What is the difference between pulse duration modulation and pulse density modulation?

Answer 1

Both reproduce analog waveforms by switching between 2 values at a much higher rate than the signal itself changes.

• Pulse duration modulation is a synonym for pulse-width modulation or PWM. The switching frequency is usually constant, and the width or duration of the "high" state determines the output. For instance, if the width is 100%, it is always high, and produces the highest analog value possible. If the width is 50%, it spends half of its time high and half low, producing a halfway voltage. If the width is 0%, it stays low all the time, producing the lowest possible voltage.
• Pulse density modulation uses a feedback loop to track the analog output voltage, switching low when the output is too high and high when the output is too low.

To produce a 50% voltage, with an equivalent resolution of 8-bits, a PWM waveform will turn on for 128 clock cycles and then off for 128 cycles. A PDM waveform with the same clock rate would alternate between on and off every other cycle. The average of both waveforms is 50%, but the PDM waveform switches more often. For 100% output or 0% output, they are identical.