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Standard boosts automotive Bluetooth streaming

Posted: 17 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:portable digital music? apple? ipod? bluetooth? generic media control profile?

By Ankit Jain and Bikash Chowdhury

Portable digital music is "going places" as evidenced by Apple's iPod shipping more than five million units a quarter and mobile phone vendors are launching music phones at the rate of nearly one a month. Bluetooth shipments have been growing and now touch five million units per week. Automakers, audio/video suppliers, and electronic system integrators are keen to leverage the immense popularity of portable music devices and Bluetooth for in-car automotive multimedia entertainment. However, providing a compelling user experience streaming music over automotive Bluetooth has proven very challengingprimarily due to limitations in the Bluetooth specification. However, a proposed open standard called Generic Media Control Profile (GMCP) addresses user requirements for Bluetooth streaming.

Portable digital music in automobiles
There has been an explosion in the shipment of portable music players with the worldwide market for flash and hard drive-based portable media players expected to increase from 35.9 million players shipped in 2004 to 82.3 million players shipping in 2007 (Source: IDC, December 2004). And, almost all the top mobile phone vendors have either announced or launched mobile phones that support music. With more than 700 million mobile phone shipments globally, the numbers for music phones are going to be significant.

Given the popularity of portable digital music, automakers are eager to bring portable digital music into the car. They want to enable users to enjoy their music on their portable music devices in the car using the car's speakers. The user should also be able to view the media-content-related information (such as ID3 tags) on the car head unit display, control the portable music device from the car head unit, and be able to switch between handsfree mobile phone operation (wired or wireless) and music.

Some definitions
Portable music devices: A small electronic gadget that allows users to store, carry and play music in digital form. It can be a music player that employs flash or hard disk storage technology and supports any number of digital media formatsAAC, WMA, MP3 to name a few; or it can be mobile phone that provides storage and media format decoding capabilities for storing and playing music files.

Car Head Unit: This is the part of the audio system that a user interacts with most. It consists of a panel with display and control buttons.

A2DP: Is an acronym for Advanced Audio Distribution Profile. It is part of the Bluetooth specifications for advance audio and provides specification for streaming music over Bluetooth.

AVRCP: Stands for Audio Video Remote Control Profile. It is part of the Bluetooth specifications for advanced audio and provides specification for controlling an audio music source remotely over Bluetooth.

Controller: In the context of AVRCP, a Controller governs the audio music source remotely.

Target: In the context of AVRCP, a Target is a device that is controlled; more often than not, it is the music source.

The need for Bluetooth for in-car portable music
Automakers want to support portable music devices in the car. But, the reality is that each one of these portable devices has a custom or proprietary interface making any kind of universal cabling impractical. This limits the ability of OEMs and vendors to support a reasonable, let alone a large, number of portable music devices.

Even with support for the most popular portable music players, the solution and user experience are often inelegant. Most solutions involve installing a wired connector for the music player in the car's glove compartment. Every time the user walks into the car, he or she has to open the glove compartment and install the portable music player, before enjoying the music. The expensive wiring work and the required user intervention make the solution unattractive and costly.

Wired support for a portable music player in a car

In addition, the lifetime of a car is much more than the lifetime of a portable music device. Thus, while the interfaces in these portable music devices change over time, it becomes very expensive to upgrade the car with the newest interface.

Bluetooth as a technology is an ideal solution to the above problem. It allows Bluetooth devices to form ad-hoc networks, provides a high-speed wireless connection (>700 kbps), is low powered compared to other wireless technologies, and specifies how to stream and control music (Advanced Audio Distribution Profile and Audio Video Remote Control Profile) over the wireless link. The user can bring his or her Bluetooth-enabled personal music device into the car, which connects over Bluetooth to the car head unit, and stream music to be rendered on the car speakers.

