<tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99473"> </a>GSM</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99475"> </a>WAV<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99478"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99480"> </a>Low</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99482"> </a>Low</font></td>
  </tr>
  <tr>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99484"> </a>G.723.1</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99486"> </a>WAV<br>RTP</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99489"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99491"> </a>Medium</font></td>
    <td><font face="Palatino, Times New Roman, Times, serif"><a name="99493"> </a>Low</font></td>
  </tr>
</table>



<br></font>


<a name="99495"> </a><font  size="2" face="Palatino, Times New Roman, Times, serif">Table 1-2:   Common audio formats.<br></font>


<p>
  <a name="99286"> </a><font face="Palatino, Times New Roman, Times, serif">Some formats are designed with particular applications and requirements in mind. High-quality, high-bandwidth formats are generally targeted toward CD-ROM or local storage applications. H.261 and H.263 are generally used for video conferencing applications and are optimized for video where there's not a lot of action. Similarly, G.723 is typically used to produce low bit-rate speech for telephony applications. </font>
</p>


<h3>
  <a name="98051"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Media Presentation</font>
</h3>


<p>
  <a name="96978"> </a><font face="Palatino, Times New Roman, Times, serif">Most time-based media is audio or video data that can be presented through output devices such as speakers and monitors. Such devices are the most common <em>destination</em> for media data output. Media streams can also be sent to other destinations--for example, saved to a file or transmitted across the network. An output destination for media data is sometimes referred to as a <em>data sink</em>.</font>
</p>


<h4>
  <a name="97670"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Presentation Controls</font>
</h4>


<p>
  <a name="97096"> </a><font face="Palatino, Times New Roman, Times, serif">While a media stream is being presented, VCR-style presentation controls are often provided to enable the user to control playback. For example, a control panel for a movie player might offer buttons for stopping, starting, fast-forwarding, and rewinding the movie. </font>
</p>


<h4>
  <a name="97671"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Latency</font>
</h4>


<p>
  <a name="97053"> </a><font face="Palatino, Times New Roman, Times, serif">In many cases, particularly when presenting a media stream that resides on the network, the presentation of the media stream cannot begin immediately. The time it takes before presentation can begin is referred to as the <em>start latency. </em>Users might experience this as a delay between the time that they click the start button and the time when playback actually starts.</font>
</p>


<p>
  <a name="97038"> </a><font face="Palatino, Times New Roman, Times, serif">Multimedia presentations often combine several types of time-based media into a synchronized presentation. For example, background music might be played during an image slide-show, or animated text might be synchronized with an audio or video clip. When the presentation of multiple media streams is synchronized, it is essential to take into account the start latency of each stream--otherwise the playback of the different streams might actually begin at different times.</font>
</p>


<h4>
  <a name="97112"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Presentation Quality</font>
</h4>


<p>
  <a name="97148"> </a><font face="Palatino, Times New Roman, Times, serif">The quality of the presentation of a media stream depends on several factors, including:</font>
</p>

<ul>
  <li><a name="97149"> </a><font face="Palatino, Times New Roman, Times, serif">The compression scheme used</font>
  <li><a name="97150"> </a><font face="Palatino, Times New Roman, Times, serif">The processing capability of the playback system</font>
  <li><a name="97151"> </a><font face="Palatino, Times New Roman, Times, serif">The bandwidth available (for media streams acquired over the network)</font>
</ul>

<p>
  <a name="97152"> </a><font face="Palatino, Times New Roman, Times, serif">Traditionally, the higher the quality, the larger the file size and the greater the processing power and bandwidth required. Bandwidth is usually represented as the number of bits that are transmitted in a certain period of time--the <em>bit rate</em>. </font>
</p>


<p>
  <a name="97204"> </a><font face="Palatino, Times New Roman, Times, serif">To achieve high-quality video presentations, the number of frames displayed in each period of time (the <em>frame rate</em>) should be as high as possible. Usually movies at a frame rate of 30 frames-per-second are considered indistinguishable from regular TV broadcasts or video tapes.</font>
</p>


<h3>
  <a name="96181"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Media Processing</font>
</h3>


<p>
  <a name="97243"> </a><font face="Palatino, Times New Roman, Times, serif">In most instances, the data in a media stream is manipulated before it is presented to the user. Generally, a series of processing operations occur before presentation:</font>
</p>

<ol type="1">
  <li value="1"><a name="97459"> </a><font face="Palatino, Times New Roman, Times, serif">If the stream is multiplexed, the individual tracks are extracted.</font>
  <li value="2"><a name="97465"> </a><font face="Palatino, Times New Roman, Times, serif">If the individual tracks are compressed, they are decoded.</font>
  <li value="3"><a name="97475"> </a><font face="Palatino, Times New Roman, Times, serif">If necessary, the tracks are converted to a different format.</font>
  <li value="4"><a name="97466"> </a><font face="Palatino, Times New Roman, Times, serif">Effect filters are applied to the decoded tracks (if desired).</font>
</ol>

<p>
  <a name="97483"> </a><font face="Palatino, Times New Roman, Times, serif">The tracks are then delivered to the appropriate output device. If the media stream is to be stored instead of rendered to an output device, the processing stages might differ slightly. For example, if you wanted to capture audio and video from a video camera, process the data, and save it to a file:</font>
</p>

