previous next

Chapter 2: Planning a Streaming Media Presentation

A streaming presentation can consist of one or many clips. No matter how simple or complicated your presentation, you'll need to plan your media production so you can work effectively and reach your target audience. This chapter explains the basics of how to put streaming media presentations together. If you are not yet familiar with RealSystem components, see also Appendix A.

Step 1: Decide How to Deliver Clips

The first step in creating a streaming presentation is to consider the last step: how will you deliver your clips to other people? How you plan to stream your clips can greatly affect your media production.

RealServer Streaming

RealServer is the primary vehicle for streaming clips to RealPlayer. It delivers clips at many bandwidths, ensuring that streamed presentations stay synchronized. A RealServer administrator sets up and runs each RealServer. If you will not be running RealServer yourself, check the following with your RealServer administrator:

  1. What version of RealServer is available?

    To deliver clips described in this manual, you'll need RealServer G2, 7, or 8, rather than any of the earlier versions. Make sure that your RealServer can deliver all the clips you plan to develop.

  2. How many streams can RealServer serve?

    Each RealServer has a maximum number of media streams it can send out at once. A RealServer with a maximum of 500 streams, for example, can stream video to 500 viewers simultaneously. Make sure that the RealServer you plan to use has adequate capacity for your needs.

  3. Are there any bandwidth constraints?

    The RealServer computer may lack the outgoing bandwidth to deliver a lot of high-speed clips simultaneously. If you plan to develop high-bandwidth presentations, confer with the RealServer administrator about bandwidth limitations.

  4. Where will your clips reside?

    Your clips typically reside on RealServer, whereas your Web pages are on a Web server. You'll need to know the URLs for your clips on RealServer so that you can set up your Web page hyperlinks correctly.

  5. Do any RealServer features need to be set up?

    The RealServer administrator can set up many streaming and security features, such as:

Using RealServer through an Internet Service Provider

If an Internet service provider (ISP) hosts your Web pages, contact the ISP administrator to check out the RealServer issues described above. Also find out how much disk space you will have for streaming media. Many ISPs allot you a certain amount of disk space on their servers, such as 5 or 10 MB. Although this is a generous amount for Web pages, it's not much for streaming media. A single video clip can easily take up that much space.

Web Server Downloading

Although Web servers can deliver some streaming clips, they don't have RealServer's ability to synchronize clips and keep long presentations flowing smoothly. When only a Web server is available, you can still deliver multimedia presentations, but you will not be able to use all of the features that RealSystem offers.

Additional Information
If you plan to deliver clips with a Web server, read "Limitations on Web Server Playback".

Local Playback

You can also create presentations that play back from a user's local computer. An example of this is a multimedia-enhanced book, like this online manual, written with HTML and containing links to RealSystem clips. Users download the files to their computers, playing back the media clips with RealPlayer. In this case, you produce clips as described in this production guide, except that you don't target specific network connection bandwidths. In the HTML pages, URLs point to clips on the user's computer instead of on RealServer.

Additional Information
For more on local URLs in SMIL files, see "Linking to Local Clips". See also "Creating a Ram File Manually".

Step 2: Choose Clip Types and Gather Tools

RealSystem gives you many possibilities for creating streaming media. Your presentation may consist of a single clip, or several clips that play together. As you decide what types of clips you want to stream, gather the production tools you'll need to make the clips.

Tip
Visit http://www.realstore.com to find out about the many tools available to help you create streaming media.

Audio and Video

RealAudio and RealVideo are the most popular streaming media formats. To produce them, you run an encoding tool with audio or video input from a live source, a recording format such as tape or CD, or a digitized file in a standard format such as WAV, AVI, QuickTime, or MPEG.

Audio and Video Production Tools

A streaming RealAudio or RealVideo clip results from gathering, editing, and encoding audio or video input. To carry out the initial steps of gathering and editing content, you'll need the following:

Producing RealAudio and RealVideo does not require that you use specific microphones, cameras, capture cards, or editing tools. Just ensure that your editing tools can save files in formats you can easily convert to streaming formats with your encoding tool.