Bluetooth is already installed in a large number of automobiles for handsfree operation of mobile phones. The cost of adopting Bluetooth as a solution for music streaming and control is incremental for automakersit is the cost of software alone. What makes Bluetooth even more attractive for them is its widespread adoption in multiple applications, with five million Bluetooth products shipping per week (close to 120 million Bluetooth mobile phones are expected to ship in 2005, and companies have introduced Bluetooth transmitters for portable music players including the iPod). Spurred by legislation in favor of handsfree support in automobiles, 16 million cars are expected to be Bluetooth enabled for handsfree and stereo by the year 2007. Given these impressive shipment data, it is easy to see why Bluetooth is the technology of choice for adding portable music devices to the in-car entertainment ecosystem.

Automotive Bluetootha user experience
Bob has recently purchased a new car that comes fitted with Bluetooth advanced audio technology. He walks to his car with his shining new iPod that supports Bluetooth streaming. As soon as he enters, the iPod in his coat pocket connects to the in-car music system over Bluetooth. The car display shows his favorite play list. Bob selects the song on the display and presses Play button on the car head unit. Music starts streaming from the iPod to the car speakers over Bluetooth. Bob can see details like song name, album, and artist name; he can navigate through the play list to a song of his choiceall using the display and control on the car head unit.

Bluetooth streaming music in the car

Need for a standard
Generic Media Control Profile (GMCP) is proposed to be an open standard for transfer of media content related information over Bluetooth. Developed by Impulsesoft, it leverages AVRCP (the Bluetooth SIG (Special Interest Group) specified standard for remote control of Bluetooth AV devices) and has already been implemented in a number of Bluetooth AV solutions.

The Audio Video Remote Control Profile (AVRCP) provides basic remote control functionality like Play, Pause, Next, and Forward. It does not provide a standard for exchange of media content related information like play list, song name, album name, and artist name. AVRCP does, however, provide a command (Vendor Dependent command) that allows developers to send any command. But some developers and companies have adopted non-AVRCP approaches for solving the problem. These include using AT commands over Serial Port Profile (SPP), or building additional semantics in File Transfer Profile for media objects and ID3 tags access. Such varied approach and lack of any standard has resulted in widespread interoperability issues for features that relate to display of media content related information.

While automotive applications are, no doubt, big drivers for a standard, the requirement is not limited to streaming and display in automobiles alone. According to technology research firm In-stat, while four million automobiles are going to be Bluetooth enabled for streaming music by 2007, the corresponding figure for Bluetooth stereo headphones with displays are expected to be 12 million. Clearly there is both a technology and market need for a standard like GMCP.

Anatomy of GMCP
The following requirements relating to media content related information are addressed by GMCP.

  • Target Device Query: The specification should allow the Target device to be queried for: Device information including device name, type, and class; Device settings such as play mode, shuffle, or repeat, and default language support.

  • Information Extraction: The standard should provide ability to extract ID3 tags such as song name, album name, and artist name.

  • Track Information Display: The specification should provide ability to display extracted information.

  • Navigation: The standard should provide ability to browse and select the media list by category, play list, etc.

  • Search: The specification should allow for attribute-based search such as title, artist, genre, mood, or other ID3 tags.

  • Event Notification: The standard should provide capability for Target device to send event notifications relating to status or change of status; for example, track change, forward, rewind, play status, and elapsed time.

  • Annotation: The specification should allow users (and developers) to specify labels for easy and fast access to information or track; for instance, favorites, and most recently played songs.

The GMCP standard needed to meet the following design objectives. It had to:

  • Be consistent with existing Bluetooth profiles: GMCP should not require changes to existing profiles.

  • Be backward compatible: Existing implementations that do not have GMCP should require no changes in order to work with implementations that had GMCP.

  • Leverage existing transport protocol over Bluetooth radio.

  • Be extensible: It should be possible and easy to extend GMCP to address future requirements.

  • Support asynchronous event notification: GMCP should allow Target device to send event notifications relating to status or change of status asynchronously.

  • Be OS, language, and device independent.

  • Be lightweight: GMCP needs to have low memory footprint and minimal latency.

GMCP leverages AVRCP extension for transfer of media content related information. It uses the AVRCP "VENDOR_DEPENDENT" command for extending AVRCP. The payload and transaction formats are designed in line with IEEE 1394 AV/C protocol. GMCP follows the AV/C Indirect Mode, wherein it uses rich semantic for communicating media content related information and the Controller is free to define the user interface independent of the Target display.

