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You Can't Be Too ThinThe skinny new audio format that will replace MP3s—and revolutionize Internet radio.

By Paul Boutin
Posted Friday, Jan. 21, 2005, at 7:44 PM ET

It's a given that fat broadband lines are the future of online media. But right now, for Internet radio, the future is about slimming down—creating skinny little streams of data that don't eat up too much bandwidth. The key is a new and better audio compression format called aacPlus, or sometimes HE-AAC, which has been chosen by the industry committee that standardized MP3 13 years ago (the Motion Picture Experts Group). If you've tried to listen to online stations, you know they sound grainy if they're streamed at any less than 128 kilobits per second—maybe 96 kbps if you're not fussy. That makes a broadband connection a must. But aacPlus sounds nearly as good as a CD, even when it's compressed enough to play through a dialup line. Don't take my word for it—see the results of the European Broadcasting Union's listener tests, in which aacPlus was deemed the "clear winner" at a dialup-friendly 48 kbps.

AacPlus has been around for a while—it's what XM satellite radio has used from the outset—but recently it's been gaining ground. Future digital music players will support the format just as surely as they do MP3, but you don't have to wait—you can listen to it right now. Install the free Winamp player, which added aacPlus support a few months ago. Then click through the channels on the Tuner2 Web site, which all stream aacPlus sound at 48 kbps or less. I've spent a week comparing them to the higher bandwidth stations served by the big three of Net radio—Yahoo, AOL, and MSN—and only Yahoo's 96 kbps premium subscription channels sound as good. For a tour, skip Tuner2's glut of techno stations and instead try Groove Salad for laid-back electro-lounge music, Radio Paradise for classic rock, and Sky.FM's Mostly Classical channel. (If you're on a Mac or other non-Windows computer, install the free VLC player instead of Winamp.)

It seems crazy until you try it, but Mostly Classical proves that aacPlus can sound great at 24 kbps. At 48 kbps, it's almost as crisp as a CD. At 128 kbps, it can deliver 5.1 channel surround sound. AacPlus works by combining three technologies, each of which shrinks the size of an audio signal. The first is AAC, the Advanced Audio Coding technique that Apple licensed from Dolby for iTunes. AAC analyzes the sound and throws away any data it knows human ears won't be able to hear, which is a lot. Then, aacPlus adds Spectral Band Replication, which strips out all of the music's high frequencies and replaces them with a tiny bit of analytical data. AacPlus players reconstruct the highs as a mathematical function of what's left. As a final space-saving trick, aacPlus tracks are recorded in parametric stereo. Instead of a left and a right channel, one channel is the sum of the left and right signals (L+R), and the other is their difference (L-R). This takes up less bandwidth, and the player can easily flip the two channels back to the original left and right. (Bonus trivia: This is how FM stereo broadcasts work.)

Why bother with all this trickery when cheap broadband is spreading everywhere? There are two good reasons. For station owners, lower bandwidth means lower costs. Webcasters spend most of their money paying for network traffic. A station that serves MP3 streams to a thousand listeners at a time can run up a $4,000 monthly bill. Downsizing from 128 kbps to 24 knocks that down to around $1,000. The result: More sites will be able to afford to serve more channels to more listeners, creating a sort of anti-Clear Channel effect. To get an idea just how much variety is already out there on Net radio, spend a few minutes randomly searching Shoutcast's 8,000-channel directory. You can hunt down obscure genres like Ghanaian hilife, listen to alternative newscasters on Radio Chomsky, and check what's playing on nine dozen '80s channels instead of one. Most of these stations broadcast in MP3 format. Imagine what'll happen when the cost of Webcasting goes down by a factor of two, three, or four as aacPlus players become the norm.

Just as important, low-bandwidth stations can reach us in places where we can't get a 128 kbps connection. Today that includes homes with dial-up lines, offices where co-workers share a DSL or T1 connection, and the not-so-speedy Wi-Fi link to my roof. But there's a huge audience looming on the horizon: cellular subscribers. Gadget makers plan to build aacPlus players into 3G phones, PDAs, laptops with cellular modems, maybe even car systems, which could one day tune in to cellular towers instead of satellites, and stream music that actually sounds good straight from the Net. Drivers get excited about 130 channels, but imagine how psyched they'll be with 13,000. Soon, I'll be able to listen to Groove Salad without being stuck at my desk.

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Paul Boutin is a writer living in San Francisco.
Photograph on Slate's home page © Royalty-Free/Corbis.
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