content over the data network from one or more other playback devices in communication with the playback device  200 , network devices within a local area network, or audio content sources over a wide area network such as the Internet. In one example, the audio content and other signals transmitted and received by the playback device  200  may be transmitted in the form of digital packet data containing an Internet Protocol (IP)-based source address and IP-based destination addresses. In such a case, the network interface  214  may be configured to parse the digital packet data such that the data destined for the playback device  200  is properly received and processed by the playback device  200 . 
         [0047]    As shown, the network interface  214  may include wireless interface(s)  216  and wired interface(s)  218 . The wireless interface(s)  216  may provide network interface functions for the playback device  200  to wirelessly communicate with other devices (e.g., other playback device(s), speaker(s), receiver(s), network device(s), control device(s) within a data network the playback device  200  is associated with) in accordance with a communication protocol (e.g., any wireless standard including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). The wired interface(s)  218  may provide network interface functions for the playback device  200  to communicate over a wired connection with other devices in accordance with a communication protocol (e.g., IEEE 802.3). While the network interface  214  shown in  FIG. 2  includes both wireless interface(s)  216  and wired interface(s)  218 , the network interface  214  may in some embodiments include only wireless interface(s) or only wired interface(s). 
         [0048]    In one example, the playback device  200  and one other playback device may be paired to play two separate audio components of audio content. For instance, playback device  200  may be configured to play a left channel audio component, while the other playback device may be configured to play a right channel audio component, thereby producing or enhancing a stereo effect of the audio content. The paired playback devices (also referred to as “bonded playback devices”) may further play audio content in synchrony with other playback devices. 
         [0049]    In another example, the playback device  200  may be sonically consolidated with one or more other playback devices to form a single, consolidated playback device. A consolidated playback device may be configured to process and reproduce sound differently than an unconsolidated playback device or playback devices that are paired, because a consolidated playback device may have additional speaker drivers through which audio content may be rendered. For instance, if the playback device  200  is a playback device designed to render low frequency range audio content (i.e. a subwoofer), the playback device  200  may be consolidated with a playback device designed to render full frequency range audio content. In such a case, the full frequency range playback device, when consolidated with the low frequency playback device  200 , may be configured to render only the mid and high frequency components of audio content, while the low frequency range playback device  200  renders the low frequency component of the audio content. The consolidated playback device may further be paired with a single playback device or yet another consolidated playback device. 
         [0050]    By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices including a “PLAY:1,” “PLAY:3,” “PLAY:5,” “PLAYBAR,” “CONNECT:AMP,” “CONNECT,” and “SUB.” Any other past, present, and/or future playback devices may additionally or alternatively be used to implement the playback devices of example embodiments disclosed herein. Additionally, it is understood that a playback device is not limited to the example illustrated in  FIG. 2  or to the SONOS product offerings. For example, a playback device may include a wired or wireless headphone. In another example, a playback device may include or interact with a docking station for personal mobile media playback devices. In yet another example, a playback device may be integral to another device or component such as a television, a lighting fixture, or some other device for indoor or outdoor use. 
         [0000]    b. Example Playback Zone Configurations 
         [0051]    Referring back to the media playback system  100  of  FIG. 1 , the environment may have one or more playback zones, each with one or more playback devices. The media playback system  100  may be established with one or more playback zones, after which one or more zones may be added, or removed to arrive at the example configuration shown in  FIG. 1 . Each zone may be given a name according to a different room or space such as an office, bathroom, master bedroom, bedroom, kitchen, dining room, living room, and/or balcony. In one case, a single playback zone may include multiple rooms or spaces. In another case, a single room or space may include multiple playback zones. 
         [0052]    As shown in  FIG. 1 , the balcony, dining room, kitchen, bathroom, office, and bedroom zones each have one playback device, while the living room and master bedroom zones each have multiple playback devices. In the living room zone, playback devices  104 ,  106 ,  108 , and  110  may be configured to play audio content in synchrony as individual playback devices, as one or more bonded playback devices, as one or more consolidated playback devices, or any combination thereof. Similarly, in the case of the master bedroom, playback devices  122  and  124  may be configured to play audio content in synchrony as individual playback devices, as a bonded playback device, or as a consolidated playback device. 
         [0053]    In one example, one or more playback zones in the environment of  FIG. 1  may each be playing different audio content. For instance, the user may be grilling in the balcony zone and listening to hip hop music being played by the playback device  102  while another user may be preparing food in the kitchen zone and listening to classical music being played by the playback device  114 . In another example, a playback zone may play the same audio content in synchrony with another playback zone. For instance, the user may be in the office zone where the playback device  118  is playing the same rock music that is being playing by playback device  102  in the balcony zone. In such a case, playback devices  102  and  118  may be playing the rock music in synchrony such that the user may seamlessly (or at least substantially seamlessly) enjoy the audio content that is being played out-loud while moving between different playback zones. Synchronization among playback zones may be achieved in a manner similar to that of synchronization among playback devices, as described in previously referenced U.S. Pat. No. 8,234,395. 
