Digital music storage formats
There’s a bewildering range of formats in which music can be stored. Andrew Everard makes sense of them
CD is simple: it’s one format, playable in all players, so you can be sure any disc can be put in any player, and music will come out. Make the move to storing your music on a computer, hard drive or other memory device, or indeed stream it from an online service, and you enter a bewildering world of formats, codecs, bitrates and filename extensions.
A track is stored on a standard CD as a “.cda” (CD Audio) file, but becomes a “.wav” if you rip it unchanged to a Windows computer, or an “.aiff” on a machine running Apple OS. Same file, three file extensions.
Then there are the two kinds of what is commonly called audio compression, but is more correctly referred to as “data reduction” or “bitrate reduction”.
“Lossy compression”, as it is commonly known, works by throwing away some of the data used to record the music, based on well-known psychoacoustic principles. The lower the bitrate, the more gets thrown away.
CD Audio runs at about 1.4Mbps, so clearly to get down to even high-resolution MP3 files, at 320kbps, quite a lot needs to be ditched; when you get down to 192kbps or even 128kbps, you’re only using a small fraction of the data on the original disc.
Although MP3 has become a generic term for both data-reduced music and the hardware used to play it (to most people, an Apple iPod is an MP3 player), it’s far from the only codec available to produce “compressed” music files.
Windows Media Audio has its own versions, and there are systems such as Advanced Audio Coding (or AAC), which is based on MP3 but generally felt to give better sound quality for a given bitrate, and Ogg Vorbis, which has similar characteristics but mainly finds itself used by those with an unhealthy interest in experimenting with data formats.
Furthermore, within the compression choices, you have a range of options: for MP3, for example, Apple’s iTunes offers you the choice of three quality presets, or custom data-rates as low as 16kb/s or up to 320kb/s.
The final option on iTunes is “Apple Lossless”: like Microsoft’s Windows Media Audio Lossless and FLAC (Free Lossless Audio Codec), this operates in a different manner to the data-discarding “lossy” systems, and more like file-zipping programs on a computer.
These lossless codecs keep all the data of the original music track, but pack it into less space for storage, “unzipping” or extracting it when playback is required.
Lossless files are much larger than their lossy counterparts, but still only about 50-60% of the size of the original WAV or AIFF file, and of course when uncompressed, as decoders for these formats do as music is played, they should (provided the encoder has done its job properly), be identical to the original music file, both in terms of identical data and in their sound.
I’ll not get drawn into the audiophile arguments – we’d be here all night – but the two schools of thought are a) that lossless sounds better than MP3 and other lossy formats, but not as good as the original CD; and b) files stored using lossless codecs sound every bit as good as the original CD – of course they must, since the data is identical. I suggest you try them, and make your own decision!
Audio formats at a glace:
Apple Lossless is iTunes’ star turn, and very good it sounds too. Not all non-Apple network and portable players accept it, which is why software such as Elgato’s EyeConnect is available to transcode it on the fly, and iTunes has an option to save MP3 versions of files for use on mobile devices.
FLAC is the compression codec I use most, both when ripping CDs and when buying high-resolution music downloads. It’s free, it’s cross-platform, and it seems to work with almost anything.
320kbps MP3 is my baseline choice when storing music for non-critical listening – content I know I’ll only be using in the background, or on the iPod.
WMA/AAC streams are worth seeking out when listening to internet radio. Although MP3 may be the most popular streaming format, WMA and (in particular) AAC can offer better sound for the same bitrate. Have a poke around the website of those internet stations you like, and see whether they also offer higher-quality streams.