[Laser] PDM vs PWM

[n5gui at cox.net]

Recent comments by Clint Turner joggled a few brain cells.  It may just be me, but I make a distinction between Pulse Width Modulation (PWM) and Pulse Density Modulation (PDM).



Clint used the term PWM in describing digital music players and class D amplifiers.  I am no expert in this area, but I think that PDM better describes the technique used.  Let me begin by stating that PWM is a type of PDM.  Specifically PWM sends one pulse per sample period, and that pulse has duty cycle ( on time / off time ) that is proportional to the value that the sample represents.  The more general case, PDM, can have multiple pulses per sample period, the length of pulses can vary, but are usually multiples of a fixed clock.  

Suppose that you want to represent 16 discrete levels at a sample rate of 1000 samples per second.  You need a clock speed of 15,000 per second.  For the value of zero, you have only one way to send the data, no pulses are sent during that sample period.

    000000000000000   

Also for the value of 15, there is only one way, a single pulse 15 clock cycles long sent in that sample period.

    111111111111111

It gets more complicated if you want to send the value of 7.  For what I term PWM, you send a pulse for 7 clock cycles then wait 8 clock cycles for the sample period to end.

     111111100000000

For PDM any combination of 7 clock cycles ON and 8 clock cycles OFF satisfies the requirement.

      010101010101010       or      111000010000111      or        110011001100     or  ???

How and why you would choose one pattern over another is for someone else to explain.  From what I gather PDM is easer to build in hardware than PWM, particularly when you need more resolution than the example I have used, such as for music.

I am not sure that I see an immediate application for PDM in light communication, but let me suggest the possibility of a PDM modulator.  ( I think it goes by the name Sigma Delta modulator. )  It would require a clock, a comparator, a driver circuit, and an integration unit.  A source of audio is fed to on input on the comparator.  The other input is fed by the output of the integration unit.  When the clock cycle begins, if the audio input is higher than the integration input,  an ON signal is sent to the driver.  If the audio input is lower than the integration input, an OFF signal is sent to the driver.  When the driver receives any ON signal the output device will emit light and continue until the  driver receives an OFF signal.  The integration unit increases its output for ON clock periods and decreases for OFF clock periods.  The integrator could be as simple as a capacitor fed by a resistor from the comparator.  The audio input must be limited to less than the range of the output of the integration unit.  

When properly adjusted the output of the integration unit will follow the audio input value by a small lag time.  The circuits on the receiving end of the communication system should match very accurately the output of the integration unit since its low pass filtering should perform the same function, just at a distance from the transmitter.   

It is a different way to modulate light.

James
 n5gui