In a binary system for generating sound, digital signals representing an audio analog signal are stored in memory, a first voltage level is produced for a time t 1 proportional to the value of a respective stored digital signal, and a second voltage level is produced for a time t 2 =t c -t 1 , where t c is a fixed clock signal time interval. The first and second voltage levels are successively supplied to a speaker to operate the speaker in a type of frequency modulated mode to produce sound corresponding to the audio analog signal.

RealSound is a patented (US US5054086 A) technology for the PC created by Steve Witzel of Access Software during the late 1980s. RealSound enables 6-bit digitized PCM-audio playback on the PC speaker by means of PWM drive, allowing software control of the loud speaker's amplitude of displacement. The first video games to use it were World Class Leader Board and Echelon, both released in 1988. At the time of release, sound cards were very expensive and RealSound allowed players to hear life like sounds and speech with no additional sound hardware, just the standard PC speaker.