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For the benefit of anyone laboring under the delusion that Apple's MPEG-4 AAC audio is "near CD quality", here's a quick comparison.
I re-encoded the iTunes and AAC files using QuickTime 6.3, after someone suggested that 6.1 and 6.2 had been full of bugs from having been rushed out of the door too quickly.
Indeed, QuickTime 6.3 does improve the AAC quality--enough that if you encoded anything with 6.1 and 6.2, I'd strongly suggest re-encoding with 6.3.
However, to my ears at least, LAME MP3 retains a slight edge over AAC at 160kbps, and is clearly superior at 128kbps.
With the launch of Apple's online music store and new iPods, a lot of people have bought into the hype that AAC at 128kbps is "near CD quality" and "better than MP3". I decided to try a few tests of my own instead.
First, I ripped the hit track "Emerge" by media darlings Fischerspooner from CD to AIFF using iTunes. From this, I took a preview-length slice from around 38 seconds into the track, so I hopefully wouldn't be accused of copyright violation. Remember, I'm doing this for the academic purpose of comparing the merits of different audio encoding algorithms, something which I believe falls under "fair use" copyright provisions.
I chose the Fischerspooner track for a couple of reasons. Firstly, it's reasonably representative of the kind of music I like to listen to--electronica, with stereo and phasing effects and lots of pure waveforms. Secondly, I had noticed before that it doesn't compress very easily--square and triangle waves require an infinite fourier series to represent exactly, so analog synth sounds (even if produced by digital software, as in this case) are hard to compress to MP3.
So, next I encoded to MP3 using both iTunes and LAME; with iTunes I used the best VBR setting, with LAME I tried both --alt-preset standard (which attempts to achieve CD quality) and --abr 128 (for a fair comparison with MPEG-4 AAC as sold on the Apple Music site).
After that, I encoded to MPEG-4 AAC 128kbps using the "enhanced" QuickTime 6.3 AAC encoder and iTunes.
Finally, I encoded to Ogg Vorbis using the Vorbis QuickTime component and an average bitrate of 128kbps, then again at quality 5. I picked quality 5 because my previous testing had indicated that that was about the lowest quality Ogg encoding I couldn't tell from CD audio.
Here's the original file, as a bzip2 compressed AIFF. (Use StuffIt Expander or bunzip2 at the command line to decompress):
Here are the various 128kbps files:
emerge-lame-128abr.mp3 - LAME MP3 [440KB]
emerge128kbps.ogg - Ogg Vorbis [412KB]
emerge.m4a - MPEG-4 AAC "high quality" [472KB]
Here are the 160kbps VBR files:
emerge-itunes-vbr.mp3 - The best iTunes can do in 160kbps VBR MP3 [643KB]
emerge-lame-160abr.mp3" - LAME 160kbps VBR
emergeq5.ogg - Ogg Vorbis -q5 [692KB]
emerge.mp4 - AAC at 160kbps [580KB]
And finally:
emerge-lame.mp3 - LAME's "CD quality" VBR MP3 [784KB]
I auditioned the sample files using Sennheiser HD445 headphones plugged straight into an iBook and a flat panel iMac. Testing two different Macs makes me pretty sure it's not a matter of faulty hardware, and note that I didn't use a "golden ears" high-end audio system--just a decent pair of headphones on a regular Mac.
The results are quite clear to my ears. For the 128kbps files, the LAME-encoded MP3 is the closest to the original, followed closely by the Ogg Vorbis file. The AAC file is truly awful.
I was rather surprised at this, as Vorbis is supposed to be better than MP3. I was also shocked by how bad the AAC file was.
Allowing 160kbps, Ogg Vorbis seems to overtake LAME MP3, in that it requires slightly fewer kbps to achieve a result that isn't glaringly different from the original CD. Both are better than the best iTunes can manage in MP3. AAC at 160kbps is slightly worse than LAME MP3 160kbps, but it's a close thing.
If you care about audio quality and listen to a lot of electronic music, avoid MPEG-4 AAC, particularly if you use bit rates below 160kbps. iTunes MP3 at 160kbps is better than AAC at 128kbps if you can afford the extra file size. If you really want CD quality, you'll need to use Ogg Vorbis or LAME MP3 encoding.
If encoding speed or iPod compatibility is important to you, use AAC--but don't go below 160kbps.
I am disappointed that yet again, Apple revved the iPod but didn't bother to implement Vorbis decode. I won't be buying one.