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Chapter 11: Broadcasting a Presentation

The Internet is swiftly becoming the next great broadcast medium. RealSystem lets you broadcast live or prerecorded presentations over the Internet or an intranet. This chapter provides background on using broadcast applications and RealServer to broadcast media. Refer to the documentation for RealServer and your tools for instructions on configuring a broadcast.

Tip
Real Broadcast Network™ provides full services for broadcasting small and large events. For details, see http://www.realnetworks.com/rbn/index.html.

Broadcasting vs. On-Demand Delivery

When a streaming presentation is delivered on demand, it starts from its beginning when the viewer clicks the presentation link in a Web page. Each viewer can receive the presentation at any time and use RealPlayer's controls to fast-forward or rewind through the presentation.

In a streaming broadcast, however, the user hosting the broadcast starts the presentation at a certain time. Viewers who click the presentation link join the broadcast in progress. Before the broadcast begins and after it ends, the presentation URL is not valid. During the broadcast, the RealPlayer fast-forward and rewind controls do not function.

To make an analogy, on-demand content is like a movie on videotape. The viewer can see it at any time, skip forward, rewind, and pause. A streaming broadcast, on the other hand, is like a movie shown on a television channel. As with a TV broadcast, there are two types of streaming media broadcasts:

The following illustration shows how RealServer delivers both live and prerecorded broadcasts.

Broadcasting Live or Prerecorded Content

Broadcasting Tools

To broadcast a presentation, you need the tools described in the following sections.

Source Capture Equipment

This equipment captures the broadcast content. It is typically a microphone or video camera connected to an audio or video capture card. For text, it could be a live text feed coming in over a network.

Editing Equipment

When you broadcast prerecorded content, you first write the source to a digitized file. You can then use editing software to optimize the file for broadcast. Live content, however, is sent directly from the capture equipment to the broadcast application.

Broadcast Application

A broadcast application takes the live source and encodes it in the appropriate streaming format, sending the output to RealServer. RealProducer, for example, can encode a video camera's RGB or YUV output as RealVideo in real time. A broadcast application typically runs on a separate computer that has a network connection to the RealServer computer. To broadcast prerecorded content, you typically do not need a broadcast application because RealServer can broadcast the clips itself.

RealSystem's open architecture also lets you build a broadcast application to send RealServer any type of data for broadcast. To build such an application, you need the RealSystem Software Development Kit (SDK), available at this Web address:

http://www.realnetworks.com/devzone/downlds/index.html

Sample RealText and RealPix broadcast applications are available in the RealSystem Authoring Kit, available at this Web page:

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

RealServer

RealServer streams the broadcast to RealPlayer. The RealServer administrator can give you the broadcast URL and parameters for connecting a broadcast application to RealServer. Because each RealServer has limits on its outgoing bandwidth and the number of streams it can produce, verify that your RealServer has capabilities appropriate for your anticipated audience size.

Additional Information
For information on configuring a broadcast, refer to the RealServer Administration Guide, available at http://service.real.com/help/library/servers.html.

Preparing a Broadcast

This section provides tips for preparing either a live or a prerecorded broadcast. See the manual or online help for your broadcast application for explicit instructions on how to encode and broadcast content. Note that the RealServer administrator sets up RealServer for the broadcast.

SureStream Broadcasts

Using SureStream technology, you can broadcast RealAudio or RealVideo at multiple bandwidths. Each viewer's RealPlayer selects an encoding appropriate for its connection speed. When you begin the broadcast, you specify in RealProducer that you want to use SureStream.

Multiple-Bandwidth Broadcast Through SureStream

To broadcast without using SureStream, you need to have a separate broadcast application running on a separate computer for each bandwidth connection. This is required because older codecs require more CPU power than do SureStream codecs. You then connect each broadcast application to RealServer and broadcast the different streams using different URLs.

Additional Information
For more on codecs, see "Understanding RealAudio" and "Understanding RealVideo".

Broadcast Computer CPU Power

RealProducer can broadcast a SureStream clip for several different bandwidths in real time using just a moderately powerful PC. Refer to the RealProducer manual or online help for guidelines on computer requirements for broadcasting.

