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Yesterday we covered Analog Codingshowing how we have analog in and analog out with a Class D amplifier and no magic decoding process is required as we have in PCM. The direct output of the PWM engine is a series of on and off again pulses that can be fed directly into a loudspeaker to make music. As a refresher, unlike digital audio, we have three states to a PWM output:
  1. On
  2. Off
  3. Time
Take a look at how this coding appears when we see a sine wave represented with PWM. The first tow states are obvios - you see the on's at the top and bottom and the off's as the gaps in between the ons. The time variable is how long each of the on pulses remains so. Notice anything? See how each of the pulses gets wider (time) when the sine wave approaches its top and bottom peaks and smallest at the middle line (called the zero crossing point)? This is a very simple arrangement that allows the output of the amplifier to rise when the pulse is on for a long time and fall when the pulse if very narrow. Think of it this way: if there's no pulse we get no rise in the output and our loudspeaker doesn't do anything - and if there is a long pulse we are adding energy to the loudspeaker and the cone of that speaker moves. Tomorrow we will see how this gets encoded and take it in little small steps. I figure that's easier for us both as I am sure we all have family in town and need to take audio learning in small bits - just like PWM!
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Paul McGowan

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