Signal and noise

Getting the message through: the consequences of different encodings

There are choices to be made when encoding a signal on a carrier wave. You can alter the vibrations of the source in two ways:

• By using the signal vibration to alter the frequency of the carrier vibration.
• By using the signal vibration to alter the amplitude of the carrier vibration.

There is a further choice to be made, in how the signal is encoded before the carrier wave is altered:

• The signal can be represented as many different levels (analogue).
• The signal can be represented as a series of ones and zeroes (digital).

That allows four possible combinations: AM or FM; digital or analogue.

The encoded signal then has to travel from the transmitter to the receiver, often through adverse conditions. Under these adverse conditions the encoded signal may be altered by being combined with other vibrations. So the amplitude or the frequency, or both, may be altered by noise being added to the signal.

The success in getting the information through is different for each of these four different options. That is, the robustness of the transmission as conditions get less favourable depends on the encoding. The extent to which the signal is still recoverable, or recognisable, as more and more noise is added, is one good measure of robustness. As general rules: digital is more robust than analogue, because you only have to judge between two different levels; frequency modulation is better than amplitude modulation because noise affects amplitude more readily than frequency.