Use Your Favorite Editing Tools to Prepare Audio and Video Files

RealAudio and RealVideo Encoding Tools

Some editing programs can export digitized audio and video directly to RealAudio or RealVideo. If your editing program cannot export clips or you don't want to use this feature, you can use a RealNetworks tool to encode clips from files in standard formats such as WAV, AVI, QuickTime, and MPEG. RealProducer Basic is a free tool for encoding RealAudio and RealVideo clips. RealProducer Plus is an enhanced version that offers more encoding features.

RealProducer Creates Streaming Clips

Additional Information
For more information about RealProducer, see "Getting Production Tools". You can find tools at http://www.realnetworks.com/products/index.html.

SMIL

When you want to combine two or more clips into a single presentation, you use SMIL. Pronounced "smile," SMIL is a simple markup language that tells RealPlayer how to lay out and play your clips. You can use any word processor or text editor to write SMIL. For basic information about SMIL, see "Writing SMIL Files". For details, see Chapter 6.

Animation

With Macromedia Flash, you can build anything from streaming cartoons to e-commerce applications. To create a soundtrack, you can encode a RealAudio clip that streams along with the Flash clip. See Chapter 5 for details about producing Flash animation for RealPlayer. Learn more about Flash from Macromedia's Web site at:

http://www.macromedia.com/software/flash/

Images

Streaming multimedia presentations played with RealPlayer can include still images in the following formats:

Images in SMIL Presentations

To add images to streaming presentations as backgrounds or buttons, for example, simply incorporate the images by using SMIL. This way, you can specify exactly where images appear in relation to your clips. You can also use SMIL to turn images into hyperlinks.

RealSlideshow Presentations

When you want to create a streaming slideshow, the easiest solution is to use RealSlideshow™ or RealSlideshow Plus. These tools have drag-and-drop interfaces that let you quickly build your slideshow, which can include text captions, audio narrations, and background music. RealNetworks' partners will even host your streaming slideshow. Get RealSlideshow at:

http://www.realnetworks.com/products/index.html

RealPix Markup

Streaming slideshows are based on the RealPix markup language. Instead of using RealSlideshow, you can write your own markup to assemble images into a RealPix presentation that has eye-catching special effects such as dissolves and zooms. If you have RealPlayer installed and are viewing this document from your local computer rather than over the Internet, you can view a RealPix sample. Learn the markup from RealPix Authoring Guide:

http://service.real.com/help/library/encoders.html

Text

Unlike HTML, SMIL does not display text directly. To show text in RealPlayer, you can add text to any image, video, or animation clip. Or you can use RealText, which streams text at specific times within a presentation. RealText lets you subtitle videos, for example, or create hypertext links. If you have RealPlayer installed and are viewing this document from your local computer rather than over the Internet, you can see an example of RealText. Learn the RealText markup from RealText Authoring Guide:

http://service.real.com/help/library/encoders.html

RealPlayer's Update Features

RealPlayer's plug-in and autoupdate technologies ensure that your clips can reach the widest audience possible. RealPlayer plug-ins function like Web browser plug-ins. If RealPlayer doesn't have a plug-in needed to play a particular streaming clip, it downloads that plug-in from the Internet. RealPlayer can even use its autoupdate technology to upgrade itself to a new version when necessary.

RealPlayer Downloads Plug-ins it Needs from the Internet

Tip
RealPlayer's plug-in technology lets it play many types of clips developed by RealNetworks' partners. Check http://www.realnetworks.com/devzone for news about other types of streaming media.

Compatibility with Earlier Versions of RealPlayer

Plug-in and autoupdate technologies were introduced with RealPlayer G2. Earlier versions of RealPlayer cannot upgrade themselves, so they cannot play all the clips described in this production guide. Generally, you don't need to be concerned with backward compatibility because most RealPlayer users upgrade to the latest release. The following table summarizes which versions of RealPlayer offer which features. RealPlayer 4.0, for example, plays only RealAudio and RealVideo.

