Sound recording and reproduction is an
electrical or
mechanical inscription and re-creation of
sound waves, such as spoken
voice, singing,
instrumental music, or sound effects. The two main classes of sound recording technology are
analog recording and
digital recording. Acoustic analog recording is achieved by a small
microphone diaphragm that can detect changes in atmospheric pressure (
acoustic sound waves) and record them as a graphic representation of the sound waves on a medium such as a
phonograph (in which a stylus senses grooves on a record). In
magnetic tape recording, the sound waves vibrate the microphone diaphragm and are converted into a varying
electric current, which is then converted to a varying
magnetic field by an
electromagnet, which makes a representation of the sound as magnetized areas on a plastic tape with a magnetic coating on it. Analog sound reproduction is the reverse process, with a bigger
loudspeaker diaphragm causing changes to atmospheric pressure to form acoustic sound waves. Electronically generated sound waves may also be recorded directly from devices such as an
electric guitar pickup or a
synthesizer, without the use of acoustics in the recording process other than the need for musicians to hear how well they are playing during recording
sessions.
Digital recording and reproduction converts the analog sound signal picked up by the microphone to a digital form by a process of digitization, allowing it to be stored and transmitted by a wider variety of media. Digital recording stores audio as a series of binary numbers representing samples of the amplitude of the audio signal at equal time intervals, at a sample rate so fast that the human ear perceives the result as continuous sound. Digital recordings are considered higher quality than analog recordings not necessarily because they have higher fidelity (wider frequency response or dynamic range), but because the digital format can prevent much loss of quality found in analog recording due to noise and electromagnetic interference in playback, and mechanical deterioration or damage to the storage medium. A digital audio signal must be reconverted to analog form during playback before it is applied to a loudspeaker or earphones.
The automatic reproduction of music can be traced back as far as the 9th century, when the Banu Musa brothers invented "the earliest known mechanical musical instrument", in this case a hydropowered organ which played interchangeable cylinders automatically. According to Charles B. Fowler, this "cylinder with raised pins on the surface remained the basic device to produce and reproduce music mechanically until the second half of the nineteenth century."[1] The Banu Musa also invented an automatic flute player which appears to have been the first programmable machine.[2]
In the 14th century, Flanders introduced a mechanical bell-ringer controlled by a rotating cylinder. Similar designs appeared in barrel organs (15th century), musical clocks (1598), barrel pianos (1805), and musical boxes (1815). All of these machines could play stored music, but they could not play arbitrary sounds, could not record a live performance, and were limited by the physical size of the medium. The first device that could record sound mechanically (but could not play it back) was the phonautograph, developed in 1857 by Parisian inventor Édouard-Léon Scott de Martinville. The earliest known recordings of the human voice were phonautograms also made in 1857. These earliest known recordings include a dramatic reading in French of Shakespeare's Othello and music played on a guitar and trumpet. The recordings consist of groups of wavy lines scratched by a stylus onto fragile paper that was blackened by the soot from an oil lamp [3]. One of his phonautograms of Au Clair de la Lune, a French folk song, was digitally converted to sound in 2008. [3]. While this is an interesting playback that sounds like a girl singing, the creator of this recording, Patrick Feaster of Indiana University in Bloomington, reports that phonautograms his team had previously transcribed, using a laser as a virtual stylus, had been played back at twice the actual speed. What sounded like a girl singing the French folksong was actually Léon Scott singing, Feaster concluded in May, 2009. Since the above recording was recovered, the same team have since recovered a recording of a 435-Hz tuning fork (at that time the French standard concert pitch for A' — now 440 Hz). The tuning fork is barely audible.
