As computers became more powerful, they began to be used for more applications, which required more storage space for both programs and data. Hard disk capacity increased at about the same rate as processor power, but not fast enough to keep up with the demands of applications that created very large files, such as digital audio, digital video, and digital photography.
Lossless formats helped somewhat, but in the case of audio, lossless compression is able to reduce files to only about half their original size. For example, a Red Book Audio CD can hold 74 minutes of audio. If the audio were copied to a computer’s hard drive, it would take up 650 MB of space. Lossless compression might reduce the file to 325 MB, but that’s still quite large.
Lossy formats such as MP3 and AAC were developed to achieve much larger reductions in file size than are possible with lossless compression. By discarding unnecessary and redundant information, lossy formats can squeeze an audio file to about one tenth of its original size without losing much quality. With MP3, the 74 minutes of audio from the CD in the previous example would use only 60 MB of space. (See Chapter 10 for more information on how lossy audio formats work.)
DPCM (Differential Pulse Code Modulation) and ADPCM (Adaptive Differential Pulse Code Modulation) are simple forms of lossy compression based on PCM. These formats were used to save space in the days when the capacity of hard disks was measured in megabytes ...