RealPlayer Features, from the Latest Version of RealPlayer to the Earliest
Feature 8 7 G2 5 4 3 2 1
RealAudio streaming X X X X X X X X
RealVideo streaming X X X X X
Flash 2.0 streaming X X X X
Flash 3.0 and 4.0 streaming X
RealPix streaming X X X
RealText streaming X X X
SMIL presentations X X X
Plug-ins for additional clip types X X X
Autoupdate X X X

This table covers only general clip compatibility, not codec compatibility. The latest version of RealProducer creates RealAudio and RealVideo clips with codecs that RealPlayer 4 and earlier versions cannot play back. RealProducer has a compatibility option for RealPlayer 5, making that the earliest version of RealPlayer you can reach with current production methods.

Protection of Copyrighted Content

Unlike a Web browser, RealPlayer does not store clips in a disk cache or allow users to copy or download still images. This helps you keep copyrighted material secure when you stream clips from RealServer, though not from a Web server. A RealProducer option lets you encode RealAudio and RealVideo clips so that viewers can record the clips on their computers.

Step 3: Develop a Bandwidth Strategy

Any computer connected to a network has a connection bandwidth, which is a maximum speed at which it can receive data. Web users with 28.8 Kbps modems, for example, can view only those presentations that stream less than 28.8 Kb of data per second. Presentations that stream more data than that per second may stall because the data cannot get over the modems fast enough to keep the clips flowing. These presentations will not cause problems for users with faster connections, though.

Successfully targeting your audience's connection bandwidth is crucial for developing streaming media. Viewers don't like to wait more than a few seconds for playback to begin after they click a link. And if your clips sputter because they use too much bandwidth, viewers are not likely to stay tuned. Developing a bandwidth strategy helps ensure that clips play back quickly and don't stall. You can also devise ways to deliver good clips to users with slow connections, and great clips to those with fast connections.

Presentation Data Must Fit RealPlayer's Bandwidth

Buffering

For each streaming clip, RealPlayer keeps a "buffer" that acts as a data reservoir. Data enters the buffer as it streams to RealPlayer, leaving the buffer as RealPlayer plays the clip. The buffer helps ensure that lapses of available bandwidth don't stall the presentation. If network congestion halts the flow of data for a few seconds, for example, RealPlayer keeps the clip playing with the buffered data. Your goal is to minimize initial buffering and eliminate rebuffering.

Initial Buffering (Preroll)

RealPlayer buffers a few seconds of data before a clip plays. Also called "preroll," initial buffering is required for every clip. Developing clips that use an appropriate amount of bandwidth keeps preroll to an acceptable level. You want preroll to be low—less than 15 seconds for each clip. RealAudio and RealVideo encoding tools set a low preroll for you. With other clips, though, how you create the clip determines its preroll.

Tip
By using SMIL, you can mask the preroll between clips. Once you're familiar with SMIL, refer to "Smoothing Transitions Between Clips".

Rebuffering

When clip data has stopped coming in and the clip buffer is empty, RealPlayer has to halt clip playback to store data again, or "rebuffer." Sometimes this is unavoidable because the viewer's available bandwidth drops for too long. When developing a multiclip presentation, though, you need to consider timelines carefully so that you don't inadvertently cause rebuffering, which can happen if too many clips fight for too little bandwidth.

Audience Bandwidth Targets

Your streaming presentations should never consume all of your audience's connection bandwidth. They must always leave bandwidth for network overhead, error correction, resending lost data, and so on. Otherwise, they may require frequent rebuffering. The following table recommends maximum streaming speeds for common network connections. To reach 28.8 Kbps modems, for example, a presentation should stream no more than 20 Kb of data per second.

