Sound recording and reproduction is the electrical storage and re-creation of sound waves. In a digital recording, sound signals picked up by a microphone are converted to a digital form by a process of digitization. The recording can then be stored in a variety of media, including CDs, DVDs and Blu-ray discs or on computers, portable media players and smartphones. A digital audio signal must be changed back to an electrical signal during playback before it is sent to loudspeakers or earphones. The signal causes a diaphragm inside each speaker to form sound waves.

First recording machine

The US inventor Thomas Edison (1847–1931) adapted the parts of a long-distance telephone he had invented to produce the first recording machine, which he called the phonograph. The first recorded sound he made on it was heard in 1878, when Edison recited the nursery rhyme “Mary had a little lamb”.

His phonograph was like a telephone, only with the vibrating parts connected to a steel needle. As Edison spoke into a horn, the needle "wrote" the pattern of vibrations on a piece of tinfoil wrapped round a drum, which was turned at the same time. When the needle was returned to the beginning of its written message and the drum turned again, the pattern cut in the foil made it vibrate in the same way. The telephone parts also vibrated and the sound came back out of the drum.

Alexander Graham Bell's Volta Laboratory made improvements in the 1880s, replacing the foil-wrapped drum with a wax-coated cardboard cylinder, and a needle that moved from side to side across it.

Gramophone records

Gramophone (a name which later replaced the phonograph) records or discs were the chief method for recording and replaying sound for most of the 20th century. Developed by US inventor Emile Berliner, who invented them in 1887, gramophone records had completely replaced the phonograph cylinder by the 1920s. The 1920s also saw the introduction of the electric gramophone in which the vibrating needle created an electrical signal, which was then amplified and turned into sound through a loudspeaker.

The very earliest records, manufactured from 1889 onwards, were made of hard rubber. In around 1895, a material called shellac was introduced, and became standard. By 1925, the speed of the record on the turntable became fixed at 78 rpm (revolutions per minute). A lighter material, a type of plastic called vinyl, replaced shellac by the end of the 1930s.
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The 33 1/3 rpm "long play" (LP) record first went on sale in 1948, with the smaller 45 rpm following in 1949. The slower speed and narrower grooves allowed for more music to be recorded on to both sides of the records. Both became standard until the introduction of CDs in the 1980s.

Tape recorder

Tape recording was developed in the 1940s. To record on to tape, the electrical signal from a microphone, representing the original sound waves, is sent to an electromagnet, which creates a pattern in the tiny magnetic particles that coat the tape. This pattern recreates the signal as the tape plays, and the signal is amplified before going to a loudspeaker.

Tape recording gave performing artists, record producers and the general public the power to record and re-record audio sounds with little loss in quality. Cassette recorders, for both recording and playing back music, became popular from the 1970s onwards. Sales of video cassette recorders (VCR), which made use of similar technology for recording and playing back TV programmes and films, boomed in the 1980s.

Digital recording

Most sound (and video) recording is now done digitally. A microphone turns the sound into an electrical signal, which is then digitized more than 44,000 times a second to create a long string of binary numbers. The binary numbers can be stored in computer memory or on a compact disc (CD), DVD or Blu-ray disc.

Compact disc

Both a compact disc (CD) and digital versatile disc (DVD) are plastic and aluminium discs measuring 12 centimetres (4.7 inches) in diameter. On a disc, the binary digits 0 and 1 are represented by flat areas (called lands) or shallow pits in the surface.

There may be about 3 billion pits in a single spiral track that starts at the centre and unwinds to the edge of the disc. A typical pit is one thousandth of a millimetre long.