         [0054]    As suggested above, the zone configurations of the media playback system  100  may be dynamically modified, and in some embodiments, the media playback system  100  supports numerous configurations. For instance, if a user physically moves one or more playback devices to or from a zone, the media playback system  100  may be reconfigured to accommodate the change(s). For instance, if the user physically moves the playback device  102  from the balcony zone to the office zone, the office zone may now include both the playback device  118  and the playback device  102 . The playback device  102  may be paired or grouped with the office zone and/or renamed if so desired via a control device such as the control devices  126  and  128 . On the other hand, if the one or more playback devices are moved to a particular area in the home environment that is not already a playback zone, a new playback zone may be created for the particular area. 
         [0055]    Further, different playback zones of the media playback system  100  may be dynamically combined into zone groups or split up into individual playback zones. For instance, the dining room zone and the kitchen zone  114  may be combined into a zone group for a dinner party such that playback devices  112  and  114  may render audio content in synchrony. On the other hand, the living room zone may be split into a television zone including playback device  104 , and a listening zone including playback devices  106 ,  108 , and  110 , if the user wishes to listen to music in the living room space while another user wishes to watch television. 
         [0000]    c. Example Control Devices 
         [0056]      FIG. 3  shows a functional block diagram of an example control device  300  that may be configured to be one or both of the control devices  126  and  128  of the media playback system  100 . As shown, the control device  300  may include a processor  302 , memory  304 , a network interface  306 , and a user interface  308 . In one example, the control device  300  may be a dedicated controller for the media playback system  100 . In another example, the control device  300  may be a network device on which media playback system controller application software may be installed, such as for example, an iPhone™, iPad™ or any other smart phone, tablet or network device (e.g., a networked computer such as a PC or Mac™). 
         [0057]    The processor  302  may be configured to perform functions relevant to facilitating user access, control, and configuration of the media playback system  100 . The memory  304  may be configured to store instructions executable by the processor  302  to perform those functions. The memory  304  may also be configured to store the media playback system controller application software and other data associated with the media playback system  100  and the user. 
         [0058]    In one example, the network interface  306  may be based on an industry standard (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). The network interface  306  may provide a means for the control device  300  to communicate with other devices in the media playback system  100 . In one example, data and information (e.g., such as a state variable) may be communicated between control device  300  and other devices via the network interface  306 . For instance, playback zone and zone group configurations in the media playback system  100  may be received by the control device  300  from a playback device or another network device, or transmitted by the control device  300  to another playback device or network device via the network interface  306 . In some cases, the other network device may be another control device. 
         [0059]    Playback device control commands such as volume control and audio playback control may also be communicated from the control device  300  to a playback device via the network interface  306 . As suggested above, changes to configurations of the media playback system  100  may also be performed by a user using the control device  300 . The configuration changes may include adding/removing one or more playback devices to/from a zone, adding/removing one or more zones to/from a zone group, forming a bonded or consolidated player, separating one or more playback devices from a bonded or consolidated player, among others. Accordingly, the control device  300  may sometimes be referred to as a controller, whether the control device  300  is a dedicated controller or a network device on which media playback system controller application software is installed. 
         [0060]    The user interface  308  of the control device  300  may be configured to facilitate user access and control of the media playback system  100 , by providing a controller interface such as the controller interface  400  shown in  FIG. 4 . The controller interface  400  includes a playback control region  410 , a playback zone region  420 , a playback status region  430 , a playback queue region  440 , and an audio content sources region  450 . The user interface  400  as shown is just one example of a user interface that may be provided on a network device such as the control device  300  of  FIG. 3  (and/or the control devices  126  and  128  of  FIG. 1 ) and accessed by users to control a media playback system such as the media playback system  100 . Other user interfaces of varying formats, styles, and interactive sequences may alternatively be implemented on one or more network devices to provide comparable control access to a media playback system. 
         [0061]    The playback control region  410  may include selectable (e.g., by way of touch or by using a cursor) icons to cause playback devices in a selected playback zone or zone group to play or pause, fast forward, rewind, skip to next, skip to previous, enter/exit shuffle mode, enter/exit repeat mode, enter/exit cross fade mode. The playback control region  410  may also include selectable icons to modify equalization settings, and playback volume, among other possibilities. 
