Pulse-width modulation

Pulse-width modulation is a way to represent data over a communications channel. With pulse-width modulation, the value of a sample of data is represented by the length of a pulse.

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 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 resistive waste. An RC filter can be used to smooth the pulse train into a steady analog voltage. This is how the data on a CD or other digital media is converted into an analog audio signal.