For pulse density modulation if X is too big – say, if you have a minimum time increment of 1 microsecond, and your period is 256 microseconds, and X is greater than 128 – the line idles ...

Pulse Width Modulation (PWM) is a fundamental technique in power electronics that facilitates the efficient control of power delivery by modulating the width of voltage or current pulses.

PDM — pulse density modulation — is similar to PWM and, like PWM, requires a filter that could range from a simple RC network to an active filter. You can see the result in the video below.