         [0062]    The playback zone region  420  may include representations of playback zones within the media playback system  100 . In some embodiments, the graphical representations of playback zones may be selectable to bring up additional selectable icons to manage or configure the playback zones in the media playback system, such as a creation of bonded zones, creation of zone groups, separation of zone groups, and renaming of zone groups, among other possibilities. 
         [0063]    For example, as shown, a “group” icon may be provided within each of the graphical representations of playback zones. The “group” icon provided within a graphical representation of a particular zone may be selectable to bring up options to select one or more other zones in the media playback system to be grouped with the particular zone. Once grouped, playback devices in the zones that have been grouped with the particular zone will be configured to play audio content in synchrony with the playback device(s) in the particular zone. Analogously, a “group” icon may be provided within a graphical representation of a zone group. In this case, the “group” icon may be selectable to bring up options to deselect one or more zones in the zone group to be removed from the zone group. Other interactions and implementations for grouping and ungrouping zones via a user interface such as the user interface  400  are also possible. The representations of playback zones in the playback zone region  420  may be dynamically updated as playback zone or zone group configurations are modified. 
         [0064]    The playback status region  430  may include graphical representations of audio content that is presently being played, previously played, or scheduled to play next in the selected playback zone or zone group. The selected playback zone or zone group may be visually distinguished on the user interface, such as within the playback zone region  420  and/or the playback status region  430 . The graphical representations may include track title, artist name, album name, album year, track length, and other relevant information that may be useful for the user to know when controlling the media playback system via the user interface  400 . 
         [0065]    The playback queue region  440  may include graphical representations of audio content in a playback queue associated with the selected playback zone or zone group. In some embodiments, each playback zone or zone group may be associated with a playback queue containing information corresponding to zero or more audio items for playback by the playback zone or zone group. For instance, each audio item in the playback queue may comprise a uniform resource identifier (URI), a uniform resource locator (URL) or some other identifier that may be used by a playback device in the playback zone or zone group to find and/or retrieve the audio item from a local audio content source or a networked audio content source, possibly for playback by the playback device. 
         [0066]    In one example, a playlist may be added to a playback queue, in which case information corresponding to each audio item in the playlist may be added to the playback queue. In another example, audio items in a playback queue may be saved as a playlist. In a further example, a playback queue may be empty, or populated but “not in use” when the playback zone or zone group is playing continuously streaming audio content, such as Internet radio that may continue to play until otherwise stopped, rather than discrete audio items that have playback durations. In an alternative embodiment, a playback queue can include Internet radio and/or other streaming audio content items and be “in use” when the playback zone or zone group is playing those items. Other examples are also possible. 
         [0067]    When playback zones or zone groups are “grouped” or “ungrouped,” playback queues associated with the affected playback zones or zone groups may be cleared or re-associated. For example, if a first playback zone including a first playback queue is grouped with a second playback zone including a second playback queue, the established zone group may have an associated playback queue that is initially empty, that contains audio items from the first playback queue (such as if the second playback zone was added to the first playback zone), that contains audio items from the second playback queue (such as if the first playback zone was added to the second playback zone), or a combination of audio items from both the first and second playback queues. Subsequently, if the established zone group is ungrouped, the resulting first playback zone may be re-associated with the previous first playback queue, or be associated with a new playback queue that is empty or contains audio items from the playback queue associated with the established zone group before the established zone group was ungrouped. Similarly, the resulting second playback zone may be re-associated with the previous second playback queue, or be associated with a new playback queue that is empty, or contains audio items from the playback queue associated with the established zone group before the established zone group was ungrouped. Other examples are also possible. 
         [0068]    Referring back to the user interface  400  of  FIG. 4 , the graphical representations of audio content in the playback queue region  440  may include track titles, artist names, track lengths, and other relevant information associated with the audio content in the playback queue. In one example, graphical representations of audio content may be selectable to bring up additional selectable icons to manage and/or manipulate the playback queue and/or audio content represented in the playback queue. For instance, a represented audio content may be removed from the playback queue, moved to a different position within the playback queue, or selected to be played immediately, or after any currently playing audio content, among other possibilities. A playback queue associated with a playback zone or zone group may be stored in a memory on one or more playback devices in the playback zone or zone group, on a playback device that is not in the playback zone or zone group, and/or some other designated device. 
         [0069]    The audio content sources region  450  may include graphical representations of selectable audio content sources from which audio content may be retrieved and played by the selected playback zone or zone group. Discussions pertaining to audio content sources may be found in the following section. 
