RealSystem gives you the power to create compelling, complex multimedia presentations streamed over a network. It includes RealServer, the most advanced streaming media server available, and RealPlayer, the world's most popular desktop application for playing streaming media files.
RealSystem streams multimedia presentations over a network. This lets the presentation start playing back shortly after the computer begins to receive data. In contrast, a Web server downloads files, and the computer must receive the files in their entirety before starting playback. A downloaded video clip, for example, may not begin playback for several minutes. A streamed video clip, in contrast, begins to play within seconds.
RealMedia gives you many choices for combining media into a streaming presentation. In choosing which formats to stream, you need to take a careful look at bandwidth as described in the next section. This helps you decide, for example, what size to make a streaming video.
Chapter 3 discusses the audio formats you can stream:
Chapter 4 describes the video formats you can stream:
RealFlash, which pairs Macromedia Flash animation with a RealAudio soundtrack, lets you stream animated presentations over a network. See Chapter 5 for details.
RealMedia lets you stream any JPEG image along with video or audio. In addition, you can use RealPix to create eye-catching slide shows with special effects such as dissolves and zooms. If you have the latest version of ReaPlayer installed, click here to view a RealPix sample. For more information, download RealPix Content Creation Guide from http://www.real.com.
RealMedia includes RealText, streaming text you can format and display at specific times within a presentation. You can use RealText to add subtitles to a video, for example, or lay out text from a live source to create a real-time stock ticker. If you have the latest version of ReaPlayer installed, click here to see a static example of a RealText stock ticker. For details, see RealText Content Creation Guide, available at http://www.real.com.
RealSystem easily extends to stream nearly any type of file or live event. Check http://www.real.com for the availability of plug-ins that let RealSystem stream additional video and audio formats, as well as exciting new types of media.
A network connection's bandwidth is a crucial factor in developing a streaming multimedia presentation. To let Web users with 28.8 Kbps modems view your presentation, for example, you have to develop a presentation that requires less than 28.8 Kilobits of data per second.
As the first step in developing your presentation, target a bandwidth as described below and create content with that connection speed in mind. This helps ensure that the presentation streams smoothly. If you don't hit your bandwidth mark, the presentation may pause intermittently as RealPlayer waits for the necessary data to arrive.
Total bandwidth is the upper limit on how much data can pass through a network connection per second. Internet bandwidth is described in Kilobits per second (Kbps). A 28.8 Kbps modem, for example, can receive data at any speed up to 28.8 Kbps. Bandwidth is analogous to a speed limit, such as 60 m.p.h. A presentation's bit rate is analogous to car speed. Based on variables such as weather and traffic, a car may be able to travel only 30 m.p.h. Due to network congestion and server load, a 28.8 Kbps modem may receive 11 Kbps of data one minute, 25 Kbps of data another.
When you drive on a highway, you have no control over weather and traffic that makes you slow down. Under good conditions, though, you can observe the speed limit. Likewise with your presentation, you have no control over server load and network congestion when a Web user views your presentation. You can, however, ensure that your presentation does not exceed the user's bandwidth. On the highway, breaking the speed limit gets you a ticket. On the Internet, exceeding bandwidth stalls your presentation.
For example, a 28.8 Kbps connection can still play a presentation that requires a 56 Kbps stream. But the modem takes around two seconds to receive the data that RealPlayer has to play every second. In other words, data has to be displayed at a rate faster than which it comes in over the modem. Consequently, RealPlayer does not begin playback until it receives and stores ("buffers") enough data to play the presentation without halting. For a long presentation, this may take a few minutes. Viewers are not likely to wait for such a slow starting presentation.
Designing content suitable for viewers' available bandwidth is crucial to delivering a compelling multimedia presentation. Because most Internet users have 28.8 Kbps modems, content available to the public should target that bandwidth. If your presentation is for high-speed intranet use only, you may be able to target a higher minimum bandwidth. Delivering content suited for low bandwidth ensures that your presentation flows smoothly for all viewers, helping you reach the largest audience possible.
The target bandwidth of a RealMedia presentation is the maximum bandwidth available for a particular connection, such as 28.8 Kbps. The presentation's total bit rate must be at or below the target bit rate. The total bit rate comprises two main parts:
If your target bit rate is 28.8 Kbps, for example, take 75% of that rate as the bandwidth available for your streaming files. For a 28.8 Kbps connection, you have approximately 20 Kbps total for your presentation. The following table lists the recommended maximum presentation bit rate for streaming files over different network connections.
Once you know the bit rate available for your streaming files, you can begin to develop your bandwidth strategy. If you want to stream just one file for your presentation, your strategy is straightforward. Suppose you want to create an audio file that Web users with 14.4 Kbps modems can play. You can simply create a RealAudio file that consumes 8 Kbps of bandwidth. Then anyone with a 14.4 Kbps or higher connection can listen to your presentation.
An exciting part of RealMedia, though, is that it allows you to put together different types of data in one presentation, such as slide shows with audio voiceovers, or video with scrolling subtitles. When multiple files play together in your presentation, you need to consider how much presentation bandwidth to allot to each file. This is also true if you have a single file that has multiple streams, such as a video that contains a visual track and an audio track.
