Media control for Generic Audio Content Control
This document describes how to run the media control functionality, using the shell, both as a client and as a server.
The media control server consists of to parts. There is a media player (mpl) that contains the logic to handle media, and there is a media control service (mcs) that serves as a GATT-based interface to the player. The media control client consists of one part, the GATT based client (mcc).
The media control server may include an object transfer service (ots) and the media control client may include an object transfer client (otc). When these are included, a richer set of functionality is available.
The media control server and client both implement the Generic Media Control Service (only), and do not use any underlying Media Control Services.
Note that in the examples below, in many cases the debug output has been removed and long outputs may have been shortened to make the examples shorter and clearer.
Also note that this documentation does not list all shell commands, it
just shows examples of some of them. The set of commands is
explorable from the mcc shell and the mpl shell, by typing
mpl and pressing TAB. A help text for each command can be
found by doing
mcc <command> help or or
A media player has a name and an icon that allows identification of the player for the user.
The content of the media player is structured into tracks and groups. A media player has a number of groups. A group contains tracks and other groups. (In this implementation, a group only contains tracks, not other groups.) Tracks can be divided into segments.
An active player will have a current track. This is the track that is playing now (if the player is playing). The current track has a title, a duration (given in hundredths of a second) and a position - the current position of the player within the track.
There is also a current group (the group of the current track), a parent group (the parent group of the current group) and a next track.
The media player is in a state, which will be one of playing, paused, seeking or inactive. When playing, playback happens at a given playback speed, and the tracks are played according to the playing order, which is one of the playing orders supported. Track changes are signalled as notifications of the track changed characteristic. When seeking (fast forward or fast rewind), the track position is moved according to the seeking speed.
The opcodes supported tells which operations are supported by the player by writing to the media control point. There is also a search control point that allows to search for groups and tracks according to various criteria, with the result returned in the search results.
Finally, the content control ID is used to associate the media player with an audio stream.
Media Control Client (MCP)
The media control client is used to control, and to get information from, a media control server. Control is done by writing to one of the two control points, or by writing to other writable characteristics. Getting information is done by reading characteristics, or by configuring the server to send notifications.
Using the media control client
Before use, the media control client must be initialized by the
To achieve a connection to the peer, the
bt commands must be
bt init followed by
bt advertise on (or
bt connect if the server is advertising).
When the media control client is connected to a media control server,
the client can discover the server’s Generic Media Control Service, by
giving the command
mcc discover_mcs. This will store the
handles of the service, and (optionally, but default) subscribe to all
After discovery, the media control client can read and write characteristics, including the media control point and the search control point.
uart:~$ bt init Bluetooth initialized uart:~$ mcc init MCC init complete uart:~$ bt advertise on Advertising started Connected: F6:58:DC:27:F3:57 (random)
Service discovery (GMCS and included OTS):
uart:~$ mcc discover_mcs <dbg> bt_mcc.bt_mcc_discover_mcs: start discovery of MCS primary service <dbg> bt_mcc.discover_primary_func: [ATTRIBUTE] handle 0x00ae <dbg> bt_mcc.discover_primary_func: Primary discovery complete <dbg> bt_mcc.discover_primary_func: UUID: 2800 <dbg> bt_mcc.discover_primary_func: UUID: 8fd7 <dbg> bt_mcc.discover_primary_func: Start discovery of MCS characteristics <dbg> bt_mcc.discover_mcs_char_func: [ATTRIBUTE] handle 0x00b0 <dbg> bt_mcc.discover_mcs_char_func: Player name, UUID: 8fa0 <dbg> bt_mcc.discover_mcs_char_func: [ATTRIBUTE] handle 0x00b2 <dbg> bt_mcc.discover_mcs_char_func: Icon Object, UUID: 8fa1 <dbg> bt_mcc.discover_mcs_char_func: [ATTRIBUTE] handle 0x00b4 <dbg> bt_mcc.discover_mcs_char_func: Icon URI, UUID: 8fa2 <dbg> bt_mcc.discover_mcs_char_func: [ATTRIBUTE] handle 0x00b6 <dbg> bt_mcc.discover_mcs_char_func: Track Changed, UUID: 8fa3 <dbg> bt_mcc.discover_mcs_char_func: Subscribing - handle: 0x00b6 [...] <dbg> bt_mcc.discover_mcs_char_func: [ATTRIBUTE] handle 0x00ea <dbg> bt_mcc.discover_mcs_char_func: Content Control ID, UUID: 8fb5 <dbg> bt_mcc.discover_mcs_char_func: Setup complete for MCS <dbg> bt_mcc.discover_mcs_char_func: Start discovery of included services <dbg> bt_mcc.discover_include_func: [ATTRIBUTE] handle 0x00af <dbg> bt_mcc.discover_include_func: Include UUID 1825 <dbg> bt_mcc.discover_include_func: Discover include complete for MCS: OTS <dbg> bt_mcc.discover_include_func: Start discovery of OTS characteristics <dbg> bt_mcc.discover_otc_char_func: [ATTRIBUTE] handle 0x009c <dbg> bt_mcc.discover_otc_char_func: OTS Features [...] <dbg> bt_mcc.discover_otc_char_func: [ATTRIBUTE] handle 0x00ac <dbg> bt_mcc.discover_otc_char_func: Object Size Discovery complete <dbg> bt_otc.bt_otc_register: 0 <dbg> bt_otc.bt_otc_register: L2CAP psm 0x 25 sec_level 1 registered <dbg> bt_mcc.discover_otc_char_func: Setup complete for OTS 1 / 1 uart:~$
Reading characteristics - the player name and the track duration as examples:
uart:~$ mcc read_player_name Player name: My media player 4d 79 20 6d 65 64 69 61 20 70 6c 61 79 65 72 |My media player uart:~$ mcc read_track_duration Track duration: 6300
Note that the value of some characteristics may be truncated due to being too long to fit in the ATT packet. Increasing the ATT MTU may help:
uart:~$ mcc read_track_title Track title: Interlude #1 (Song for uart:~$ gatt exchange-mtu Exchange pending Exchange successful uart:~$ mcc read_track_title Track title: Interlude #1 (Song for Alison)
Writing characteristics - track position as an example:
The track position is where the player “is” in the current track. Read the track position, change it by writing to it, confirm by reading it again.
uart:~$ mcc read_track_position Track Position: 0 uart:~$ mcc set_track_position 500 Track Position: 500 uart:~$ mcc read_track_position Track Position: 500
Controlling the player via the control point:
Writing to the control point allows the client to request the server to do operations like play, pause, fast forward, change track, change group and so on. Some operations (e.g. goto track) take an argument. Currently, raw opcode values are used as input to the control point shell command. These opcode values can be found in the mpl.h header file.
Send the play command (opcode “1”), the command to go to track (opcode “52”) number three, and the pause command (opcode “2”):
uart:~$ mcc set_cp 1 Media State: 1 Operation: 1, result: 1 Operation: 1, param: 0 uart:~$ mcc set_cp 52 3 Track changed Track title: Interlude #3 (Levanto Seventy) Track duration: 7800 Track Position: 0 Current Track Object ID: 0x000000000104 Next Track Object ID: 0x000000000105 Operation: 52, result: 1 Operation: 52, param: 3 uart:~$ mcc set_cp 2 Media State: 2 Operation: 2, result: 1 Operation: 2, param: 0
Using the included object transfer client
When object transfer is supported by both the client and the server, a larger set of characteristics is available. These include object IDs for the various track and group objects. These IDs can be used to select and download the corresponding objects from the server’s object transfer service.
Read the object ID of the current group object:
uart:~$ mcc read_current_group_obj_id Current Group Object ID: 0x000000000107
Select the object with that ID:
uart:~$ mcc ots_select 0x107 Selecting object succeeded
Read the object’s metadata:
uart:~$ mcc ots_read_metadata Reading object metadata succeeded <inf> bt_mcc: Object's meta data: <inf> bt_mcc: Current size :35 <inf> bt_otc: --- Displaying 1 metadata records --- <inf> bt_otc: Object ID: 0x000000000107 <inf> bt_otc: Object name: Joe Pass - Guitar Inte <inf> bt_otc: Object Current Size: 35 <inf> bt_otc: Object Allocate Size: 35 <inf> bt_otc: Type: Group Obj Type <inf> bt_otc: Properties:0x4 <inf> bt_otc: - read permitted
Read the object itself:
The object received is a group object. It consists of a series of records consisting of a type (track or group) and an object ID.
uart:~$ mcc ots_read_current_group_object <dbg> bt_mcc.on_group_content: Object type: 0, object ID: 0x000000000102 <dbg> bt_mcc.on_group_content: Object type: 0, object ID: 0x000000000103 <dbg> bt_mcc.on_group_content: Object type: 0, object ID: 0x000000000104 <dbg> bt_mcc.on_group_content: Object type: 0, object ID: 0x000000000105 <dbg> bt_mcc.on_group_content: Object type: 0, object ID: 0x000000000106
The search control point takes as its input a sequence of search control items, each consisting of length, type (e.g. track name or artist name) and parameter (the track name or artist name to search for). If the result is successful, the search results are stored in an object in the object transfer service. The ID of the search results ID object can be read from the search results object ID characteristic. The search result object can then be downloaded as for the current group object above. (Note that the search results object ID is empty until a search has been done.)
