pulse density modulation (PDM)
Pulse Density Modulation(PDM) is one of several pulse modulations. This type of modulation can be regarded as a combination of pulse width modulation and pulse position modulation( PPM), since the pulse width changes and also the relation of the pulses to the clock signal. In this method, the relative pulse density corresponds to the signal amplitude of the analog signal.
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In a PDM-modulated bit stream, the 1 corresponds to a positive analog level, the 0 to a negative one. The analog signal is obtained from the bit stream by limiting the bit stream in a low-pass filter. Pulse density modulation is used in Direct Stream Digital( DSD) in the Super Audio CD.