Which File Format (Codec) to Choose
Digital media is stored in files. There are various encoding methods or file formats, each associated with a different Codec (which stands for Compressor-Decompressor, i.e. the program which creates or reads the file). There are different codecs because different codecs are optimised for different uses, such as highest qality or highest compression.
For a very good guide to which format to choose check out this Guide to File Formats. If audio quality is your primary concern (and if you are reading this, it probably is!) then check out this How-To for Best Audio Quality.
Codecs noted as Lossy mean that some information is removed when the music is encoded to reduce file size. Lossless means that no information is lost when encoding.
FLAC (Free Lossless Audio Codec)
FLAC is a free open-source lossless audio codec:
+ Lossless + Freeware and OpenSource + Supported natively by SlimServer / SqueezeBox + Supports Vorbis-style tags + No DRM protection - Most hardware and some software players don't support FLAC - Must be transcoded (automatically) to play on SlimServer with older (SB1/SLIMP3) hardware
Download the FLAC codec here.
+ Freeware and OpenSource + Heavily tested + Compatible with almost all players + Supports Id3-style tags + No DRM protection + Transparent to most people at higher bit rates. - Lossy
AAC (Advanced Audio Coding)
AAC is a lossy codec that is sounds good at low bitrites (small file sizes) and is supported by Apple on its iPods and in its iTunes application:
+ Freeware + Supported by iPod and several other hardware players + Supports proprietary + Supports tagging - Lossy - May contain DRM protection - Must be transcoded (automatically) to play on SlimServer
ALAC (Apple Lossless Audio Codec)
Apple's lossless MPEG4 audio encoder is called ALAC and it outputs files with the extension .m4a.
+ Lossless + Freeware + Supported by iPod and some other physical players + Supports tagging - May contain DRM protection - Must be transcoded (automatically) to play on SlimServer
Apple's iTunes supports encoding to ALAC.
WMA (Windows Media Audio)
+ Both Lossy and Lossless (WMA Pro) support (as of version 9) + Freeware + Supported by many non-iPod hardware digital audio players + Supports tagging (though the tags are non-standard) - May contain DRM protection
+ Lossless + Supported by virtually all players and editors + Played natively by SlimServer - No tagging support (Wav64 does support tagging) - Uncompressed (thus large file sizes) - Must be transcoded (automatically) to play on SlimServer with older (SB1/SLIMP3) hardware
Other File Formats
There are at least a dozen other audio file formats for specific purposes. For more information of other common types check out Hydrogen Audio's Codec Forum.