<ol type="1">
  <li value="1"><a name="97487"> </a><font face="Palatino, Times New Roman, Times, serif">The audio and video tracks would be captured.</font>
  <li value="2"><a name="97488"> </a><font face="Palatino, Times New Roman, Times, serif">Effect filters would be applied to the raw tracks (if desired).</font>
  <li value="3"><a name="97489"> </a><font face="Palatino, Times New Roman, Times, serif">The individual tracks would be encoded.</font>
  <li value="4"><a name="97490"> </a><font face="Palatino, Times New Roman, Times, serif">The compressed tracks would be multiplexed into a single media stream.</font>
  <li value="5"><a name="99267"> </a><font face="Palatino, Times New Roman, Times, serif">The multiplexed media stream would then be saved to a file.</font>
</ol>

<h4>
  <a name="99216"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Demultiplexers and Multiplexers</font>
</h4>


<p>
  <a name="99218"> </a><font face="Palatino, Times New Roman, Times, serif">A demultiplexer extracts individual tracks of media data from a multiplexed media stream. A <em>mutliplexer</em> performs the opposite function, it takes individual tracks of media data and merges them into a single multiplexed media stream. </font>
</p>


<h4>
  <a name="97514"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Codecs</font>
</h4>


<p>
  <a name="97515"> </a><font face="Palatino, Times New Roman, Times, serif">A codec performs media-data compression and decompression. When a track is encoded, it is converted to a compressed format suitable for storage or transmission; when it is decoded it is converted to a non-compressed (raw) format suitable for presentation. </font>
</p>


<p>
  <a name="97538"> </a><font face="Palatino, Times New Roman, Times, serif">Each codec has certain input formats that it can handle and certain output formats that it can generate. In some situations, a series of codecs might be used to convert from one format to another.</font>
</p>


<h4>
  <a name="97513"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Effect Filters</font>
</h4>


<p>
  <a name="97529"> </a><font face="Palatino, Times New Roman, Times, serif">An effect filter modifies the track data in some way, often to create special effects such as blur or echo.</font>
</p>


<p>
  <a name="97472"> </a><font face="Palatino, Times New Roman, Times, serif">Effect filters are classified as either pre-processing effects or post-processing effects, depending on whether they are applied before or after the codec processes the track. Typically, effect filters are applied to uncompressed (raw) data.</font>
</p>


<h4>
  <a name="96188"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Renderers</font>
</h4>


<p>
  <a name="99184"> </a><font face="Palatino, Times New Roman, Times, serif">A renderer is an abstraction of a presentation device. For audio, the presentation device is typically the computer's hardware audio card that outputs sound to the speakers. For video, the presentation device is typically the computer monitor.</font>
</p>


<h5>
  <a name="99185"> </a><i><font color="#003366" face="Palatino, Times New Roman, Times, serif">Compositing</font></i>
</h5>


<p>
  <a name="99186"> </a><font face="Palatino, Times New Roman, Times, serif">Certain specialized devices support <em>compositing</em>. Compositing time-based media is the process of combining multiple tracks of data onto a single presentation medium. For example, overlaying text on a video presentation is one common form of compositing. Compositing can be done in either hardware or software. A device that performs compositing can be abstracted as a renderer that can receive multiple tracks of input data.</font>
</p>


<h3>
  <a name="99183"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Media Capture</font>
</h3>


<p>
  <a name="97259"> </a><font face="Palatino, Times New Roman, Times, serif">Time-based media can be captured from a live source for processing and playback. For example, audio can be captured from a microphone or a video capture card can be used to obtain video from a camera. Capturing can be thought of as the <em>input</em> phase of the standard media processing model. </font>
</p>


<p>
  <a name="97427"> </a><font face="Palatino, Times New Roman, Times, serif">A capture device might deliver multiple media streams. For example, a video camera might deliver both audio and video. These streams might be captured and manipulated separately or combined into a single, multiplexed stream that contains both an audio track and a video track.</font>
</p>


<h4>
  <a name="97429"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Capture Devices</font>
</h4>


<p>
  <a name="97280"> </a><font face="Palatino, Times New Roman, Times, serif">To capture time-based media you need specialized hardware--for example, to capture audio from a live source, you need a microphone and an appropriate audio card. Similarly, capturing a TV broadcast requires a TV tuner and an appropriate video capture card. Most systems provide a query mechanism to find out what capture devices are available.</font>
</p>


<p>
  <a name="97376"> </a><font face="Palatino, Times New Roman, Times, serif">Capture devices can be characterized as either push or pull sources. For example, a still camera is a pull source--the user controls when to capture an image. A microphone is a push source--the live source continuously provides a stream of audio.</font>
</p>


<p>
  <a name="97436"> </a><font face="Palatino, Times New Roman, Times, serif">The format of a captured media stream depends on the processing performed by the capture device. Some devices do very little processing and deliver raw, uncompressed data. Other capture devices might deliver the data in a compressed format.</font>
</p>


<h4>
  <a name="97358"> </a><font color="#003366" face="Palatino, Times New Roman, Times, serif">Capture Controls</font>
</h4>


<p>
  <a name="97311"> </a><font face="Palatino, Times New Roman, Times, serif">Controls are sometimes provided to enable the user to manage the capture process. For example, a capture control panel might enable the user to specify the data rate and encoding type for the captured stream and start and stop the capture process. </font>
</p>
</blockquote>
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