pulse number modulation (PNM)
Pulse Number Modulation (PNM) is one of several pulse modulations.
In this type of modulation, the modulated information content is represented by a number of narrow pulses within a time slot. The time slots are triggered by the clock signal and have a fixed duration. A higher signal level can be represented by a higher number of pulses, a lower level by a lower number. The pulses always have the same spacing and amplitude.