Creating MP3 FilesThe MP3 (short for MPEG-1 Layer III) compression protocol is the most popular Web-delivery format for music. The MPEG algorithm dramatically reduces audio-file size by using Fast Fourier Transforms to divide a track's frequency spectrum into discrete bands. The MP3 encoding process employs psychoacoustic principles to reduce file size with minimal effect on the frequencies heard by the human ear.
To create an MP3 file, you need an encoding program, often referred to as a ripper. A variety of shareware and commercial applications are available on the Web, but I usually use AudioCatalyst from Xing Technology (www.xingtech.com) and the PC version of WaveConvert from Waves (www.waves.com) with an MPEG-encoding plug-in installed. QDesign (www.qdesign.com) makes an especially efficient codec for professional use.
WaveConvert is the Swiss Army knife of audio-conversion utilities. AudioCatalyst is more specific, geared toward pulling songs off a CD and putting them on your hard drive in MP3 format. Both programs allow you to select from a range of output bit rates-the higher the rate, the better the audio quality (and the larger the file). The MPEG codec is notorious for smearing sharp transients, such as the notes from a snare drum. Consequently, a rock tune is best encoded at a higher bit rate. On the other hand, you could successfully use a lower bit rate on a recording of a choir.
The standard bit rate for Red Book (16-bit, 44.1 kHz stereo) audio is 128 Kbps, which results in about a 10:1 compression ratio. If you start with a mono file and limit the bandwidth a little with an EQ, you can use lower bit rates without generating audio artifacts (often heard as electronic bubbling noises). Voice-overs, with their small frequency range and lack of hard attacks, can usually be compressed at 32 Kbps or lower, resulting in an amazing 20:1 or better ratio. AudioCatalyst also features a "variable bit-rate" format that will, for example, encode the sounds of a car crash at a high rate and the ensuing silence at a lower rate in the same file. This efficient scheme results in a consistently better sound, but the format is not supported by all MPEG players.
An MP3 file can be posted on a Web site for download, but its behavior depends on the user's platform and plug-ins. On the PC, Microsoft's Internet Explorer automatically streams any MP3 file it encounters, as does the Apple QuickTime 4 plug-in. Setting up streaming on a Mac using Netscape's Navigator browser is a little more problematic. On my system (a Power Mac G3/300 MHz running OS 8.5 and using a T1 line for Internet access), I have to download an entire file and then play it using a "helper" application.
Some Web software, such as Beatnik and Macromedia Flash 4, have built-in MPEG-compression utilities. These keep the output files small and allow several low-bandwidth delivery methods, including streaming and interactivity. For more information, check out the MPEG Pointers and Resources page at www.mpeg.org.