- How Digital Sampling Works
- Lossy Compressed Formats
- Lossless Compressed Formats
- Non-Compressed Formats
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Lossy Compressed Formats
Lossy compression works by sampling the original file and removing those ranges of sounds that the average listener can't hear. A lossless encoder uses complex algorithms to determine what sounds a human is able to hear, based on accepted psychoacoustic models, and chops off those sounds outside this range. You can control the sound quality and the size of the resulting file by selecting different sampling rates for the data. The less sampling going on, the smaller the file size—and the lower the sound quality.
The problem with shrinking files to this degree, of course, is that by making a smaller file, you've dramatically reduced the sampling rate of the music. This results in music that sounds compressed; it won't have the high-frequency response or the dynamic range (the difference between soft and loud passages) of the original recording. To many users, the sound of the compressed file will be acceptable, much like listening to an FM radio station. To other users, however, the compression presents an unacceptable alternative to high-fidelity reproduction.
The most popular lossy compressed format today is the MP3 format, although there are lots of other formats that work in the same fashion. Here's a list of available lossy formats:
- Advanced Audio Coding (AAC)—Also known as MPEG-4 AAC, this is the proprietary audio format used by Apple's iTunes and iPod. AAC offers slightly better sound quality than MP3 files along with strong digital rights management (DMA), to prevent unauthorized use. Unfortunately, most non-Apple music players won't play AAC-format songs—but if you're an iPod user, this is the format you'll be using.
- ATRAC3 (OMA, OMG)—OMG stands for Open Magic Gate, which is the digital rights management wrapper Sony uses for its proprietary ATRAC3 digital audio format. (For what it's worth, ATRAC3 stands for Adaptive TRansform Acoustic Coding 3.) The ATRAC3 and newer ATRAC3plus formats offer good sound and decent compression, along with strong digital rights management. Incompatible with everything except Sony products.
- Liquid Audio (LAT, LQT, LSL)—An MP3 competitor from Liquid Audio, somewhat popular in the late 1990s but not widely used today.
- MP3 (MP3)—Short for MPEG-1 Level 3, the MP3 format remains the most widely-used digital audio format today, with a decent compromise between small file size and sound quality. The primary advantage of MP3 is its universality; unlike most other file formats, just about every digital music player and player program can handle MP3-format music.
- mp3PRO (MP3)—This is an improved version of the original MP3 format, introduced in 2001 but not in wide use today. mp3PRO files use the same file extension as regular MP3 files.
- OGG Vorbis (OGG)—An open-source encoding technology originally known as "Squish," OGG Vorbis was designed as a substitute for MP3 and WMA. It uses variable bitrate compression, which encodes different parts of a song with higher or lower compression, to produce better quality when needed.
- QuickTime Audio (MOV)—Essentially the same MPEG-4 technology as the AAC format,
- RealAudio Media (RA, RM, RMA)—Proprietary format used by Real Networks, designed particularly for real-time streaming audio feeds.
- Windows Media Audio (WMA)—Microsoft's digital audio format is promoted as an MP3 alternative with similar audio quality at half the file size. That may be stretching it a bit, but WMA does typically offer a slightly better compromise between compression and quality than you find with MP3 files. It also provides strong digital rights management.