Digital Data, Analog Signals

The most common situation for this type of situation is the transmission of digital data through the telephone network, designed to receive, switch and transmit analog signals in the voice frequency range. Modems allow the conversion of digital data into analog signals and vice versa.

Coding or Modulation Techniques

There are three encoding or modulation techniques which are:

• Amplitude Displacement (ASK)
• Frequency Shift (FSK)
• Phase Shift (PSK)

ASK.

The two binary values ​​are represented by two different amplitudes of the carrier. The 'binary 1' is represented by the presence of the carrier at constant amplitude and the 'binary 0' is represented by the absence of the carrier.

It is a rather inefficient modulation technique, it is typically used at 1200 bps at most, it is used for the transmission of digital data in optical fibers.

FSK.

The two binary values ​​are represented by two different frequencies close to the carrier frequency.  FSK is less sensitive than ASK.  On telephone-quality lines, it is used at speeds of 1200 bps. It is also frequently used in radio transmissions at higher frequencies (from 3 to 30 Mhz), it can also be used in local area networks that use coaxial cable.

P.S.K.

The phase of the carrier signal is shifted to represent the digital data. The phase shift can be given by different techniques:

• DPSK:  A phase shift occurs each time a 'binary 1' is transmitted, otherwise the phase remains constant. It is carried out in two phases 0º and 180º, we can obtain up to 2 signals.
• QPSK: Instead of using a 180º phase shift it uses phase shifts corresponding to multiples of 90º (90º,180º,270º,360º). So each signal element represents 2 bits instead of 1.