Media Control Service

The Bluetooth SIG has established the following conventions for use of the words shall, must, will, should, may, can, is, and note in the development of specifications:

For clarity of the definition of those terms, see Core Specification Volume 1, Part E, Section 1.

Where a field in a packet, Protocol Data Unit (PDU), or other data structure is described as "Reserved for Future Use" (irrespective of whether in uppercase or lowercase), the device creating the structure shall set its value to zero unless otherwise specified. Any device receiving or interpreting the structure shall ignore that field; in particular, it shall not reject the structure because of the value of the field.

Where a field, parameter, or other variable object can take a range of values, and some values are described as "Reserved for Future Use," a device sending the object shall not set the object to those values. A device receiving an object with such a value should reject it, and any data structure containing it, as being erroneous; however, this does not apply in a context where the object is described as being ignored or it is specified to ignore unrecognized values.

When a field value is a bit field, unassigned bits can be marked as Reserved for Future Use and shall be set to 0. Implementations that receive a message that contains a Reserved for Future Use bit that is set to 1 shall process the message as if that bit was set to 0, except where specified otherwise.

The acronym RFU is equivalent to Reserved for Future Use.

When a field value is an enumeration, unassigned values can be marked as “Prohibited.” These values shall never be used by an implementation, and any message received that includes a Prohibited value shall be ignored and shall not be processed and shall not be responded to.

Where a field, parameter, or other variable object can take a range of values, and some values are described as “Prohibited,” devices shall not set the object to any of those Prohibited values. A device receiving an object with such a value should reject it, and any data structure containing it, as being erroneous.

“Prohibited” is never abbreviated.

MCS exposes characteristics that describe a single media player. A media player in this context is a device, or part of a device, that allows another device to control the media that is played. For example, a media player could be a television, a set-top box, a radio, a phone running a radio or music application, a phone running a podcast application, or a similar type of device or application. MCS does not directly manage the audio transport. The description of the media player is comprised of the following:

Each media player should have a name that allows the user to identify the media player. To help identify a media player, an icon may also be exposed. This icon may either be stored locally in an included OTS [3] or be available on the internet at a provided Uniform Resource Locator (URL).

MCS provides characteristics that describe the media player and allows the media player to be controlled.

The media player has an object called the current track. Some properties of the current track object are exposed through the service as characteristics:

The current track object is also exposed through the optionally included OTS and can be accessed by using the Current Track Object ID. The current track always represents the media for which the player is currently playing, paused, or seeking.

If the media player has no current track, then the media player is inactive.

The current track has a track position as well as a track duration. The track position is the current time within the media file from which the current track is being played. The track duration is the length (measured in time) of the media file. The track position can be set to anywhere from the beginning to the end of the media file. It cannot be assumed that the track duration is valid; therefore, the track duration should only be considered a guide to the length of the current track.

When the current track is being played, usually the media is played at the playback speed, which is an adjustable value that may be faster or slower than the nominal, real-time speed of the track.

For streaming media, which does not have a known duration, the duration of the current track has a known reserved value.

Repositioning within the current track can be done relative to the start of the media, relative to the end of the media, or relative to the current track position. Repositioning never moves the track position outside the bounds of the media itself.

To help with repositioning, a track segments object can be exposed that allows different segments within the current track to be discovered. Each segment includes a name and an absolute offset in time from the start of the current track to the start of the segment. Therefore, this offset can be used as an absolute track position to move to the start of a segment.

The media player can set a playback speed such that media is played at a speed slightly faster or slightly slower than real time. Some media players do not support all the possible playback speeds, and many media players only support playback speeds on a certain subset of tracks. The Playback Speed characteristic exposes the current playback speed.

The media player can support fast forward and fast rewind at very high speeds, which is known as the current track seeking speed. During seeking, the current track audio may not be used and only the current track position is updated. The media player can support any seeking speeds. By using the Fast Forward and Fast Rewind opcodes of the Media Control Point, a client can request fast forward or fast rewind multiple times. Each time the server receives a request, the media player may move to a higher fast forward or rewind speed, or the seeking speed may wrap to the lowest speed.

If a media player is playing content organized by tracks, the server defines which track is the next track in the playing order. By exposing this next track information to the client, the client can pre-load the next track's information so that when the current track changes to the next track, the client can instantly update the information shown to the user.

To enable this, MCS has an object called the next track, which can be exposed by using the Next Track Object ID characteristic. The next track is the track that is played once the current track is finished. If the next track has no value, then the next track is determined by the playing order. If there is no next track, because no additional tracks exist in the playing order after the current track, then the next track is invalid. If the next track is written, then the client is overriding the deterministic next track and selecting a different next track. Once the next track becomes the current track, the normal playing order is used for the next track unless the next track is written again. This allows a client to perform a jukebox style of functionality. Writing the next track may also affect the current group and the playing order when the next track becomes the current track.

The media player has an object called the current group. Details of the current group are exposed by using the Current Group Object characteristic. The current group is a sequence of object records that exposes the set of tracks, and possibly other groups, within the current group. Information about a group is only available through the included OTS.

The current group is used to determine the next track by using the play order. Only tracks within a single group can be played automatically.

