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Encoding and modulation of digital signals

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 into electrical signal shape. When the signal has to be sent, it has to be modulated. Pulse Code Modulation (PCM) Probably the simplest and the most common modulation method is pulse code modulation (PCM). In CPM, coding information is modulated as voltage levels. If the supply voltage is 5V, then binary”1″ is represented as +5V while 0 as 0V level. Only two voltage levels are used to modulate the signal. The 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. CPM method is OK, but it has several drawbacks like the number of wires required for parallel buses. Still, probably the most dangerous problem is that transmission may experience skew problems when each data wire may have different time delays…

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