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📁 奉献给多媒体java编程者们。JMF2.1.1最新版本的用户指南。JMF是java用于基于实时多媒体的开发工具
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<title>Working with  Time-Based Media  </title>
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    <td align="right"><em>JMF 2.0 API Guide</em>
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<blockquote>
<div align="right">
<a name="99806"> </a><font  size="3" face="Palatino, Times New Roman, Times, serif">1 <br></font>
</div>
<div align="right">
<h2>
  <a name="99722"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Working with <br>Time-Based Media</font>
</h2>
</div>

<p>
  <a name="96367"> </a><font face="Palatino, Times New Roman, Times, serif">Any data that changes meaningfully with respect to time can be characterized as time-based media. Audio clips, MIDI sequences, movie clips, and animations are common forms of time-based media. Such media data can be obtained from a variety of sources, such as local or network files, cameras, microphones, and live broadcasts. </font>
</p>


<p>
  <a name="99102"> </a><font face="Palatino, Times New Roman, Times, serif">This chapter describes the key characteristics of time-based media and describes the use of time-based media in terms of a fundamental data processing model: </font>
</p>


<a name="99564"> </a><font  size="1" face="Palatino, Times New Roman, Times, serif"><img src="images/JMFTBMa.gif" height="216" width="478">
<br></font>


<a name="98146"> </a><font  size="2" face="Palatino, Times New Roman, Times, serif">Figure 1-1:   Media processing model.<br></font>


<h3>
  <a name="100910"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Streaming Media</font>
</h3>


<p>
  <a name="97895"> </a><font face="Palatino, Times New Roman, Times, serif">A key characteristic of time-based media is that it requires timely delivery and processing. Once the flow of media data begins, there are strict timing deadlines that must be met, both in terms of receiving and presenting the data. For this reason, time-based media is often referred to as<em> streaming media</em>--it is delivered in a steady stream that must be received and processed within a particular timeframe to produce acceptable results. </font>
</p>


<p>
  <a name="98079"> </a><font face="Palatino, Times New Roman, Times, serif">For example, when a movie is played, if the media data cannot be delivered quickly enough, there might be odd pauses and delays in playback. On the other hand, if the data cannot be received and processed quickly enough, the movie might appear jumpy as data is lost or frames are intentionally dropped in an attempt to maintain the proper playback rate.</font>
</p>


<h4>
  <a name="97896"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Content Type</font>
</h4>


<p>
  <a name="96387"> </a><font face="Palatino, Times New Roman, Times, serif">The format in which the media data is stored is referred to as its <em>content type. </em>QuickTime, MPEG, and WAV are all examples of content types. Content type is essentially synonymous with file type--content type is used because media data is often acquired from sources other than local files. </font>
</p>


<h4>
  <a name="96773"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Media Streams</font>
</h4>


<p>
  <a name="97922"> </a><font face="Palatino, Times New Roman, Times, serif">A <em>media stream</em> is the media data obtained from a local file, acquired over the network, or captured from a camera or microphone. Media streams often contain multiple channels of data called <em>tracks</em>. For example, a Quicktime file might contain both an audio track and a video track. Media streams that contain multiple tracks are often referred to as <em>multiplexed </em>or<em> complex</em> media streams. <em>Demultiplexing</em> is the process of extracting individual tracks from a complex media stream.</font>
</p>


<p>
  <a name="96517"> </a><font face="Palatino, Times New Roman, Times, serif">A track's <em>type </em>identifies the kind of data it contains, such as audio or video. The <em>format</em> of a track defines how the data for the track is structured. </font>
</p>


<p>
  <a name="96575"> </a><font face="Palatino, Times New Roman, Times, serif">A media stream can be identified by its location and the protocol used to access it. For example, a URL might be used to describe the location of a QuickTime file on a local or remote system. If the file is local, it can be accessed through the FILE protocol. On the other hand, if it's on a web server, the file can be accessed through the HTTP protocol. A <em>media locator</em> provides a way to identify the location of a media stream when a URL can't be used.</font>
</p>


<p>
  <a name="96583"> </a><font face="Palatino, Times New Roman, Times, serif">Media streams can be categorized according to how the data is delivered: </font>
</p>