Maximum Streaming Rates
Target Audience Maximum Streaming Rate
14.4 Kbps modem 10 Kbps
28.8 Kbps modem 20 Kbps
56 Kbps modem 34 Kbps
64 Kbps ISDN 45 Kbps
112 Kbps dual ISDN 80 Kbps
Corporate LAN 150 Kbps
256 Kbps DSL/cable modem 225 Kbps
384 Kbps DSL/cable modem 350 Kbps
512 Kbps DSL/cable modem 450 Kbps

For any other connection speed, calculate the maximum streaming speed as:

Multiclip Presentations

When several clips are played together, their streaming speeds added together should not exceed the connection maximum. For example, RealPix and RealAudio clips streaming at 12 and 8 Kbps, respectively, can play in parallel over 28.8 Kbps modems because together they stream at 20 Kbps. However, they cannot play back together if they stream at 12 and 16 Kbps, respectively, because the 28 Kbps total streaming speed leaves the modem no bandwidth for overhead. Such a presentation would likely require frequent rebuffering.

Streaming at Less than the Maximum Speed

Your presentations do not have to stream at the maximum speeds listed in the preceding table. In some cases, you may want your clips to stream at less than the maximum:

Clip Bandwidth Characteristics

To reach your target audiences with your clips, you need to understand your clips' bandwidth characteristics.

RealAudio and RealVideo

A RealAudio and RealVideo encoding tool can turn your source audio or video file into a clip that streams to any target connection with little preroll. But if the tool has to squeeze a file down too much to reach a low-bandwidth target, clip quality may degrade. So although the clip will stream well, you might not like the results. To ensure good-quality playback, keep your streaming bandwidths in mind when creating source files, especially when you plan to reach dial-up modem users.

Additional Information
See "Understanding RealAudio" and "Understanding RealVideo".

Flash

Macromedia Flash streams well at low bandwidths, making it an attractive alternative to video. Low streaming speed doesn't affect Flash's visual quality as it can with video. At low bandwidths, though, you may not be able to include as many items in your animated scenes as when streaming at high bandwidths. After you develop a Flash clip for RealPlayer, you tune it to stream at a specific bit rate. For more on this, see "Flash Bandwidth Characteristics".

RealText and SMIL

Because RealText and SMIL files are plain text, they use little bandwidth. You generally don't need to be concerned about how they affect a presentation's bandwidth consumption.

RealPix (Slideshows)

RealPix bandwidth use depends on the image sizes and how soon each image must appear in the clip's timeline. At higher bandwidths, you can use larger images and display them at shorter intervals. By varying image size and the RealPix timeline, you gain a lot of control over bandwidth use. RealSlideshow always ensures that images stream at a rate appropriate for your target audience. If you write RealPix markup by hand, though, you need to be careful not to overload a connection's bandwidth.

Images in SMIL Presentations

JPEG, GIF, or PNG images in a SMIL presentation stream at 12 Kbps. See "Defining Image Options" for instructions on changing this streaming bit rate.

Reaching Multiple Audiences

To provide good content for users with slower connections, and great content for those with faster connections, you can use two methods, combining them if needed:

Either way, you add to your Web page just one link for all visitors. You don't need separate links for modems and DSL connections, for example.

SureStream RealAudio and RealVideo

With RealSystem's SureStream technology, you can encode a RealAudio or RealVideo clip for multiple bandwidths. For example, you can encode a single RealAudio music clip for 28.8 Kbps modems, 56 Kbps modems, 112 Kbps dual ISDN, 256 Kbps DSL, and so on. The clip's playback quality improves with each faster speed. When a viewer clicks a link to a SureStream clip, RealPlayer and RealServer determine which stream to use based on the available bandwidth, as shown in the following illustration.

SureStream Clip Encoded for Multiple Bandwidths

RealServer and RealPlayer can even adjust this choice to compensate for network conditions. If a fast connection becomes bogged down because of high network traffic, RealServer switches to a lower-bandwidth stream to prevent the presentation from stalling. When the congestion clears, RealServer switches back to the higher-bandwidth stream. RealPlayer doesn't need to rebuffer data during this shifting.