         [0000]    d. Example Audio Content Sources 
         [0070]    As indicated previously, one or more playback devices in a zone or zone group may be configured to retrieve for playback audio content (e.g. according to a corresponding URI or URL for the audio content) from a variety of available audio content sources. In one example, audio content may be retrieved by a playback device directly from a corresponding audio content source (e.g., a line-in connection). In another example, audio content may be provided to a playback device over a network via one or more other playback devices or network devices. 
         [0071]    Example audio content sources may include a memory of one or more playback devices in a media playback system such as the media playback system  100  of  FIG. 1 , local music libraries on one or more network devices (such as a control device, a network-enabled personal computer, or a networked-attached storage (NAS), for example), streaming audio services providing audio content via the Internet (e.g., the cloud), or audio sources connected to the media playback system via a line-in input connection on a playback device or network devise, among other possibilities. 
         [0072]    In some embodiments, audio content sources may be regularly added or removed from a media playback system such as the media playback system  100  of  FIG. 1 . In one example, an indexing of audio items may be performed whenever one or more audio content sources are added, removed or updated. Indexing of audio items may involve scanning for identifiable audio items in all folders/directory shared over a network accessible by playback devices in the media playback system, and generating or updating an audio content database containing metadata (e.g., title, artist, album, track length, among others) and other associated information, such as a URI or URL for each identifiable audio item found. Other examples for managing and maintaining audio content sources may also be possible. 
         [0073]    The above discussions relating to playback devices, controller devices, playback zone configurations, and media content sources provide only some examples of operating environments within which functions and methods described below may be implemented. Other operating environments and configurations of media playback systems, playback devices, and network devices not explicitly described herein may also be applicable and suitable for implementation of the functions and methods. 
       III. EXAMPLE METHODS FOR CONTROLLING A PLAYBACK DEVICE 
       [0074]    As discussed above, some examples described herein involve, among other things, a playback device receiving one or more messages from a control device. The one or more messages may identify (i) a playlist and (ii) an application identifier associated with an application in use by the control device. Then, based on receiving the one or more messages, the playback device may populate a playback queue of the playback device with one or more media items associated with the playlist. The playback device may also associate the received application identifier with the playback queue. Additionally, the playback device may send, to the control device, a queue identifier that represents the playback queue. 
         [0075]    Methods  500  and  1400  shown respectively in  FIGS. 5 and 14  present example methods that can be implemented within an operating environment involving, for example, the media playback system  100  of  FIG. 1  and one or more of the playback device  200  of  FIG. 2 . Methods  500  and  1400  may include one or more operations, functions, or actions as illustrated by one or more of blocks  502 - 508  and  1402 - 1404 . 
         [0076]    Although the blocks of methods  500  and  1400  are illustrated in sequential order, these blocks may also be performed in parallel, and/or in a different order than those described herein. Also, the various blocks may be combined into fewer blocks, divided into additional blocks, and/or removed based upon the desired implementation. 
         [0077]    In addition, for the methods  500  and  1400  and other processes and methods disclosed herein, the flowchart shows functionality and operation of one possible implementation of present embodiments. In this regard, each block may represent a module, a segment, or a portion of program code, which includes one or more instructions executable by a processor for implementing specific logical functions or steps in the process. The program code may be stored on any type of computer readable medium, for example, such as a storage device including a disk or hard drive. The computer readable medium may include non-transitory computer readable medium, for example, such as computer-readable media that stores data for short periods of time like register memory, processor cache and Random Access Memory (RAM). The computer readable medium may also include non-transitory media, such as secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media may also be any other volatile or non-volatile storage systems. The computer readable medium may be considered a computer readable storage medium, for example, or a tangible storage device. In addition, for the methods  500  and  1400  and other processes and methods disclosed herein, each block in  FIG. 5  and  FIG. 14  may represent circuitry that is wired to perform the specific logical functions in the process. 
         [0078]    At block  502 , the method  500  includes receiving, by a playback device, one or more messages from a control device. The one or more messages may identify (i) a playlist and (ii) an application identifier associated with an application. The application (perhaps in use by the control device) may cause the control device to send the one or more messages to the playback device. The one or more messages may represent a command to populate a playback queue at (or accessible to) the playback device with one or more media items of the playlist associated with the application. The application identifier may be included in the one or more messages so that the playback device may associate the playback queue with the application. 
         [0079]    Referring to  FIG. 6 , a playback device  602  may receive one or more messages  610  from a first control device  608 A. The one or more messages  610  may include data such as uniform resource identifiers (URIs) or other data representing respective storage locations of audio or video files corresponding to the playlist. In one example, the one or more messages  610  may include messages received sequentially, with each respective message including data corresponding to one audio or video file represented by the playlist. But, data corresponding to multiple audio or videos files represented by the playlist may also be sent simultaneously. 