<ul>
  <li><a name="96599"> </a><font face="Palatino, Times New Roman, Times, serif">Pull--data transfer is initiated and controlled from the client side. For example, Hypertext Transfer Protocol (HTTP) and FILE are pull protocols.</font>
  <li><a name="96601"> </a><font face="Palatino, Times New Roman, Times, serif">Push--the server initiates data transfer and controls the flow of data. For example, Real-time Transport Protocol (RTP) is a push protocol used for streaming media. Similarly, the SGI MediaBase protocol is a push protocol used for video-on-demand (VOD). </font>
</ul>

<h4>
  <a name="98245"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Common Media Formats</font>
</h4>


<p>
  <a name="99306"> </a><font face="Palatino, Times New Roman, Times, serif">The following tables identify some of the characteristics of common media formats. When selecting a format, it's important to take into account the characteristics of the format, the target environment, and the expectations of the intended audience. For example, if you're delivering media content via the web, you need to pay special attention to the bandwidth requirements.</font>
</p>


<p>
  <a name="101753"> </a><font face="Palatino, Times New Roman, Times, serif">The CPU Requirements column characterizes the processing power necessary for optimal presentation of the specified format. The Bandwidth Requirements column characterizes the transmission speeds necessary to send or receive data quickly enough for optimal presentation.</font>
</p>


<a name="99569"> </a><font  size="1" face="Palatino, Times New Roman, Times, serif">

<table border="1" bordercolorlight="#FFFFFF" bordercolordark="#000000"
       cellpadding="5" cellspacing="0">
  <caption><b><i><font face="Palatino, Times New Roman, Times, serif"></font></i></b></caption>
  <tr bgcolor="#CCCCCC"><div align="center">
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99319"> </a>Format</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99321"> </a>Content Type</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99323"> </a>Quality</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99325"> </a>CPU Requirements</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99327"> </a>Bandwidth Requirements</font></th>
  </div></tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99329"> </a>Cinepak</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99331"> </a>AVI<br>QuickTime</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99333"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99335"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99337"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99339"> </a>MPEG-1</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99341"> </a>MPEG</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99343"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99345"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99347"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99349"> </a>H.261</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99351"> </a>AVI<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99354"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99356"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99358"> </a>Medium</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99360"> </a>H.263</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99362"> </a>QuickTime<br>AVI<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99365"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99367"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99369"> </a>Low</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99371"> </a>JPEG</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99373"> </a>QuickTime<br>AVI<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99377"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99379"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99381"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99383"> </a>Indeo</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99385"> </a>QuickTime<a name="99386"> </a>AVI</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99388"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99390"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99392"> </a>Medium</font></td>
  </tr>
</table>



<br></font>


<a name="101714"> </a><font  size="2" face="Palatino, Times New Roman, Times, serif">Table 1-1:   Common video formats.<br></font>


<a name="101792"> </a><font  size="1" face="Palatino, Times New Roman, Times, serif">

<table border="1" bordercolorlight="#FFFFFF" bordercolordark="#000000"
       cellpadding="5" cellspacing="0">
  <caption><b><i><font face="Palatino, Times New Roman, Times, serif"></font></i></b></caption>
  <tr bgcolor="#CCCCCC"><div align="center">
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99405"> </a>Format</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99407"> </a>Content Type</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99409"> </a>Quality</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99411"> </a>CPU Requirements</font></th>
    <th><font face="Palatino, Times New Roman, Times, serif" color="#003366"><a name="99413"> </a>Bandwidth Requirements</font></th>
  </div></tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99415"> </a>PCM</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99417"> </a>AVI<br>QuickTime<br>WAV</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99421"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99423"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99425"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99427"> </a>Mu-Law</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99589"> </a>AVI<br>QuickTime<br>WAV<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99434"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99436"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99438"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99440"> </a>ADPCM<br>(DVI,<br>IMA4)</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99442"> </a>AVI<br>QuickTime<br>WAV<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99447"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99449"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99451"> </a>Medium</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99453"> </a>MPEG-1</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99455"> </a>MPEG</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99457"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99459"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99461"> </a>High</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99463"> </a>MPEG<br>Layer3</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99465"> </a>MPEG</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99467"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99469"> </a>High</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99471"> </a>Medium</font></td>
  </tr>
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