This implementation has a working implementation of the search functionality interface and the server-side search control point parameter parsing. But the actual searching is faked, the same results are returned no matter what is searched for.
There are two commands for search, one (
allows to input the search control point parameter (the sequence of
search control items) as a string. The other (
set_scp_ioptest) does preset IOP test searches and takes the round
number of the IOP search control point test as a parameter.
Before the search, the search results object ID is empty
uart:~$ mcc read_search_results_obj_id Search Results Object ID: 0x000000000000 <dbg> bt_mcc.mcc_read_search_results_obj_id_cb: Zero-length Search Results Object ID
Run the search corresponding to the fourth round of the IOP test:
The search control point parameter generated by this command and parameter has one search control item. The length field (first octet) is 16 (0x10). (The length of the length field itself is not included.) The type field (second octet) is 0x04 (search for a group name). The parameter (the group name to search for) is “TSPX_Group_Name”.
uart:~$ mcc set_scp_ioptest 4 Search string: 00000000: 10 04 54 53 50 58 5f 47 72 6f 75 70 5f 4e 61 6d |..TSPX_G roup_Nam| 00000010: 65 |e | Search control point notification result code: 1 Search Results Object ID: 0x000000000107 Search Control Point set
After the successful search, the search results object ID has a value:
uart:~$ mcc read_search_results_obj_id Search Results Object ID: 0x000000000107
Media Control Service (MCS)
The media control service (mcs) and the associated media player (mpl) typically reside on devices that can provide access to, and serve, media content, like PCs and smartphones.
As mentioned above, the media player (mpl) has the player logic, while the media control service (mcs) has the GATT-based interface. This separation is done so that the media player can also be used without the GATT-based interface.
Using the media control service and the media player
The media control service and the media player are in general controlled remotely, from the media control client.
Before use, the media control client must be initialized by the
As for the client, the
bt commands are used for connecting -
bt init followed by
bt connect <address> <address
bt advertise on if the server is advertising).
uart:~$ bt init Bluetooth initialized uart:~$ mpl init [Large amounts of debug output] uart:~$ bt connect F9:33:3B:67:D2:A7 (random) Connection pending Connected: F9:33:3B:67:D2:A7 (random)
Control is done from the client.
The server will give debug output related to the various operations performed by the client.
Example: Debug output by the server when the client gives the “next track” command:
[00:13:29.932,373] <dbg> bt_mcs.control_point_write: Opcode: 49 [00:13:29.932,403] <dbg> bt_mpl.mpl_operation_set: opcode: 49, param: 536880068 [00:13:29.932,403] <dbg> bt_mpl.paused_state_operation_handler: Operation opcode: 49 [00:13:29.932,495] <dbg> bt_mpl.do_next_track: Track ID before: 0x000000000104 [00:13:29.932,586] <dbg> bt_mpl.do_next_track: Track ID after: 0x000000000105 [00:13:29.932,617] <dbg> bt_mcs.mpl_track_changed_cb: Notifying track change [00:13:29.932,708] <dbg> bt_mcs.mpl_track_title_cb: Notifying track title: Interlude #4 (Vesper Dreams) [00:13:29.932,800] <dbg> bt_mcs.mpl_track_duration_cb: Notifying track duration: 13500 [00:13:29.932,861] <dbg> bt_mcs.mpl_track_position_cb: Notifying track position: 0 [00:13:29.933,044] <dbg> bt_mcs.mpl_current_track_id_cb: Notifying current track ID: 0x000000000105 [00:13:29.933,258] <dbg> bt_mcs.mpl_next_track_id_cb: Notifying next track ID: 0x000000000106 [00:13:29.933,380] <dbg> bt_mcs.mpl_operation_cb: Notifying control point - opcode: 49, result: 1
Some server commands are available. These commands force notifications of the various characterstics, for testing that the client receives notifications. The values sent in the notifications caused by these testing commands are independent of the media player, so they do not correspond the actual values of the characteristics nor to the actual state of the media player.
Example: Force (fake value) notification of the track duration:
uart:~$ mpl duration_changed_cb [00:15:17.491,058] <dbg> bt_mcs.mpl_track_duration_cb: Notifying track duration: 12000