If the current group is changed while the current track is playing, the current track is changed to the first track of the current playing order.

The media player has a playing order that determines the next track for a given current track within a current group. The playing order can be any of the following:

Beyond what is defined in this specification, the organization of group objects and track objects are left up to the implementation. The group and track structure should be set up so that also clients with limited functionality (e.g., clients lacking the functionality to read and write Object IDs) are able to navigate the structure and access all tracks and groups.

In particular, clients that support only the “play” opcode; the “play” and “next track” opcodes; or the “play,” “next track,” and “next group” opcodes should be able to access all groups and tracks available.

The media player has a state machine that represents the states and transitions of the control of the media player. The states include inactive, playing, seeking, and paused.

The media player allows for searches to be performed over the set of tracks and groups. The search results are returned in a search results object that is exposed through the included OTS. The search results object is identical to an ordinary group object and can be selected as the current group.

The Media Player Name characteristic returns its associated value when it is read by a client.

The Media Player Name characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection. If the characteristic value is greater than (ATT_MTU-3), then the first (ATT_MTU-3) octets shall be included in the notification.

The Media Player Icon Object ID characteristic returns its associated value when it is read by a client.

The Media Player Icon URL characteristic returns its associated value when read by a client.

The Track Changed characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

A server device shall notify this characteristic to the client device depending on the value of the Client Characteristic Configuration descriptor when the track changes.

Any characteristic that will change in the transition between the current track and the next track shall be updated before notifying the Track Changed characteristic.

The Track Title characteristic returns its associated value when it is read by a client.

The Track Title characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

A server device shall notify this characteristic to the client device depending on the value of the Client Characteristic Configuration descriptor when the Track Title value is changed. If the characteristic value is longer than (ATT_MTU-3), then the first (ATT_MTU-3) octets shall be included in the notification.

The Track Duration characteristic returns its associated value when it is read by a client.

If the Track Duration characteristic supports notification, the Track Duration characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Track Duration characteristic supports notification, a server device shall notify this characteristic to the client device when the Track Duration characteristic value changes.

The Track Position characteristic returns its associated value when it is read by a client.

When the Track Position characteristic is written, the server may adjust the current playing position of the current track. If the value is zero or greater, then the current playing position shall be set to the offset from the start of the track. If the value is less than zero, then the current playing position shall be set to the offset from the end of the track and the value of the Track Position characteristic shall be set to the offset from the start of the track to the new playing position. If the value written does not correspond to a valid track position, the server shall set the Track Position characteristic to a valid value.

If the Track Position characteristic supports notification, the Track Position characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Track Position characteristic supports notification, a server device shall notify this characteristic to the client device based on the value of the Client Characteristic Configuration descriptor when the Track Position characteristic value is changed, when the track is paused, or when the playback speed is changed. As an exception to this, to avoid an excessive number of notifications, the Track Position should not be notified when the Media State is set to “Playing” and playback happens at a constant speed. The Track Position should be notified when the Media State characteristic is set to “Seeking”. The notification frequency while seeking is determined by the server implementation.

The Playback Speed characteristic returns its associated value (p) when read by a client.

When the Playback Speed characteristic is written, the server shall adjust the playback speed of the current track. If the server supports the value written, the server shall set the Playback Speed characteristic to the value written. If the server does not support the value written, the server shall set the Playback Speed characteristic to a supported value. If the value written is not supported and is greater than the existing Playback Speed characteristic, then the server should set the Playback Speed characteristic to the next higher supported playback speed. If the value written is not supported and is less than the existing Playback Speed characteristic, then the server should set the Playback Speed characteristic to the next lower supported playback speed.

For streaming media, the playback speed may be fixed to 1x normal speed.

If the Playback Speed characteristic supports notification, the Playback Speed characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Playback Speed characteristic supports notification, a server device shall notify this characteristic to the client device depending on the value of the Client Characteristic Configuration descriptor when the Playback Speed characteristic value is changed or written.

The Seeking Speed characteristic returns its associated value when it is read by a client.

If the Seeking Speed characteristic supports notification, the Seeking Speed characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Seeking Speed characteristic supports notification, a server device shall notify this characteristic to the client device depending on the value of the Client Characteristic Configuration descriptor when the Seeking Speed characteristic value is changed.

The Current Track Segments Object ID characteristic returns its associated value when it is read by a client.

The Current Track Object ID characteristic returns its associated value when it is read by a client.

When the Current Track Object ID characteristic is written, the server shall change the current track to that identified by the value and act as described in the Goto Track command in Section 3.18.1.16.

If the Current Track Object ID characteristic supports notification, the Current Track Object ID characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Current Track Object ID characteristic supports notification, a server device shall notify this characteristic to the client device when the Current Track Object ID characteristic value changes, except when the client changes the value.

The Next Track Object ID characteristic returns its associated value when it is read by a client.

When the Next Track Object ID characteristic is written, the server shall change the next track to that identified by the value written to the characteristic by the client.

If the Next Track Object ID characteristic supports notification, the Next Track Object ID characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Next Track Object ID characteristic supports notification, a server device shall notify this characteristic to the client device when the Next Track Object ID characteristic value changes, except when the change is because of the client writing the value to the characteristic.