         [0080]    The playback device  602  may retrieve and/or receive the audio or video files associated with the playlist for playback by such data. For instance, the data representing storage locations (e.g., “URI 1 -URI 5 ”) could include IP addresses or domain names of servers storing the audio or video files and/or memory addresses of the audio or video files within various networked storage media. In another example, the one or more messages  610  may include the audio or video files themselves instead of data representing storage locations where the playback device may retrieve the audio or video files. Regardless of the form of playlist-identifying data that may be received as part of the one or more messages  610 , the identifying data may be stored at a playback queue  606 A of the playback device  602  so that playback of the audio or video files associated with the playlist may be performed by accessing the playback queue  606 A. 
         [0081]    The application identifier (e.g., “App  1 ”) included within or identified by the one or more messages  610  may be any data that could be used to distinguish one control application executable by the first control device  608 A from another control application executable by any other control device (or another control application executable by the first control device  608 A). For example, the first control device  608 A may send the application identifier in the one or more messages  610  to be received by playback device  602 . The application identifier may include a data string such as “App  1 ” that may identify the application in use by the first control device  608 A. In another example, an application identifier that identifies the application could include an arbitrary or random data string such as “D3L1M1D.” In some examples, an application identifier may also be referred to as a “Queue Owner ID,” or a “QueueOwnerID.” Any example application identifier or data string that is distinguishable from other application identifiers and can be associated with a given application is possible. The application identifier (e.g., “App  1 ”) that may be included as part of the received one or more messages  610  may be stored at a memory location  604  of the playback device  602  so that the playback queue  606 A may be associated with the control application that caused the playback queue  606 A to be created or populated. 
         [0082]    Referring back to  FIG. 5 , at block  504  the method  500  includes, based on receiving the one or more messages, populating a playback queue with one or more media items associated with the playlist. The playback queue is associated with the playback device. Again referring to  FIG. 6 , the playback queue  606 A may represent a memory space physically located at the playback device  602  or at another location accessible by the media playback system that includes the playback device  602  and the first control device  608 A. For example, the playback queue  606 A may be populated with audio or video files suitable for playback of media corresponding to the playlist. In another example, the playback device  602  may populate or load the playback queue  606 A with one or more uniform resource identifiers (e.g., URI 1 -URI 5 ) that represent respective storage locations of the audio or video files corresponding to the playlist, so that the playback device  602  may use the represented storage locations to retrieve the audio or video files corresponding to the playlist. 
         [0083]    Referring back to  FIG. 5 , at block  506  the method  500  includes, based on receiving the one or more messages, associating the received application identifier with the playback queue. Referring again to  FIG. 6 , the playback device  602  may store the received application identifier (e.g., “App  1 ”) at a memory location  604  designated for storing an application identifier corresponding to the playback queue  606 A. By storing the application identifier in a designated memory location, the stored application identifier may be compared to application identifiers received subsequently from the first control device  608 A (or other control devices). Based on comparing subsequently received application identifiers with the stored application identifier that represents the playback queue  606 A (or other playback queues), the playback device  602  may allow or deny playback queue access to various control applications being executed by various control devices. 
         [0084]    Referring back to  FIG. 5 , at block  508  the method includes, based on receiving the one or more messages, sending, to the control device, a queue identifier that represents the playback queue. As shown in  FIG. 6 , the playback device  602  may generate the queue identifier  612 A as a data string such as “Q 1 ” and store the queue identifier  612 A at memory location  605 . However, a queue identifier could include any data that can distinguish a corresponding playback queue. In some examples, the queue identifier may be referred to as a “QueueID,” or “Queue ID.” After receiving the queue identifier  612 A from the playback device  602 , the first control device  608 A may use the queue identifier  612 A to access and/or alter the playback queue  606 A represented by the queue identifier  612 A. 
         [0085]    Functions that may occur in addition to events depicted in  FIG. 6  are shown in  FIG. 7 .  FIG. 7  may depict an “attach” function, where the first control device  608 A queries the playback device  602  to determine if the playback queue contains media items compatible with a given control application. (It should be noted that in some examples a playback queue may contain media items compatible with more than one control application so that the playback queue may be associated with one or more application identifier.) After sending the queue identifier, the playback device  602  may receive, from the first control device  608 A, an additional message  611  that includes the application identifier (e.g., “App  1 ”). The playback device  602  may determine that the application identifier included in the additional message  611  is associated with the playback queue  606 A. For example, the application identifier included in the additional message  611  may match the application identifier stored at memory location  604  of the playback device  602 , or the playback device may otherwise store or access data indicating that the received application identifier is associated with the playback queue  606 A. Based on the determination, the playback device  602  may send to the first control device  608 A the queue identifier  612 A that represents the playback queue  606 A. The playback device  602  may also send, to the first control device  608 A, an indication that the application identifier included in the additional message  611  is associated with the playback queue  606 A (e.g., UPnP error code “0”). 
