With the pulse density modulation (PDM), a virtual analog signal given as a binary vector can be represented by a single binary signal. In contrast to pulse code modulation, where the different amplitudes are used to define the signal, here the density of the pulses is decisive and corresponds to the previous amplitude of the encoded signal. The result is a more or less wide rectangular pulse. Unlike PWM the basic frequency / period is not fixed but a result of the momentary signal,making the PDM significantly superior - especially regarding its use in the audio field. As an example the 1-bit audio technology uses this method.

Following a typical sine wave, the pulse density modulation results in a visibly high density of signals in the top amplitude area.

Delta Sigma Conversion

 The PDM bit stream is obtained from the incoming signal by using the delta-sigma modulation. A suitable method is used to superimpose noise or a dedicated signal on the incoming signal and round it, or a comparator is used to compare it with a reference signal to form a one-bit quantizer. Depending on the amplitude of the analog input signal and the target value, either a 1 or 0 is generated. These quantization errors result in noise, which can be optimized by the process. One aspect of this is the summation of the error and feed it back to the input - another is the use of several stages of such comparator and integrator units.

The error is regulated to 0 in the medium term. Depending on the structure of the modulator, it is thus possible to control the noise spectrum. The aim is to move the errors into a frequency range that can be filtered well.