Analogue and Digital Transmission

Analogue transmission

In analogue transmission, the signal is constantly varying in amplitude either above or below some central value, and can take any level between two limits which are equally spaced either side of the central value.

The information is carried in the instantaneous value of the signal, relative to the central value.

Any noise on the transmission channel adds on to the signal, and modifies it.

Any non-linearity in the transmission channel changes the shape of the signal, and produces a distortion.

If the signal exceeds the maximum or minimum values, the signal is clipped, and thus is distorted.

A signal like this would be found on an old fashioned telephone line, or on the cable between an amplifier and loudspeakers.

This is sometimes known as baseband modulation.

Modulation

Alternatively, the wanted signal can be used to modulate a carrier - three methods are commonly shown in text books, but only the first two are generally used.

• Amplitude modulation

• Frequency modulation

• Phase modulation

The transmission of this modulated carrier is another type of analogue transmission, and suffers the same problems as above.

However signals which are frequency modulated or phase modulated are less susceptible to noise, clipping, and amplitude distortion than either amplitude modulated signals or baseband modulated signals.

Digital transmission

In digital transmission, the signal alternates between two states, such as a high voltage and a low voltage.

In some forms of digital transmission, the information is carried in the number of times that the signal goes from the high voltage to the low voltage, and from the low voltage to the high voltage.

In other forms of digital transmission, the information is carried in the length of time the signal stays at either the high voltage or the low voltage, or both.

The simplest form of digital transmission is where a logical 1 is represented by a period of time at the high voltage, and a logical 0 is represented by the same period of time at the low voltage.

A signal like this would be found on a simple serial link connecting two computers beside each other.

Unfortunately, computer techno-buffs also use the expression baseband modulation to describe this type of digital transmission, even though it has got nothing to do with analogue transmission.

The advantage of digital transmission over analogue transmission is that, because the signal is only either a high voltage or a low voltage, quite high values of noise, clipping, or distortion can be ignored by the receiver, and the original signal can be correctly recovered.

Alternatively, just like analogue signals, digital signals can be used to modulate a carrier.

Although this process does increase the susceptibility to noise and distortion, it is still better than using analogue signals.

© 1998 Ron Turner

Return to the data transmission home page