First time visitors to a broadcast television station are invariably amazed at the quality of the pictures they see on the station's TV monitors. The picture is incredibly sharp, crystal clear and free of noise, and other picture problems. However, televisions in peoples' homes show a picture with some noise and distortion, as a result of the degradation of the signal on its journey from the TV station to the home.

Radio, television and telephone systems at present use analogue signals to carry the picture and sound information. An analogue signal is a signal whose frequency is directly proportional to the data that it represents. The diagram below shows a typical analogue voice signal:

It is apparent that the signal is complex and in order to consider the individual characteristics of an analogue signal, it is best to examine a simple analogue signal, as shown below:

Basically, an analogue signal can vary any of the following three characteristics:

• Amplitude - This characteristic is proportional to the voltage and can have a positive, zero or negative value. It is represented by the height of the peaks in the diagram above and is a measurement of the strength of the signal.
• Frequency - Refers to the number of signals which pass a point in one second.
• Phase - A measure of how far the signal has travelled through its cycle. A signal which is a quarter of the way through its cycle is said to have travelled through 90 degrees.

As the telephone system was designed for analogue transmission it is well suited to the carriage of voice messages. On the other hand, data exiting the computer or a terminal is in digital form. A digital signal is a signal which can only consist of two values and is commonly known as a square wave. This can be seen in the diagram below:

It should be noted that there are only two states - one representing binary zero and the other representing binary one. When nothing is being transmitted over the line it is said to be in an idle state, yet often the idle state corresponds to the state representing binary zero. Hence there must be a method of distinguishing between these two - flow control.

Generally when there is a choice digital signals will be selected over analogue due to the fact that:

• When analogue signals are corrupted or distorted the original values cannot be reconstructed
• Digital signals can be reformed if corruption occurs
• Compression is able to be employed on digital signals

However, the problem still remains that when digital data is to be transmitted between computers this must be send over analogue telephone lines. Consequently, it follows that there must be a method of converting digital data to analogue form and vice versa - this is carried out by a MODEM.