That revolution is the portable MP3 player.  Digital sound files contain a lot of information and therefore are quite large.  Since MP3 technology is capable of compressing a sound file to about a tenth its original size, it makes storage and transfer of music files much more convenient.  Two major related innovations derived from that capability:  online music-sharing systems (e.g., Napster, Grokster, Kazaa, etc.) and portable MP3 players.  Instead of cassettes or CDs, MP3 players can use digital storage devices, such as flash memory chips (just like those used in digital cameras).  Thus, they can hold much more music in a smaller package without significant loss of sound quality (at least to most ears).

Apple's iPod goes one step further, using a tiny hard drive instead of flash memory.  The current top-of-the-line iPod has a capacity of 40 gigabytes, larger than most desktop computers sold just a couple of years ago.  Using a proprietary compression technology similar to MP3, that iPod (which fits easily into a shirt pocket - although it's a bit heavy to carry there) is capable of holding 10,000 songs, about a month's worth of uninterrupted listening!  The new iPod Mini (see picture above) in five cool colors is even smaller and lighter (though not that much cheaper!) and still holds 1,000 songs.

Perhaps the most significant aspect of the portable music player, from the Walkman to the iPod, has been its cultural impact, which has been studied by British media scholar, Dr. Michael Bull.  Bull's book, Sounding out the City: Personal Stereos and the Management of Everyday Life (see link below), is the definitive treatment of this subject.  The book was published in 2000, before the advent of the iPod, but his next book, coming this fall, will examine the iPod, together with cell phones and car stereo systems.

Pocket Calculator's Classic Walkman Museum - Arguably the best source of information about the Walkman on the Internet; includes sections on the history of the Walkman, a virtual museum with photos and detailed information on dozens of portable music players, a forum for collectors, and a database that seeks to catalog every portable stereo manufactured between 1979 and 1989.

Walkman entry on Wikipedia - short, but with useful cross-references to other Wikipedia entries.

Timeline of Sony Firsts - including the introduction of the first Walkman (the TPS-L2 personal stereo), "changing the way the American public listens to music," as the site immodestly - but accurately -states.

History of Sony's portable CD player - on the Sony site.

"From Walkman to IPod: Making the Leap to the Next Revolutionary Mobile Audio Product," by Tim Bajarin - on ABC News.com.  A brief account of the evolution of portable music players written shortly after the iPod was released (Dec. 14, 2001).

A Brief History of Portable Music - a rambling, tongue-in-cheek history from marching bands through boom boxes to mini-disks and mp3s on seethedonkey.com, a blog by Canadian Lee Zanello.

"Bull Session with Professor iPod," - by Leander Kahney on Wired News, Feb. 25, 2004.  An interview with Michael Bull, one of the few experts (perhaps the only expert) on the social impact of personal music systems.

Sounding Out the City: Personal Stereos and the Management of Everyday Life, by Michael Bull, on Amazon.com. The book on the social impact of personal music players.  Paperback, $23.95.

MP3 Overview - by Christopher Jones (July 27, 2000) on Webmonkey.  Everything you need to know about MP3s.

What is MP3, how does it work, what is MPEG? - technical but easily understandable answers to these questions and more on a site for Mac users.

Paper on MP3 - by Yuchin Chang.  An overview of this sound file compression technology, including its history, speculation on future development, and a useful set of links.

Apple's original iPod press release - on a site devoted to the history of recording technology.

Introducing iPod Mini - By far the coolest new portable music player, the iPod Mini comes in a range of colors, is considerably smaller than its big brother, and is still capable of holding 1,000 tunes.  Cool comes at a price, however:  despite its much smaller capacity, the mini costs nearly as much as the original iPod.

"How do you like them Apples?" - "The fortunes of the innovative computer company have been transformed by the success of its digital music player, the iPod," reports David Teather in The Guardian Unlimited (UK), January16, 2004. 

Inside the iPod - a comment on the origins of the iPod on Random Walks, a group blog established by Adam Rice.

"iPod's 'dirty secret' wins Web fans," - by Matthew Broersma on cNet News, November 26, 2003.  Short item about the iPod battery replacement controversy.

"For iPod, 6 Flavors of Flattery," by David Pogue, The New York Times (Circuits), February 12, 2004.  IPod's success has spawned a crop of wannabes from Samsung, Dell, Gateway, Rio, Creative Labs, and iRiver. Some of them look pretty good. (Note:  This page was free through the link above as of today (February 29, 2004), but it may end up behind The Times's pay-per-view firewall soon.)