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* STG's Review of RealAudio



RealAudio provides a clean, easy way of vending real-time audio over the Internet. This document provides a step-by-step, critical account of the installation, preparation and maintainance required for our being able to provide real-time audio presentations at the HT&T site.

Installation of the Real Audio Server

After obtaining binaries for the RealAudio server from Progressive Networks (, we performed trial installations on two STG servers: Swansong (a 50 MHZ dual-processor SparcStation 20 running Solaris 2.4); and Goon (a 200 MHZ Pentium Pro running RedHat Linux 4.0). We tried both versions 2 and 3 (beta 2) of the Real Audio server.

Installation was straightforward, involving little more than de-archiving the distribution files and modifying a few items in the configuration files. The Sparc 3b2 installation was especially clean and fast, and superior to the Linux 2.0 installation procedure, which required more intervention on the part of the installer. We have recommended that Progressive Networks restructure their Linux distribution, using RedHat Software's freely available RPM program installation/maintenance tool.

Running the Server

The RealAudio server was nearly as straightforward to run as it was to install. All we had to do was create scripts that started the server at boot-time: one for our Solaris server and another for our RedHat Linux server. We have recommended that Progressive Networks include auto-configuring startup scripts as part of their standard Unix distribution, and make them installable at the discretion of the system administrator.

Preparing the Real Audio File

The audio portions of the presentations at HT&T96 were captured on a DAT recorder, while the visual portions were captured on video tape. The audio was digitized using a DAT player connected directly to a Macintosh 8500 running SoundEdit 16 (the unedited digital file for a presentation of approximately 60 minutes was 318MB). The digital file was then edited to enhance clarity.

We reviewed the video tape in order to recover the URLs used during the presentation, and prepared the events file that allows Web pages to be synchronized with the Real Audio file.

The preparation of the Real Audio file was the least automated of the procedures described in this document. Software that would capture audio as well as time cues for the Web pages used during an on-line presentation, would be extremely useful in the production of Real Audio files.

Vending Real-Time Audio

Vending the Real Audio files was perhaps the least straightforward part of setting up the RealAudio server. Raw, digital audio files are extremely large, and require creative and severe compression in order to be viable candidates for "streaming audio" (i.e. audio played on the fly by users) on the Internet. Currently, the use of proprietary compression algorithms is what makes technologies like Real Audio possible; the "lossy" algorithms used in the Real Audio Encoder can compress raw audio files by a factor of thirty or more.

In our particular case, we used the RealAudio Encoder on a Macintosh 8500 to reduce the digitized audio from Kenneth Foote's one-hour presentation to as little as 3.5 MB (the size of 2.0-format files encoded for 14.4 kbps links). The file we actually have online (which is encoded for links of 28.8 kbps, or better, and requires version 3.0 of the Real Audio Player) is approximately 7 MB.

To allow people to access the Real Audio presentation over the Web, we had to prepare a .ram file (i.e., a brief Web-accessible file with a .ram filename extension), which contains one or more "pnm" URLs understood by RealAudio clients. In our case, the .ram file simply contains the one pnm-URL for Foote's presentation.

We also prepared an events file, which lists the Web pages to which Foote referred during his talk, along with their time offsets. Using the cevents utility provided along with the RealAudio server, we compiled the events file into an .rae file (this file allows the RealAudio server to tell RealAudio clients what URLs to display on the local browser and when, during the presentation, to display them).

As soon as the compiled Real Audio file (HTTkfreal.ram) is requested, a connection between the user's RealAudio client and the server is established, and the presentation begins.

One thing worth adding here is that although the RealAudio cevents utility offers a way to compile slideshows in which URLs display at specific time offsets in a given audio presentation, it offers no way of doing the reverse, namely of allowing users to go from URLs to specific points in the presentation where those URLs are discussed. In order to obtain this functionality, we found it necessary to write our own version of the cevents compiler.

Providing Instructions for Users

In order for users to hear RealAudio over the Internet, they must have a RealAudio client. Because many users don't have such a client, we decided to provide brief instructions on how to get and install a RealAudio client. We believe that everyone vending RealAudio files over the Internet should do something like this; we think it is a mistake to assume that users either have, or know where to get and how to install, a RealAudio client.

General Evaluation and Comments

In general, the RealAudio server is a fine piece of work. It installs well, runs well (we have had no crashes to date), and inter-operates well with existing Web servers and clients. In addition, it provides nice, clean cross-platform support (MacOS, Windows, Linux, and Unix), and offers very useful features, such as the slide-show capability mentioned above. The most useful feature, of course, is that RealAudio sound files start playing as soon as the link gets established between the user's client and the RealAudio server.

Finally, we note that the RealAudio server is based on proprietary (or on as yet still unformalized) specifications, and on the assumption that people are using slow, mainly modem-based links. Although RealAudio may see increased competition, in the next few years, from systems that utilize public-domain compression algorithms and protocols, as well as from technologies based on IP multicast (made possible by changes in the network infrastructure, e.g., MBone), slow transmission media are going to be with us for the foreseeable future, and thus it is likely that RealAudio, and streaming protocols like it, will remain a useful technology.

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The Forum on Hypermedia Teaching & Technology is sponsored by NetTech, the Education Alliance at Brown, and STG. E-mail technical questions, or co mments on this page to Richard Goerwitz at Richard_Goerwitz@Brown.EDU