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Mobile Media API (MMAPI) background


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Mobile Media API (MMAPI) background

MMAPI defines the superset of the multimedia capabilities that are present in MIDP 2.0. It started life as JSR 135

A D V E R T I S E M E N T
and is currently at version 1.1. The current version includes some documentation changes and security updates, and is distributed as an optional jar file in the J2ME wireless toolkit 2.2. Although the release notes for the toolkit state that MMAPI 1.1 is bundled, the actual version is 1.0. I have blogged about this before and have submitted an official bug with Sun.

The MMAPI is built on a high-level abstraction of all the multimedia devices that are possible in a resource-limited device. This abstraction is manifest in three classes that form the bulk of operations that you do with this API. These classes are the Player and Control interfaces, and the Manager class. Another class, the DataSource abstract class, is used to locate resources, but unless you define a new way of reading data you will probably never need to use it directly.

In a nutshell, you use the Manager class to create Player instances for different media by specifying DataSource instances. The Player instances thus created are configurable by using Control instances. For example, almost all Player instances would theoretically support a VolumeControl to control the volume of the Player. Figure 1 shows this process.

Figure 1
Figure 1. Player creation and management

Manager is the central class for creating players and it provides three methods to indicate the source of media. These methods, all static, are createPlayer(DataSource source), createPlayer(InputStream stream, String type) and createPlayer(String locator). The last method is interesting because it provides a URI style syntax for locating media. For example, if you wanted to create a Player instance on a web based audio file, you can use createPlayer("http://www.yourwebsite.com/audio/song.wav"). Similarly, to create a media Player to capture audio, you can use createPlayer("capture://audio"); and so on. Table 4.1 shows the supported syntax with examples.

Media Type Example syntax
Capture audio "capture://audio" to capture audio on the default audio capture device or "capture://devmic0?encoding=pcm" to capture audio on the devmic0 device in the PCM encoding
Capture video "capture://video" to capture video from the default video capture device or "capture://devcam0?encoding=rgb888&width=100&height=50" to capture from a secondary camera, in rgb888 encoding mode and with a specified width and height
Start listening in on the built-in radio "capture://radio?f=105.1&st=stereo" to tune into 105.1 FM frequency and stereo mode
Start streaming video/audio/text from an external source "rtp://host:port/type" where type is one of audio, video or text
Play tones and MIDI "device://tone" will give you a player that you can use to play tones or

"device://midi" will give you a player that you can use to play MIDI

Table 4.1. List of supported protocols and example syntax

A list of supported protocols for a given content type can be retrieved by calling the method getSupportedProtocols(String contentType) which returns a String array. For example, if you call this method with the argument "audio/x-wav" it will return an array with three values in it: http, file and capture for the wireless toolkit. This lets you know that you can retrieve the content type "audio/x-wav", by using http and file protocols, and capture it using the capture protocol. Similarly, a list of supported content types for a given protocol can be accessed by calling the method getSupportedContentTypes(String protocol). Thus, calling getSupportedContentTypes("capture") will return audio/x-wav and video/vnd.sun.rgb565 for the wireless toolkit, indicating that you can capture standard audio and rgb565 encoded video. Note that passing null in any of these methods will return all supported protocols and content types respectively.

Once a Player instance is created using the Manager class methods, it needs to go through various stages before it can be used. Upon creation, the player is in an UNREALIZED state and must be REALIZED and PREFETCHED before it can be STARTED. Realization is the process in which the player examines the source or destination media resources and has enough information to start acquiring them. Prefetching happens after realization and the player actually acquires these media resources. Both realization and prefetching processes may be time- and resource-consuming, but doing them before the player is started ensures that there is no latency when the actual start happens. Once a player is started, using the start() method, and is processing media data, it may enter the PREFETCHED state again when the media processing stops on its own (because the end of the media was reached, for example), you explicitly call the stop() method on the Player instance, or when a predefined time (called TimeBase) is reached. Going from STARTED to PREFETCHED state is like pausing the player, and calling start() on the Player instance restarts from the previous paused point (if the player had reached the end of the media, this means that it will restart from the beginning).

Good programming practice requires that you call the realize() and prefetch() methods before you call the start() method to avoid any latency when you want the player to start. The start() method implicitly calls the prefetch() method (if the player is not in a PREFETCHED state), which in turn calls the realize() method (if the player is not in a REALIZED state), but if you explicitly call these methods first, you will have a Player instance that will start playing as soon as you call start(). A player can go into the CLOSED state if you call the close() method on it, after which the Player instance cannot be reused. Instead of closing, you can deallocate a player by calling deallocate(), which returns the player to the REALIZED state, thereby releasing all the resources that it would have acquired.

Figure 2 shows the various states and the transitions between them.

Figure 2
Figure 2. Media player states and their transitions

Notification of the transitions between different states can be delivered to attached listeners on a player. To this end, a Player instance allows you to attach a PlayerListener by using the method addPlayerListener(PlayerListener listener). Almost all transitions states are notified to the listener via the method playerUpdate(Player player, String event, Object eventData).

A player also enables control over the properties of the media that it is playing by using controls. A control is a media processing function that may be typical for a particular media type. For example, a VideoControl controls the display of video, while a MIDIControl provides access to MIDI devices' properties. There are, of course, several controls that may be common across different media, VolumeControl being an example. Because the Player interface extends the Controllable interface, it provides means to query the list of the available controls. You do this by calling the method getControls(), which returns an array of Control instances, or getControl(String controlType), which returns an individual Control (null if the controlType is not supported).

As I said earlier, MIDP 2.0 contains a subset of the broad MMAPI 1.1. This is to ensure that devices that only support MIDP 2.0 can still use a consistent method of discovery and usage that can scale if the broader API is present. The subset only supports tones and audio with only two controls for each, ToneControl and VolumeControl. Additionally, datasources are not supported, and hence, the Manager class in MIDP 2.0 is simplified and does not provide the createPlayer(DataSource source) method.

In the next few sections, you will learn how to play audio and video from a variety of sources in your multimedia MIDlets.



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