Pulse-width modulation of a signal or power source involves the modulation of its duty cycle to either convey information over a communications channel or control the amount of power sent to a load.
In telecommunications, the width of the pulses correspond with specific data values encoded at one end and decoded at the other.
Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation).
_ _ _ _ _ _ _ _ | | | | | | | | | | | | | | | | Clock | | | | | | | | | | | | | | | | __| |____| |____| |____| |____| |____| |____| |____| |____
_ __ ____ ____ _ Data | | | | | | | | | | | | | | | | | | | | _________| |____| |___| |________| |_| |___________
Data 0 1 2 4 0 4 1 0
The inclusion of a clock signal is not necessary as the leading edge of the data signal can be used as the clock if a small offset is added to the data value in order to avoid the lack of a pulse for zero values.
PWM is also used to vary the total amount of power delivered to a load without the losses normally incurred when a power source drops its output voltage through resistive means. In a PWM system the solid state switch being used to control the flow of current is either not conducting any current or has very low voltage drop across it (and is conducting high curent); since the instantaneous power disappated in the switch is the product the current and the voltage at any given time, this means no power would be disappated if the switch were ideal. With a sufficiently high modulation rate, simple RC filters are often used to smooth the pulse train into a steady analog voltage. This method is commonly used in DC motor speed control.
PWM power limits are also used in the simple light dimmers common in homes. In this case the electricity being modulated is AC. Simple adjustment to the brightness of the light can be implemented by setting at what voltage in the AC cycle the dimmer begins to conduct electricty to the light bulb (using a triac). Because the duty cycle of the modulation is the same as the AC frequency of the line (60hz in the United States) the human eye sees only the average intensity (see flicker fusion).
(main article: switched-mode power supply)
PWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.
One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.
PWM is sometimes used in sound synthesis, in particular subtractive synthesis, as it gives a nice effect similar to chorus or slightly detuned oscillators played together. The ratio between the high and low level is typically modulated with a lfo.