GMCP protocol model

GMCP is a session-based standard. It expects an explicit start and end of a session. Either the Controller or the Target could start or end a session. Within a session, GMCP commands may be context-sensitive (i.e. a command can be interpreted and executed relative to prior commands).

A GMCP session

GMCP supports the following Protocol Data Unit (PDU) or message types:

  • Configuration: This message type is needed for starting and ending a GMCP session, as well as for obtaining Target device information like play mode, and default language support.

  • Enumeration: This message type is needed for browsing the play list on the Target device, as well as for user (developer) defined labels for annotation.

  • Media Information: This allows for extracting track information or ID3 tags like song name, album name, and artist name. This message type also provides a mechanism to query for information like play status, track duration, and elapsed time.

  • Notification: This message type is used by Controller device to register for event notifications from the Target device; for example, track change, track position change, forward, rewind, and play status change.

  • Response: This message type allows for a generic response to the previous command.

The table below shows a map of Data Requirements and Message or PDU types addressing a Data Requirement.

Data requirement-message type map

Alternatives to GMCP
There can be multiple alternatives to the design choice made by GMCP. For example, the Data Requirements could be achieved by:

  • SPP Approach using AT commands over Serial Port Profile (SPP).

  • OBEX Approach building additional semantics in File Transfer Profile for media objects and ID3 tags access.

The two approaches above suffer from a number of limitations that relate to performance and functionality. The SPP Approach requires inclusion of the RFCOMM protocol layer and SPP, in addition to a need for creation of a new payload format. The OBEX Approach requires inclusion of OBEX protocol layer and FTP. In addition, it suffers from the lack of support for asynchronous event notification. GMCP does not suffer from the above limitations.

Benefits of GMCP

  • Provides improved user experience: GMCP addresses the requirements of providing a standard for transfer of media content related information over Bluetooth. Any device that needs to support display of media content related information can leverage GMCP. Examples include portable music players, music mobile phones, PCs, handhelds as Target device, and in-car entertainment systems, stereo headsets with displays, remote controls with displays, and other accessories such as Controller devices. Users can now browse a play list, search for specific songs, and view song information, such as song name and artist name, over Bluetooth.

  • Promotes interoperability: By design, GMCP allows different music sources to send media content related information to any Bluetooth device capable of controlling it and displaying such information.

  • Standards based: GMCP is based on existing standards, AVRCP and 1394, and leverages ID3 and UTF-8 fontsresulting in lower cost of development and adoption, and higher levels of standardization and interoperability.

  • Extends and complements AVRCP: By leveraging AVRCP, GMCP ensures that change of state and media content related information are synchronized with little or minimal overhead. Any other approach will incur overhead cost to ensure that change of state initiated by AVRCP and the information relating changed state requested by GMCP are in synch.

  • OS, language and device independent: GMCP has been designed to be OS, language and device independent, and therefore is highly portable.

  • Designed for small footprint: GMCP leverages AVRCP that needs to be present, by default, as part of the AV application rather than using a non AV profile like SPP or FTP. This results in low memory footprint and more efficient, low cost solution.

Future of GMCP
Impulsesoft is promoting GMCP as an open standard for transfer of media content related information over Bluetooth. As with all standards, adoption by the industry consortium is a necessary condition for the success of GMCP. Impulsesoft is working with multiple OEMs in Europe and North America for promoting GMCP to the Bluetooth SIG.

Portable digital music is here to stay. The past few months have seen top mobile phone vendors announcing music mobile phones. Apple's iPod has been showing impressive shipment figures. It is little wonder then, that portable digital music is finding its way into automobiles. However, automakers, AV suppliers and electronic system integrators are faced with the problem of plentytoo many portable music devices to support.

Bluetooth is the ideal technology that addresses this problem, but lack of a standard for transfer of media-content related information over Bluetooth is affecting user experience and interoperability. Impulsesoft's proposed an open-standard Generic Media Control Profile, GMCP, promises to fill the void in the Bluetooth specification for AVRCP.

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