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What's the difference between pulse-width modulation (PWM) and variable pulse width (VPW)?

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  • \$\begingroup\$ Where did you find the terms? (esp. VPW)? \$\endgroup\$ – jippie Sep 1 '15 at 16:18
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    \$\begingroup\$ What in the other questions concerning these terms and Wikipedia articles is missing to make this distinction? \$\endgroup\$ – PlasmaHH Sep 1 '15 at 16:26
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    \$\begingroup\$ I'd need to see the term in use, but my guess would be that VPW is PWM, but with a variable frequency. \$\endgroup\$ – Jarrod Christman Sep 1 '15 at 16:36
  • \$\begingroup\$ @JarrodChristman Pretty much exactly that. \$\endgroup\$ – Asmyldof Sep 1 '15 at 17:06
  • \$\begingroup\$ @jippie: They are two different modulations of SAE J1850 (OBD-II protocol). \$\endgroup\$ – xOneca Sep 4 '15 at 22:47
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The word 'width' in Pulse Width Modulation is a bit misleading. PWM actually encodes an analog value as the ratio of High or On time ('pulse width') to the total period of a rectangular waveform.

VPW is another name for Pulse Length or Pulse Duration Modulation (PDM). It encodes an analog value in the absolute width of the pulse. The period between pulses is unimportant.

PDM is used in RC servos, which typically respond to a pulse width varying from 1 to 2ms. The nominal repetition frequency is 50Hz (20ms between pulses) but most servos will work between 40~70Hz - and some go up to almost 500Hz (the theoretical maximum repetition frequency of a 2ms pulse).

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  • \$\begingroup\$ But Pulse Duration Modulation in Wikipedia redirects to PWM... \$\endgroup\$ – xOneca Sep 4 '15 at 23:10
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    \$\begingroup\$ According to the Alliance for Telecommunications Industry Solutions (ATIS) PWM is deprecated and replaced by PDM, but almost everybody still calls it PWM. J1850 VPW uses a form of PDM that alternates high and low pulses (end of high pulse is start of low pulse and vice versa). J1850 PWM encodes each bit in a pulse whose width is 1/3 or 3/2 of a fixed period (similar to 'regular' PWM). \$\endgroup\$ – Bruce Abbott Sep 5 '15 at 3:59
  • \$\begingroup\$ That comment explains very well what I wanted to know. But since I did a generic question, I accept your answer as-is. Thanks! \$\endgroup\$ – xOneca Sep 9 '15 at 13:37

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