We know that digital signals are stored as binary numbers in memory cells. Let’s see how these digital signals can be transferred in other words – how they can be encoded in to electrical signal shape. When signal has to be sent it has to be modulated. Pulse Code Modulation (PCM) Probably simplest and most common modulation method is pulse code modulation (PCM). In CPM coding information is modulated as voltage levels. If supply voltage is 5V, then binary”1″ is represented as +5V while 0 as 0V level. Only two voltage levels are used to modulate signal. Signal can be sent in parallel (bus mode) or serial mode. In parallel mode whole (let’s say 8 bit) signal can be sent at once via 8 wires, while in serial mode data has to be sampled in time. This transmission method is most popular in embedded systems, for instance, ADC sends converted data to DSP using CPM.