Suppose you want to stream a RealVideo clip at 28.8 Kbps. How much bandwidth should you give to the visual track and to the audio track? The answer depends on the content. Because music has a greater frequency range than voice, a music video requires more audio data than a "talking heads" interview. Hence a soundtrack with music consumes more bandwidth than one that uses just speech.
The more you increase the audio track's bandwidth, however, the more you have to decrease the visual track's bandwidth. If you start with a huge video source file, your RealVideo encoding tool may discard a lot of the source data to make the encoded RealVideo data fit a certain bandwidth. Although the RealVideo file will be playable, you may not like the results. Motion might appear too jerky, for example, or fast-moving images might not resolve visually.
The point here is that you always need to think about bandwidth as you create your streaming media content. Your bandwidth target greatly affects how you create content. If you know you'll have only a small bandwidth for video, for example, you can optimize the visual content to display in a small window at a slow frame rate. You may need to jettison panoramic and fast action shots that won't fare well under these constraints.
This manual provides tips for creating content with bandwidth targets in mind. With some practice, you will quickly learn how to balance bandwidth requirements with presentation quality.
As described above, you can create a RealAudio file consuming 8 Kbps of bandwidth that anyone with a 14.4 Kbps or higher connection can play. This file will have good quality sound, but the same source file encoded for 16 Kbps will have better sound. Encoded at 32 Kbps, the file will have even greater frequency response and dynamic range.
To provide good content for users with slower connections and great content for those with faster connections, you can use two methods, and even mix them depending on your needs. With the first method, you create a single file that targets different bandwidths. In the second method, you create separate files for each bandwidth target and let RealPlayer choose which set of files to play. Either way, you add to your Web page just one link for all visitors. You don't need separate links for modem and ISDN connections, for example.
With RealAudio and RealVideo, you can encode different versions of a source file for different bandwidths. For example, you can encode a music file in RealAudio for 28.8 Kbps modems, 56 Kbps modems, and 112 Kbps dual ISDN connections. In your Web page, you link to this single file. When a user clicks the link to play the file, RealPlayer communicates its available bandwidth to RealServer, which then chooses the encoding to use.
RealServer and RealPlayer can even adjust this choice to compensate for network conditions. If a fast connection becomes bogged down because of network traffic, RealServer seamlessly switches to a lower bandwidth encoding to prevent the presentation from stalling. When the network congestion clears, RealServer switches back to the higher bandwidth encoding.
If your presentation uses file types other than RealAudio or RealVideo, you can create multiple versions of the files for different bandwidths. When you assemble your final presentation, you use the SMIL file to designate a bandwidth connection for each of the different file groups. When a user clicks your Web page link, RealPlayer receives the SMIL file and chooses which group to play based on its own connection speed.
Because each connection speed uses a different set of files, RealServer cannot switch between the different encodings as it can with a single, multiply encoded file. RealServer employs other techniques, however, to compensate for network congestion. Its advanced stream thinning capabilities let it drop low priority data to temporarily decrease the presentation bandwidth. When the congestion clears up, it continues to stream all the presentation data.
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Additional Information |
For an overview of SMIL, see "Writing a SMIL File". "Setting Bandwidth Choices" explains how to use a SMIL file to designate different bandwidth groups. |
After you choose your media types and bandwidth target, you gather and edit your source material. The quality of your source and tools greatly affects the outcome of your streaming presentation. For example, a streaming soundtrack reflects the quality of the microphones used to capture the audio, as well as the sophistication of the hardware and software used to digitize and edit the sound file. Always use high quality source materials and good editing tools for your source files.
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Additional Information |
See Chapters 3 and 4 for tips on capturing high quality audio and video. |
Streaming multimedia presentations follow a timeline. In a two-minute video, for instance, each frame corresponds to a specific point in a two-minute timeline. The video's soundtrack is typically two minutes long as well. The first second of audio meshes with the first second of the video, and so on through both tracks' timelines. Video production software lets you coordinate the visual and audio tracks to a single timeline. You then create one file that contains these two internally coordinated tracks.
If you produce separate files through different software programs, however, you need to be aware of how timelines relate. Suppose you create a video file that has just a visual track. Through another software program you then produce a soundtrack. You need to note in this case how the clips' timelines relate. Should they start playing together? Or should one clip be delayed? When you assemble the presentation, you can use the SMIL file to specify playback times for each file.
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Additional Information |
For an overview of SMIL, see "Writing a SMIL File". |
Editing tools typically save data in a proprietary format, letting you export the data to a different format as needed. For example, most image editing programs let you export a graphic as a JPEG or GIF. If your editing program does not export the source file to the streaming format you want, you need an additional encoding tool to convert the format.
In addition, you typically need to compress the streaming file. File formats such as RealVideo are always compressed, so the conversion process involves compression. Other file formats, such as WAV, are uncompressed until you compress them. So with these file formats, compression involves another step after converting the source file to a streaming format.