         [0086]    Functions that may also occur in addition to functions depicted in  FIG. 6  are shown in  FIG. 8 .  FIG. 8  may depict another “attach” action, where a second control device  608 B queries the playback device  602  to determine if contents of the playback queue  606 A are compatible with a given control application. After sending the queue identifier, the playback device  602  may receive an additional message  611  from the second control device  608 B. The additional message  611  may include a second application identifier (e.g., “App  2 ”) that is associated with a second application in use by the second control device  608 B. Then, the playback device  602  may determine that the application identifier included in the additional message  611  is not associated with the playback queue  606 A. For example, the application identifier included in the additional message (e.g., “App  2 ”) may be compared to the application identifier stored at memory location  604  (e.g., “App  1 ”), or the playback device  602  may otherwise store or access data indicating that the received application identifier is not associated with the playback queue  606 A. Based on the determination, the playback device  602  may send to the second control device  608 B an indication  612 C that the application identifier included in the additional message  611  is not associated with the playback queue  606 A (e.g., UPnP error code “1”). 
         [0087]    Functions that may also occur in addition to functions depicted in  FIG. 6  are shown in  FIGS. 9A and 9B .  FIGS. 9A and 9B  may depict a second control device  608 B populating a playback queue  606 B at the playback device  602  and the first playback device  608 A discovering that the playback queue  606 B is associated with the first control application (i.e., the first application identifier  611 ) even though playback queue  606 B has been populated by the second control device  608 B. (In  FIGS. 9A and 9B , the playback queues  606 A and  606 B may represent the same playback queue, although at  606 B updated contents of the playback queue may be depicted and at  606 A previous contents of the playback queue may be depicted, for reference.) 
         [0088]    As shown in  FIG. 9A , after sending the (first) queue identifier the playback device  602  may receive one or more messages  610 B from a second control device  608 B that cause the playback device  602  to populate the playback queue  606 B with one or more media items associated with a second playlist. The one or more media items (e.g., URI 6 -URI 10 ) associated with the second playlist are associated with the application, perhaps in use by the second control device, and may be included in the one or more messages  610 B. The one or more messages  610 B may also include the application identifier (e.g., “App  1 ”). The playback device  602  may associate the application identifier with the playback queue  606 B. For example, the application identifier (e.g., “App  1 ”) may be stored at memory location  604  for future comparison with received application identifiers. The playback device  602  may also send the second queue identifier  612 B (e.g., “Q 2 ”) to the second control device  608 B. 
         [0089]    As shown in  FIG. 9B , the playback device  602  may then receive from the first control device  608 A an additional message  611  that includes the application identifier (e.g., “App  1 ”) and determine that the application identifier included in the additional message  611  is associated with the playback queue  606 B. For example, the playback device  602  may compare the application identifier stored at memory location  604  with the application identifier included in the additional message  611  or the playback device  602  may otherwise store or access data indicating that the received application identifier is associated with the playback queue  606 B. Finally, based on the determination, the playback device  602  may send to the first control device  608 A a second queue identifier  612 B (e.g., “Q 2 ”) that represents the playback queue  606 B. The playback device  602  may also send, to the first control device  608 A, an indication that the application identifier included in the additional message  611  is associated with the playback queue  606 B (e.g., UPnP error code “0”). 
         [0090]    Functions that may also occur in addition to events depicted in  FIG. 6  are shown in  FIG. 10 .  FIG. 10  may depict a second control device  608 B using the (first) control application to discover a compatible playback queue at the playback device  602 . (It should be noted that within various examples, multiple control devices may use respective instances of the same controller application. Herein, description of multiple control devices running the same control application should not be interpreted to mean that the control devices are running the control application in concert or in a dual-processor configuration, unless context clearly dictates otherwise.) After sending the queue identifier, the playback device  602  may receive, from the second control device  608 B, an additional message  611  that includes the application identifier (e.g., “App  1 ”). The playback device  602  may then determine that the application identifier included in the additional message  611  is associated with the playback queue  606 A. Based on the determination, the playback device  602  may send to the second control device  608 B the queue identifier  612 A (e.g., “Q 1 ”) that represents the playback queue  606 A. The playback device  602  may also send, to the second control device  608 B, an indication that the application identifier included in the additional message  611  is associated with the playback queue  606 A (e.g., UPnP error code “0”). 