Archiving Broadcasts

Using RealProducer, you can write a live broadcast to a file. RealServer can also archive the broadcast to a file. The latter option may be the better solution if your broadcast is long and your RealProducer computer has limited disk space. The RealServer administrator can set up RealServer to archive the live broadcast.

Broadcast URL and RealServer Parameters

The RealServer administrator can give you the parameters you need to connect a broadcast application to RealServer. The administrator will also provide the broadcast URL or URLs.

Trial Runs

When you broadcast live content, you don't get a second chance. It's good practice to perform a trial run to ensure that the equipment works properly and that the broadcast results are what you expect. Because you can't edit a live broadcast the way you can a prerecorded file, it's important to set your audio levels and plan your video shots carefully in advance.

Additional Information
For pointers on recording audio, see "Capturing Audio". For tips on capturing video, see "Recording Video".

During both the trial run and the live broadcast, view the broadcast output with RealPlayer. When RealPlayer connects, check that the buffering time for receiving a live stream does not exceed 10 seconds. Throughout the presentation, keep an eye on the broadcast quality. If you experience problems during your trial run, you may need to either reduce the number of SureStream streams or run RealProducer on a more powerful computer.

Advertising an Event

If you are broadcasting content that has wide public appeal, advertise your event on RealGuide (http://realguide.real.com/), RealNetworks' online guide to streaming media.

Additional Information
See "Advertising on RealGuide".

Using SMIL with a Broadcast

You can use SMIL to coordinate live streams or create a presentation that adds prerecorded content to a live broadcast. This section provides tips on using SMIL with broadcasts.

Additional Information
See Chapter 6 for more on SMIL. Chapter 9 explains how to include ads in broadcasts.

SMIL Limitations for Broadcasts

Within a SMIL file, you treat a broadcast like any other clip. For example, you can assign broadcast streams to SMIL regions and group a broadcast with on-demand clips in a <seq> or <par> group. However, you cannot use some SMIL features with broadcast streams:

Including On-Demand Clips with Broadcasts

Using SMIL, you can easily embed a broadcast in a multiclip presentation. SMIL can deliver an on-demand RealPix slideshow along with live RealAudio, for example, when both are in a <par> group. It cannot synchronize the on-demand clip with the live stream, however. This is because the on-demand clip's timeline starts when the viewer requests the presentation, whereas the broadcast stream's timeline starts when the broadcast begins.

To illustrate this, suppose that viewer A requests the presentation 2 minutes after the broadcast begins, and viewer B requests it 4 minutes after the broadcast begins. At 10 minutes into the broadcast, both viewers hear the same audio, but viewer A's RealPix clip is at its 8-minute mark, whereas viewer B's clip is at its 6-minute mark. Hence the relationship between the two timelines varies for each viewer.

Synchronizing Multiple Broadcast Streams

If you have multiple broadcast streams for a single presentation, you can synchronize the streams by using SMIL. This compensates for the fact that you typically cannot start two broadcast applications at exactly the same time. For example, you might start a live RealText encoding application a few seconds before a live RealVideo stream. When it receives the streams, RealPlayer delays playing the RealText stream until it can synchronize it with the RealVideo stream.

Tip
RealPlayer synchronizes broadcast streams to audio. For this reason, start your audio/video broadcast application last. This way, RealPlayer will have received and buffered the other streams by the time it receives the audio stream.

The broadcast streams are timestamped according to the broadcast computers' internal clocks. You should therefore run broadcast applications either on the same computer or on different computers that have synchronized clocks. To make RealPlayer synchronize streams, add a ?wallclock=name option to the broadcast source tag URLs in the SMIL file, as shown in the following example:


<par>
<video src="rtsp://realserver.example.com/encoder/video.rm?wallclock=sync"/>
<textstream src="rtsp://realserver.example.com/live/text.rt?wallclock=sync"/>
</par>

All wallclock attributes should use the same name value (sync in the example above), which can be any name you choose. You do not need the wallclock attribute for an on-demand clip or a single broadcast stream.

Warning
The wallclock attribute works with all RealNetworks clip types, including RealAudio, RealVideo, RealPix, and RealText. It may not function with other clip types broadcast by RealServer, however.


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This file last updated on 12/12/00 at 15:34:15.
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