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April 30, 2003

THE AAC FILE FORMAT: ANSWERS.

AAC is the file format used in the new online iTunes Music Store. The store is online now for Mac users and will be available to Windows users later this year.

Exactly what restrictions does it have on playback?

• You can play the same track on three different computers at the same time. You can change your mind about which three computers are “authorized” to play the song anytime you want.
• On a network, you can stream the song to (and play the song on) five different computers at the same time. None of the other computeers needs to be authorized in any way; it’s just a limit of five at a time.

Can you convert it to AIFF for use in a normal CD player?

Yes. iTunes already has the ability to burn AAC files to a CD in AIFF format suitable for playing on most consumer audio CDs.

Does it really sound better than MP3s? The only scientific study I’ve seen so far is here. They compared AAC, MP3, and other file formats. Their conclusion:

• The AAC codec is the only one in the tests which was evaluated in the range “Excellent” at 64 kbit/s, for all the audio items evaluated.[.....]

That’s not very useful in this case, since most MP3s and AACs are encoded at rates higher than 64 kbits/s.

However, personal assessments from various users appear today via Macintouch. Here are a few excerpts from these comments:

FYI, I’m an audio engineer with over 25 years experience recording and mixing all kinds of sound and music. I downloaded “New Frontier” by Donald Fagen, an impeccably recorded song, and auditioned it switching between the original CD, an MP3 ripped from that CD at 160 Kbs VBR, and Apple’s ACC MP4 file. I couldn’t hear a farthling’s worth of difference between any of them, and this is on a studio reference monitor system.

My Professional opinion: The sound quality is excellent, and a darn sight better than the crap found on Morpheus, KaZaa, et al. Plus no spyware and ad banners and bogus files.

Some songs encode well in AAC and some don’t. If you’re not obsessed with high fidelity—if MP3 sounds fine to you, if you’re satisfied with the sound from your FM radio—then the iTunes Music Store is all you need.

I doubt that the casual listener is going to notice a huge difference in sound quality, if they are used to listening to 128kbps MP3s. If they are sticklers for sound quality, and are used to encoding their MP3s at 160kbps or above, they may notice lower quality with AACs bought from Apple.

AAC is the audio codec of choice for XM satellite radio, and XM achieves very good sound quality (better than FM radio) at bitrates as low as 25 kbit/s. (No, that’s not a typo.)

Most people appear to agree that AAC is better than MP3. However, it appears that AAC, while excellent, may not yet be equivalent to the full AIFF file you get when you buy a CD.