Audio compression software uses two types of compression, lossless and lossy, to create smaller digital music files. Lossless compression retains the quality of the original audio recording. All of the 1s and 0s in the audio track are accounted for, and all of them are re-created perfectly when the compressed music is decoded. That means there is absolutely no quality loss compared to the original audio file, but lossless compression is extremely inefficient. It generally crunches files down to half their original size, meaning that with lossless compression, a 74-minute CD will consume about 325MB of space on your hard drive.
Lossy compression gives up some quality to achieve much smaller file sizes than lossless compression. Lossy formats use different algorithms to strip some of the frequencies and other data out of a song that don’t necessarily make any difference to our ears. A track compressed using a lossy algorithm may not have the dynamic range of the original track and may not sound quite as crisp, but it’s possible to create perfectly acceptable music using a bit rate as low as 128Kbps for most types of audio. Audio that doesn’t have a lot of dynamic range, such as audio books, can be recorded at bit rates as low as 64Kbps and remain virtually indistinguishable from the original audio CD. Using lossless compression, you could fit more than two hours worth of perfect audio on a single CD. Not bad, but using a bit rate of 128Kbps, you can squeeze more than 10 hours’ worth of music on a CD or 20 hours’ worth of audio books recorded at 64Kbps.
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