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Making MP3s, Part 1

Although MP3 is turning into a great way to expose yourself to new music – like the new single "Icicle" from local Michigan band Troll for Trout, or Alan Parsons’ "Dr. Evil Trance Remix" of the title track from his new album – half the fun is in rolling your own. Happily, there are no fewer than five separate Macintosh applications available for creating your own MP3s. With one exception, all of them let you encode MP3 files directly from an audio CD – and they’ll do it faster than real-time with a reasonably speedy CD-ROM drive and processor.

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Making MP3 files of CDs you already own and playing them back on your own equipment is perfectly legal. Making MP3 files of music you’ve created and giving them away is also legal. But uploading and downloading "bootleg" MP3s (songs encoded from commercial albums without the artist’s or record label’s permission) is illegal. Remember, it’s up to you to keep your use of MP3 players and encoders on the light side of the Force.

We donned headphones and put together a four-minute AIFF audio file containing several different styles of music, and next week we’ll tell you how quickly our five contenders encoded MP3 files and how these files sounded. But first, a journey into the psychology of sound.

Why Encoders Matter — When you make a 128 kilobit per second (Kbps) MP3 file from an audio CD, the encoded file is less than 10 percent of the size of the original, which means that the encoder essentially discards over 90 percent of the original data. It’s been known for decades that our sense of hearing is as much between our ears as it is in them. By taking advantage of our knowledge of how humans perceive sound (the science of psychoacoustics), it is possible to extract the most important parts of an audio signal and encode them with high fidelity, using lower fidelity for less noticeable parts of the sound, or discarding such parts altogether. This is the basic principle behind MP3 and other lossy audio compression schemes, such as the QDesign Music Codec built into QuickTime.

One interesting fact about the MPEG standard (of which MP3 is only one small part) is that the specification says nothing at all about how an MPEG encoder should work – it only defines the format required by the decoder. This means that developers are free to innovate their own encoding schemes – as long as the resulting file has the right format, it can be decoded by any MP3 player. Competition, the theory goes, will drive developers of MP3 encoder software to develop better and better psychoacoustic simulations. Better encoders mean better-sounding MP3 files – and the best part is that you don’t need new playback software to enjoy the improvement, just a new version of the file.

So, counterintuitively, the software used to create an MP3 file can have as much or more effect on its sound quality than the software you use to listen to it. Although some MP3 playback programs have built-in equalizers and other enhancements to allow you to shape the sound to your liking, all software MP3 players sound pretty much the same with those features turned off.

The good news is that the encoders we tested produced listenable MP3s at bitrates of 128 Kbps and higher regardless of the style of music. Bitrate is just a fancy word for how many bits are required to encode a second of music. The more bits you use, the less audio information you have to throw away, and thus the better the resulting file sounds, all other things being equal. If the bitrate of an MP3 or QuickTime file is lower than the bitrate of your modem (generally 56 Kbps or lower), and the planets are aligned just right, you can actually play back the file as it downloads. Most stereo MP3s you’ll find on the Internet are encoded at 128 Kbps or higher, which means you’ll need ISDN or better to listen to them in real-time.

In naked ear tests, you’d be hard pressed to notice any differences between the files encoded by our selection of audio bit-crunchers. With headphones, some minor differences become apparent, although nothing earth-shattering was revealed until we conducted a torture test, encoding stereo files at bitrates of 64 Kbps and lower. At this point, a number of encoding inaccuracies (commonly referred to as "artifacts") became apparent as the encoders struggled to decide which parts of the sound were least important and thus disposable. It was obvious which had the best psychoacoustic models under the hood. Tune in next week to see how the different encoders fared in our tests, including AudioCatalyst, SoundJam MP, N2MP3, MVP, and the free MP3 Encoder.

[Jerry Kindall is the founder of Manual Labor, a technical writing and Web design firm specializing in the Macintosh. His music collection includes, at last count, over 900 CDs.]


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