Editing Pulse-width modulation (section)

===Audio effects and amplification===
Varying the duty cycle of a pulse waveform in a synthesis instrument creates useful timbral variations. Some synthesizers have a duty-cycle trimmer for their square-wave outputs, and that trimmer can be set by ear; the 50% point (true square wave) is distinctive because even-numbered harmonics essentially disappear at 50%. Pulse waves, usually 50%, 25%, and 12.5%, make up the [[Video game music|soundtracks of classic video games]]. The term PWM as used in sound (music) synthesis refers to the ratio between the high and low level being secondarily modulated with a [[low-frequency oscillator]]. This gives a sound effect similar to [[Chorus (audio effect)|chorus]] or slightly detuned oscillators played together. (In fact, PWM is equivalent to the sum of two [[sawtooth wave]]s with one of them inverted.)<ref>{{Cite web|url=https://www.soundonsound.com/techniques/synthesizing-strings-pwm-string-sounds|title=Synthesizing Strings: PWM & String Sounds|website=www.soundonsound.com}}</ref> 

[[Class-D amplifier]]s produce a PWM equivalent of a lower frequency input signal that can be sent to a [[loudspeaker]] via a suitable filter network to block the carrier and recover the original lower frequency signal. Since they switch power directly from the high supply rail and low supply rail, these amplifiers have efficiency above 90% and can be relatively compact and light, even for large power outputs. For a few decades, industrial and military PWM amplifiers have been in common use, often for driving [[servomotor]]s. Field-gradient coils in [[MRI]] machines are driven by relatively high-power PWM amplifiers.

Historically, a crude form of PWM has been used to play back [[PCM]] digital sound on the [[PC speaker]], which is driven by only two voltage levels, typically 0&nbsp;V and 5&nbsp;V. By carefully timing the duration of the pulses, and by relying on the speaker's physical filtering properties (limited frequency response, self-inductance, etc.) it was possible to obtain an approximate playback of mono PCM samples, although at a very low quality, and with greatly varying results between implementations. The [[Sega 32X]] uses PWM to play sample-based sound in its games.

In more recent times, the [[Direct Stream Digital]] sound encoding method was introduced, which uses a generalized form of pulse-width modulation called [[pulse-density modulation]], at a high enough sampling rate (typically in the order of MHz) to cover the whole [[Acoustics|acoustic]] frequencies range with sufficient fidelity. This method is used in the [[Super Audio CD|SACD]] format, and reproduction of the encoded audio signal is essentially similar to the method used in class-D amplifiers.