I've been trying for years to find a resource with technical information about how MP3 and other internet audio distribution formats work. A book was published in 2000 called MP3: The Definitive Guide that was quite useful and informative, but it quickly went out of print and much of the information contained within it quickly became obsolete. Now, I have finally found some new resources! Check out these links:
...I would love to find technical information about the Apple formats, WMA, and RealAudio, but they seem to be industry secrets.
December 21, 2010
I originally wrote this post for a blog on another site. A reader commented with some great information. Thanks Jacob Cremer! Here's what he had to say:
Files from the iTunes store use AAC (Advanced Audio Coding), which is part of the mpeg-4 standard. AAC throws away data more intelligently than MP3s. I can still tell the difference between an AAC file and an uncompressed file. AAC audio is used in MP4 files. Apple uses the M4A extension, but the file is identical to any MP4. At this point the only disadvantage the MP4/AAC format has over MP3 is the scary name. Most software and players support it. DRM protected files from the iTunes store, however, are not compatible with anything other than iTunes or iPods.
High-Efficiency AAC is also available (free in iTunes), which has a mono channel with offset data for stereo. It can sound pretty godawful. I think you would have to mix audio in a way that specifically caters to the format in order for it to be good. Other than that, the file sizes are very impressive.
Mpeg-4 is a multimedia framework containing audio, video, 3-D, and whatever. It adds specifications as time goes by. I think of it as like a DVD where you can have menus, special features, and non-linear content. Another use would be for delivering surround sound content. MP4s contain some patented technologies. They aren’t a public domain free-for-all from NASA.
iTunes also offers ALAC (Apple Lossless Audio Codec), which is similar to FLAC. I’d say the difference is summed up in that FLAC is open source with low mainstream software support, whereas ALAC is strictly used with iTunes or Apple. While FLAC bids well to the tech savvy, anyone who uses Windows or Mac OS X can easily play an ALAC file. With a little added intelligence, an iTunes user can also transform their ALAC file into WAV, AIFF, AAC, or MP3. Out of jenkiness, iTunes may forever reject FLAC support. FLAC has a tough battle to fight and unless Amazon or Google decide to back it up and make file conversion simpler, I think the iPod will have the last say as to which format succeeds. ALAC is a monopolistic tool.
I predict the next industry scam is to sell 24-bit ALAC files via iTunes. This is the next logical and better successor to vinyl, cassettes, and CDs…
Windows Media and RealPlayer both have lossy and lossless compression formats. Windows Media DRM is a nightmare. Troubleshooting it can involve editing the Windows registry – where you can accidentally delete a line and the system won’t boot. I don’t see any advantages with either of these formats.
If you are selling your music online, I recommend bandcamp.com. They let customers pick which format to download and only hold 15% of sales (iTunes takes about 30%).
These days I’m thinking it’s unacceptable to not offer a lossless 24-bit download. If I had to pick a single lossy format, I would do AAC at 256kbps (This is the quaulity of iTunes Plus files from the iTunes store). Even Amazon 256kbps MP3 downloads aren’t desirable to me. MP3 trashes transients and discards stereo data above 16khz. The Hi Hats suck.