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Additional Information |
The chapters on producing the various file types explain the compression you need to use for the each file type. |
RealNetworks encoding tools convert files to the RealAudio or RealVideo format. RealNetworks provides free tools for converting popular sound and video formats. It also sells advanced encoding tools that help you synchronize timelines, create HTML pages, and transfer pages to a server. In addition, plug-ins for popular programs such as Adobe Premiere and Microsoft PowerPoint let you save presentations directly as RealVideo. RealNetworks' encoding tools are easy to use, and let you quickly build the presentation you want.
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Additional Information |
Check http://www.real.com for the tool that's right for you. |
Media source files are often too large to stream, even at high bandwidths. Compression reduces the data in the file, making the file smaller. To illustrate how this works, imagine your computer screen as a rectangular grid of small squares, 640 squares wide and 480 squares high. When the computer paints the screen, it acts like a workman who puts a tile in each square to create a mosaic. If you are telling the workman how to fill in the mosaic, you could say:
Suppose that the entire top row should be red tiles. You can convey easily this information without all the repetition:
This drastically reduces the amount of information needed to get the job done. File compression works much the same way. Although the many kinds of compression technology are diverse in their approaches and complexities, they all use some means to cut down the amount of data stored in a file while still retaining good playback quality.
Keep the following in mind when converting formats or compressing files:
When you have converted your presentation files to their streaming formats, you can combine the streaming files into a single file, called a container file. Popular container formats include:
RealSystem can stream either of these container formats. However, using a container format is not necessary. Keeping your files and in their native streaming formats and putting the presentation together with a SMIL file as described below gives you greater flexibility.
With your files in their streaming formats, you put the presentation together with SMIL. Pronounced "smile," SMIL stands for Synchronized Multimedia Integration Language. A SMIL file is not necessary to stream just one file. But when you have multiple files, SMIL's simple mark-up language specifies how and when the files play. Here are some of the many advantages of using SMIL:
Because your files stay separate, you avoid the additional step of merging them into a container file. To change your presentation, you edit the SMIL file rather than merging the files again into a different container file.
Because the SMIL file lists a separate URL for each file, you can put together presentations using files in any locations. You can use a video file from one server, for example, and an audio file from another.
A SMIL file can list different language options for audio or text files. To create a video with sound tracks in different languages, for example, you produce one video file with no soundtrack, then create audio files in each language. Your Web page needs just one link to the SMIL file. When a visitor clicks that link, the visitor's RealPlayer chooses the soundtrack to receive based on its language preference.
The SMIL file can also list presentation choices for different bandwidths. RealPlayer then chooses which files to receive based on its available bandwidth. You can thereby support multiple connection speeds through a single hypertext link, rather than separate links for modem users, ISDN users, T1 users, and so on.
Because a SMIL file is a simple text file, you can generate it automatically for each visitor. You can therefore create different presentation parts, then assemble a customized SMIL file based on preferences recorded in the visitor's browser.
The SMIL file lets you easily control the presentation timeline. You can delay an audio track by 2.5 seconds, for example, without changing the encoded audio file.
When your presentation includes multiple elements, such as two videos playing simultaneously, you can use SMIL layout tags to align the videos.
For commercial presentations, the SMIL file lets you insert ads into the presentation as needed.
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Additional Information |
Chapter 6 explains the SMIL file syntax. Chapter 8 explains how to set up ad rotation. |
When your presentation is ready to go, you move the streaming media files and SMIL file to RealServer for testing and delivery. To make your presentation accessible, you simply create a link to the SMIL file in your Web page. When a user clicks that link, RealPlayer launches and plays the presentation in its own window.
You can also play the presentation directly in your Web page. To do this, you use RealPlayer's Netscape plug-in or ActiveX Control, adding mark-up to your Web page to specify how the presentation displays and what RealPlayer controls appear on the page.
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Additional Information |
See Chapter 9 for information on moving files to RealServer. For more on playing back a presentation in a Web page, see Chapter 7. |
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Note |
If you use an Internet Service Provider (ISP), make sure your provider has the latest version of RealServer available. |
With RealPlayer and the browser of your choice installed, you simply click the presentation link in your Web page. RealPlayer buffers presentation files for a few seconds, then begins to play the presentation back in its own window or your browser. Free RealPlayer downloads are available from RealNetworks at http://www.real.com.
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Additional Information |
For advice on testing, see "Testing your Presentation". |
RealPlayer can play virtually any streaming file because of its plug-in technology. RealPlayer plug-ins function like Web browser plug-ins. When RealPlayer receives a streaming RealVideo movie, for example, it uses its RealVideo plug-in to play the streaming data on your computer screen. If RealPlayer doesn't have a plug-in needed to play a certain streaming file, it downloads that plug-in from the Internet.
Plug-in downloading lets you confidently develop presentations using the latest streaming file types available for RealSystem. If visitors to your Web page don't have a plug-in needed to play your presentation, they can quickly download it and view your presentation. Because RealPlayer is the world's most popular application for viewing streaming media, you can be sure that your RealMedia presentation can reach the widest audience possible.
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Additional Information |
For more information about developing RealPlayer plug-ins or building RealPlayer capabilities into another application, visit http://www.real.com. |