The Parent Group Object ID characteristic returns its associated value when it is read by a client.

The Parent Group Object ID characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

A server device shall notify this characteristic to the client device when the Parent Group Object ID characteristic value changes.

The Current Group Object ID characteristic returns its associated value when it is read by a client.

When the Current Group Object ID characteristic is written, the server shall change the current group and shall set the Current Track to the first track of the group according to the current playing order and the Track Position shall be set to 0.

If the Current Group Object ID characteristic supports notification, the Current Group Object ID characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Current Group Object ID characteristic supports notification, a server device shall notify this characteristic to the client device when the Current Group Object ID characteristic value changes, except when the change is because of the client writing the value.

See Section 2.1.7 for a further description of current group.

The Playing Order characteristic returns its associated value when it is read by a client.

When the Playing Order characteristic is written, the playing order should be changed to the value written.

If the value written is not supported, the value written is ignored and the current value does not change.

If the Playing Order characteristic supports notification, the Playing Order characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

If the Playing Order characteristic supports notification, a server device shall notify this characteristic to the client device when the Playing Order characteristic value changes.

The overall playing order is left up to the media player implementation. For example, if the end of a group is reached, the media player may continue to play from another group.

The Playing Orders Supported characteristic returns its associated value when it is read by a client.

The Media State characteristic returns its associated value when it is read by a client by using the GATT Read Characteristic Value sub-procedure.

The Media State characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

A server device shall notify this characteristic to the client device when the Media State characteristic value changes.

When the Media Control Point characteristic is written, the server shall perform the behavior defined by the first octet of that value.

If a control point is written with an opcode that is RFU or is not supported by the server, the server shall send the Media Control Point Notification with the OPCODE NOT SUPPORTED Result Code.

If the requested action of any control point write cannot be completed successfully because of an error condition within the player (a condition that is not because the player is in the Inactive state), the server shall send a Media Control Point notification with the COMMAND CANNOT BE COMPLETED Result Code.

If the Media State is Inactive when a control point write occurs, it is left up to the implementation whether to:

If the Media State is Inactive when a control point write occurs, the server shall either:

The Media Control Point characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

Table 3.8 shows the required structure of the Media Control Point Notification. The Media Control Point Notification is sent from the server to the client after each Media Control Point opcode write. If the requested opcode procedure was successfully completed, then the Result Code of the Media Control Point Notification shall be set to SUCCESS. If the requested opcode procedure was not successfully completed, the Result Code shall be set to a value from Table 3.9.

The Media Control Point Opcodes Supported characteristic returns its associated value when it is read by a client.

The Media Control Point Opcodes Supported characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

A server device shall notify this characteristic to the client device when the set of supported opcodes changes.

When the Search Control Point characteristic is written, the server shall start a search operation and populate the current or new search results object. When the search results object is fully populated, the server shall notify the Search Results Object characteristic to the client device if notifications are enabled.

The value written to the Search Control Point characteristic is a sequence of zero or more search control items. Each search control item logically generates a set of tracks and groups. The set of tracks and groups included in the search results object is a set of tracks and groups that satisfy all of the provided search control items. If zero search control point items are written to the Search Control Point, the result of the search is left up to the implementation.

For example, to discover all the tracks from the year 1999, a search would be performed for ”only tracks,” with the “earliest year” of “1999,” and the “latest year” of “1999.”

If no set of tracks or groups are generated with the provided search criteria, there are no search results and the Search Results Object characteristic shall be set to a length of 0.

The order of search results is not defined by this specification.

The Search Control Point characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

Table 3.13 shows the required structure of the Search Control Point Notification. The Search Control Point Notification is sent from the server to the client after each Search Control Point opcode write. If the requested opcode procedure was successfully completed, then the Result Code of the Search Control Point Notification shall be set to SUCCESS. If the requested opcode procedure was not successfully completed, the Result Code should be set to a value from Table 3.14.

The Search Results Object ID characteristic returns its associated value when it is read by a client.

The Search Results Object ID characteristic can be configured for notification by using the GATT Write Characteristic Descriptors sub-procedure on the Client Characteristic Configuration descriptor. When configured for notification, this characteristic can be notified while in a connection.

When the search results object is fully populated, the server shall notify the Search Results Object ID characteristic to the client device if notifications are enabled.

The Content Control ID characteristic returns its associated value when read by a client.

The CCID characteristic in each instance of the service shall persist across connections until the handle range is affected by a service change.

There shall only be one instance of the CCID characteristic in an instance of MCS.

The Segment Name Length field is an unsigned 8-bit integer that determines the length of the Segment Name in octets.

The Segment Name field is a sequence of UTF-8 octets that is n octets long, where n is the value of Segment Name Length.

The Segment Position field is a time offset from the start of the current track to the start of the segment in 0.01-second intervals represented as a 32-bit signed integer. The value of the Segment Position field shall be 0 or greater.

The enumeration of the Object Type field is defined in Table 4.3.

The Object ID field is a uint48 ID from OTS which ID identifies a Track Object (described in Section 4.3) or another Group Object.