Communications Concepts

What does a transmitted signal consist of?
There are two signals that make the signal transmitted. There is a radio signal of constant amplitude and frequency known as the carrier wave. This helps carry the information signal and is combined with it in a process called modulation.

There are two types of modulation; amplitude modulation (AM) and frequency modulation (FM).


The sound signal will be quite complex, representing a range of frequencies say of a person's voice. E.g. for telephone and radio communications the sound is transmitted in a band of audio frequencies ranging from 300 Hz to 4kHz. When this band is used to amplitude modulated the carrier, the resultant output is the carrier frequency and two sidebands which each carry the information.

The transmitted signal covers a range of frequencies that we say the transmitted signal has a bandwidth. It follows that:

bandwidth = 2 x maximum audio frequency

FM: Gives far better quality than AM because AM is very easily affected by noise and FM is not. In the FM the audio signal which carries the information that is used to modulate of the frequency The spectrum of an FM signal is more complicated than that of the AM signal.

Transmissions are organised into bands by international agreement, in this country regulated by OfCom and the Ministry of Defence. Click here to see the various bands.

Digital Communications
Signals can be coded so that only the intended person can receive them, a system called packet radio. The transmitted signal is converted to a series of very short pulses, compressed together into a short period of time, usually only milliseconds.

A digital signal is made up of combinations of 'on' (1) and 'off' (0) pulses. A pulse is made by turning 'on' the signal for a short time. Morse Code works by use of both long pulses ('dashes') which are three times as long as short pulses ('dots'). Combinations of these are strung together to make words and sentences that can be sent over very long distances.

Digital transmissions are less succumb to noise interference so can travel much further and still be detected. They take up less frequency space so more stations can use the same band.

A simple code that is often used is the binary code, a system based on just two digits 0 and 1 (as compared to decimal - 0 to 9). Each digit is called a bit (binary digit). 8 bits makes a byte. Digital signals are often made of "four-bit" words or multiples thereof i.e. 16, 32, 64..etc.

Analogue to digital conversion
The variation in voltage of the analogue signal is measured and sampled at regular intervals at a rate called it's 'sampling rate'. At this stage the signal has become pulse amplitude modulation (PAM)

The maximum analoguevoltage is then subdivided into a fixed number of levels in a process called quantisation into levels called quantisation levels.

For a digital signal, we describe it in terms of a number of bits per second i.e. it's bit rate. This is given by:
bit rate = sampling rate x number of bits per sample

The medium for which the signal travels will place a limit on the maximum frequency and on the maximum bit rate.


"PHYSICS", Dobson, Grace, Lovett - Collins Advanced Science.

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