In analog recording systems, a representation of the sound wave is stored directly in the recording medium. In digital recording what is stored is a description of the sound wave, expressed as a series of "binary" (two-state) numbers that are recorded as simple on-off signals. The methods used to encode a sound wave in numeric form and accurately reconstruct it in playback were developed during the 1950s and 1960s, notably in research at the Bell Telephone Laboratories.