         [0091]    Functions that may also occur in addition to events depicted in  FIG. 6  are shown in  FIG. 11 .  FIG. 11  may depict a second control device  608 B using a second control application to discover that the playback queue  606 A at the playback device  602  is not compatible with the second control application. After sending the queue identifier, the playback device  602  may receive, from the second control device  608 B, an additional message  611  that includes a second application identifier (e.g., “App  2 ”) associated with a second application in use by the second control device  608 B. Next, the playback device  602  may determine that the application identifier included in the additional message  611  is not associated with the playback queue  606 A. For instance, the playback device  602  may compare the application identifier included in the additional message  611  with the application identifier stored at memory location  604 . Based on the determination, the playback device  602  may send to the second control device  608 B an indication  612 C that the application identifier included in the additional message  611  is not associated with the playback queue  606 A (e.g., UPnP error code “0”). 
         [0092]    Functions that may also occur in addition to events depicted in  FIG. 6  are shown in  FIGS. 12A and 12B .  FIGS. 12A and 12B  depicts a second control device  608 B populating playback queue  606 B by way of a second control application, and the first control device  608 A discovering that the playback queue  606 B is not compatible with the first control application. (In  FIGS. 12A and 12B , the playback queues  606 A and  606 B may represent the same playback queue, although at  606 B updated contents of the playback queue may be depicted and at  606 A previous contents of the playback queue may be depicted, for reference.) 
         [0093]    As shown in  FIG. 12A , after sending the queue identifier the playback device  602  may receive, from the second control device  608 B, one or more messages  610 B that cause the playback device  602  to populate playback queue  606 B with one or more media items associated with a second playlist. The one or more messages  610 B may include one or more media items (e.g., URI 6 -URI 10 ) associated with the second playlist associated with a second application in use by the second control device  608 B. The one or more messages  610 B received from the second control device  608 B may also include a second application identifier (e.g., “App  2 ”) associated with the second application. Next, the playback device  602  may associate the received second application identifier with the playback queue  606 B. For example, the application identifier included in the one or more messages  610 B may be stored at memory location  604 . The playback device  602  may then send a second queue identifier  612 B (e.g., “Q 2 ”) to the second control device  608 B. The playback device  602  may also send, to the second control device  608 B, an indication that the application identifier included in the additional message  611  is associated with the playback queue  606 B (e.g., UPnP error code “0”). 
         [0094]    Then, as shown in  FIG. 12B , the playback device  602  may receive, from the first control device  608 A, an additional message  611  that includes the first application identifier (e.g., “App  1 ”). The playback device  602  may determine that the application identifier included in the additional message  611  is not associated with the playback queue  606 B and, based on the determination, send to the first control device  608 A an indication  612 C that the first application identifier included in the additional message  611  is not associated with the playback queue  606 B (e.g., UPnP error code “0”). 
         [0095]    Functions that may also occur in addition to events depicted in  FIG. 6  are shown in  FIGS. 13A, 13B, 13C .  FIG. 13A  depicts a control device  608  using the queue identifier  612  (e.g., “Q 1 ”) to access or alter the playback queue  606 A.  FIG. 13B  depicts the playback queue  606 A after the control device  608  has altered the playback queue  606 A.  FIG. 13C  depicts a control device  608  discovering that the provided queue identifier no longer represents a playback queue stored at the playback device  602 . 
         [0096]    As shown in  FIG. 13A , the playback device  602  may receive an additional message  614  from the control device  608  that includes (i) a command to alter the playback queue  606 A and (ii) the queue identifier  612  (e.g., “Q 1 ”) that represents the playback queue  606 A. Here, the control device  608  may be the first control device  608 A or any other control device of the media playback system. In one situation, the additional message  614  may include a command to add URIs  6 - 7  to the end of playback queue  606 A. 
         [0097]    Based on receiving the additional message  614  from the control device  608 , the playback device  602  may determine that the received queue identifier  612 A represents the playback queue  606 A. For instance, the playback device  602  may compare the received queue identifier  612 A to a queue identifier stored in a memory location  605  of the playback device  602 . The queue identifier (e.g., “Q 1 ”) stored at the memory location  605  may be known to represent the playback queue  606 A. If the queue identifier stored at the memory location  605  matches the received queue identifier  612 A or the playback device  602  otherwise determines that the received queue identifier  612 A represents the playback queue  606 A, the playback device  602  may, based on the determination, alter the playback queue  606 A according to the command and/or additional message  614 . This result is depicted in  FIG. 13B , as the playback queue  606 A includes URI 6 -URI 7  at the end of the playback queue  606 A. 
