Signals can be represented in digital or analog form, terms that refer to the methods used for encoding and storage. Analog refers to a process through which a physical trace of the signal is made. In the digital method, the signal is reduced to digits or numbers, which represent aspects of the signal or information. Digital encoding is infinitely reproducible: copies can be made without any loss of quality because the process entails nothing more than the transcription of a set of numbers. Analog duplications lose clarity—it is difficult to duplicate a physical object perfectly. An analog representation of a signal is made by allowing the signal to act directly on a physical substrate, such as light on photographic film. The quality of an analog image is determined by the sensitivity of the film and the quality of the optics of the camera—their ability to create a faithful duplicate. It was Thomas Edison (American inventor and businessman, 1847-1931), in the late nineteenth century, who developed the analog process used to cut tracks in a record album. He discovered that sound caused a diaphragm in a microphone to vibrate, which in turn moved a stylus that physically cut into the surface of a rotating wax cylinder. Sound unfolds through time, and with the phonograph, the time of the sound is mapped onto the space of the cylinder through its rotation. Through the stylus, sound acts physically and continuously on the surface of the album, creating a physical analog. Playback is the reverse process: a stylus is placed in the groove of a record and then vibrates according to its irregularities. The vibrations of the stylus are physically carried to a speaker, which produces sound. The analog recording is very deeply tied to its physical existence; when its medium changes (for example, from album to tape), the sound quality changes.
In a digital music file such as an MP3, sound is represented by binary numbers. The sound is first fed into a computer, where it is sampled and converted into numbers that denote various elements of the sound, such as frequency, timbre, and amplitude. In order to play a digital file, the computer or CD player must read the numbers, then interpret them and translate them back into analog signal to be carried to the speakers. The medium that contains the digital information is irrelevant—it could be a disc, a tape, or even paper. Both analog and digital methods must in some way create a souvenir of the event, a trace of the sound. Whereas analog record albums were stored in libraries and public or private collections, the process of digitization has moved music into the virtual network, accessible to everyone.