Switching Bandwidths During Network Congestion

Switching Between Multiple Clips with SMIL

Only RealAudio and RealVideo clips can stream at multiple bandwidths. You can create multiple versions of other clips, though, each for a different bandwidth. RealPlayer then chooses which clip to play based on a SMIL bandwidth parameter. The following illustration shows a SMIL file that lists separate high-bandwidth and low-bandwidth RealPix clips. Each RealPlayer evaluates the SMIL file and chooses the RealPix clip appropriate for its connection speed. Both presentations use the same SureStream RealAudio clip, though, which has been encoded internally for multiple bandwidths.

Bandwidth Choices through a SureStream Clip and SMIL

When you use SMIL for bandwidth choices, RealServer cannot downshift to a lower-bandwidth clip group the way it can downshift to a slower SureStream stream. RealServer employs other techniques, though, to compensate for network congestion. Its stream thinning capabilities enable it to drop low-priority data to decrease the presentation bandwidth temporarily. When the network congestion clears, RealServer continues to stream all the presentation data.

Additional Information
"Setting Bandwidth Choices" explains how to use SMIL to designate different bandwidth groups.

Step 4: Organize the Presentation Timeline

Every streaming media clip has a timeline. A RealAudio clip may play for five minutes, for example, giving it a five-minute timeline. When clips are streamed together, you have a presentation timeline as well. Before producing clips, plan the presentation timeline. Among other things, the timeline can determine the order in which you produce clips. A well-conceived timeline also helps ensure that clips do not overload a connection's bandwidth and cause rebuffering.

Clip Timelines Coordinate with a Presentation Timeline

Timeline Considerations

When you assemble a streaming media presentation, you can manipulate various aspects of clip timelines.

Clips with Internal Timelines

Audio, video, and animation have internal timelines. In a 10-minute video, for instance, each frame corresponds to a specific point in a 10-minute timeline. Each second of audio meshes with each second of the image throughout the clip's overall timeline. Your video, audio, or animation software is your main tool for manipulating the clip's timeline, which is woven into the fabric of the clip.

Clips with Variable Timelines

With RealPix or RealText, you use the markup language to control when each image or text block appears and how long it lasts. When combining clips, it's typically easier to produce audio, video, or animation first. Then set the RealPix and RealText timelines to coordinate with those clips.

SMIL Timing Commands

A SMIL file can include its own timing elements. Timing a presentation with SMIL can be as simple as having one clip start as soon as another one stops. But you can also use commands to delay playback for 10 seconds, for example, or to have a clip start playing 30 seconds into its internal timeline. SMIL's timing commands are optional, but they give you the flexibility you may need for some presentations.

Additional Information
For more on SMIL timing, see "Specifying Timing".

Timelines for Multiclip Presentations

For presentations that include multiple clips, consider how to group clips without overloading an audience's connection bandwidth. The following illustration shows poor timeline planning. At various points, RealVideo and RealPix clips playing together exceed the connection's maximum streaming speed, which is represented below by the dashed line. Illustrated by the solid line, bandwidth use peaks again when the second RealVideo clip begins to play before the first video clip finishes. This presentation requires a high preroll for clips and would likely result in rebuffering at peak points.

Poor Bandwidth Use in a Multiclip Presentation

The next illustration shows better timeline planning and bandwidth management. The presentation starts with a low-bandwidth RealText clip that does not interfere with the streaming of the images in the RealPix clip. A RealVideo clip starts after the RealPix clip has streamed all of its images and does not need any more bandwidth. The second RealVideo clip starts after the first RealVideo clip has ended, so the two clips do not compete for bandwidth.

Improved Bandwidth Use in a Multiclip Presentation

Timeline Management

When developing a streaming presentation, keep the following in mind:

Step 5: Create Your Clips

When you've decided how you'll stream clips, chosen clip types and tools, developed a bandwidth strategy, and planned a timeline, you're ready to start creating streaming presentations. The rest of this guide provides production pointers, but does not explain how to use any specific tools. Be sure to have the documentation for your production tools handy as you develop your clips.


Copyright © 1998-2000 RealNetworks
For information on RealNetworks' technical support, click here.
Comments on this document? Click here.
This file last updated on 12/12/00 at 15:34:03.
previous next