         [0098]    However, as depicted in  FIG. 13C , if the queue identifier stored at memory location  605  (e.g., “Q 3 ”) does not match the received queue identifier  612 A, the playback device  602  may determine that the received queue identifier does not represent the playback queue  606 C and send to the control device  608  an indication  616  that the received queue identifier  612 A does not represent the playback queue  606 C or that the received queue identifier  612 A is not associated with the playback queue  606 C.  FIG. 13C  may depict a scenario where another control device has deleted the playback queue  606 A and created the playback queue  606 C before the playback device  602  receives the additional message  614 . 
         [0099]    It should be noted that any of the functions depicted in  FIGS. 6-13C  may be carried out concurrently, contemporaneously, or sequentially in any possible sequence. 
         [0100]    Referring now to  FIG. 14 , at block  1402  the method  1400  includes sending, by a control device, one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application. For example, at  FIG. 6 , a control device  608 A sends one or more messages  610  to a playback device  602 . The one or more messages  610  may include an application identifier such as a data string “App  1 ” and may include data storage locations of audio or video files associated with the playlist, such as a data string that includes uniform resource identifiers URI 1 -URI 5 . 
         [0101]    At block  1404 , the method  1400  includes receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. For example, the control device  608 A of  FIG. 6  receives the queue identifier  612 A (e.g., “Q 1 ”) that represents the playback queue  606 A. 
         [0102]    As one with skill in the art will appreciate, any of the functions described above as being performed by a playback device may correspond to a related function that may be performed by a control device. In some examples, any data, message, command, or instruction received by a playback device may be sent to the playback device by the control device. Also, any data, message, command, or instruction sent by a playback device may be received by the control device. 
       IV. CONCLUSION 
       [0103]    The description above discloses, among other things, various example systems, methods, apparatus, and articles of manufacture including, among other components, firmware and/or software executed on hardware. It is understood that such examples are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of the firmware, hardware, and/or software aspects or components can be embodied exclusively in hardware, exclusively in software, exclusively in firmware, or in any combination of hardware, software, and/or firmware. Accordingly, the examples provided are not the only way(s) to implement such systems, methods, apparatus, and/or articles of manufacture. 
         [0104]    Some examples described herein involve, among other things, a playback device receiving one or more messages from a control device. The one or more messages may identify (i) a playlist and (ii) an application identifier associated with an application in use by the control device. Then, based on receiving the one or more messages, the playback device may populate a playback queue of the playback device with one or more media items associated with the playlist. The playback device may also associate the received application identifier with the playback queue. Additionally, the playback device may send, to the control device, a queue identifier that represents the playback queue. 
         [0105]    In one aspect, a method is provided. The method may involve receiving, by a playback device, one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0106]    In yet another aspect, a non-transitory computer readable medium is provided. The non-transitory computer readable medium has stored thereon instructions executable by a playback device to cause the playback device to perform functions. The functions include receiving, one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0107]    In another aspect, a playback device is provided. The playback device includes a processor and a computer readable medium storing instructions that when executed by the playback device cause the playback device to perform functions comprising: receiving one or more messages from a control device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and based on receiving the one or more messages: populating a playback queue with one or more media items associated with the playlist, where the playback queue is associated with the playback device; associating the received application identifier with the playback queue; and sending, to the control device, a queue identifier that represents the playback queue. 
         [0108]    In yet another aspect, a method is provided. The method includes sending, by a control device, one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0109]    In yet another aspect, a non-transitory computer readable medium is provided. The non-transitory computer readable medium has stored thereon instructions executable by a control device to cause the control device to perform functions. The functions include sending one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0110]    In another aspect, a control device is provided. The control device includes a processor and a computer readable medium storing instructions that when executed by the control device cause the control device to perform functions comprising: sending one or more messages to a playback device, where the one or more messages identify (i) a playlist and (ii) an application identifier associated with an application; and receiving, from the playback device, a queue identifier that represents a playback queue of the playback device. 
         [0111]    Additionally, references herein to “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one example embodiment of an invention. The appearances of this phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. As such, the embodiments described herein, explicitly and implicitly understood by one skilled in the art, can be combined with other embodiments. 
         [0112]    The specification is presented largely in terms of illustrative environments, systems, procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of data processing devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. Numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it is understood to those skilled in the art that certain embodiments of the present disclosure can be practiced without certain, specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the embodiments. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the forgoing description of embodiments. 
         [0113]    When any of the appended claims are read to cover a purely software and/or firmware implementation, at least one of the elements in at least one example is hereby expressly defined to include a tangible, non-transitory medium such as a memory, DVD, CD, Blu-ray, and so on, storing the software and/or firmware.