In a transmitting system, usually there are three kinds of coding in transmission, and three corresponding kinds of decoding in reception, as shown in the following picture:

The source is the information to transmit and it can be various.

If for instance we have a text, it’s necessary to digit it on a computer keyboard. This operation is called SOURCE CODING and consists of a conversion from an alphanumeric text into a sequence of byte constituted by 8 bits. This conversion is known as ASCII CODE.

More over there are other source codes like BAUDOT CODE or MORSE CODE. The former is a five-symbol code and was used with type printers. The latter is used with telegraph since the beginning of the century.

Channel Coding assures an error free transmission. We have already seen what kind of error transmitting channels can introduce in the signal.

This correction can be done by adding redundant bits, containing no information, but used to verify errors.

So we have CHECKING ERROR CODES and, to correct them, CORRECTING ERROR CODES.

This code is necessary to match signal and transmission line.

In numeric exchanges with digital devices and in computers, data use NRZ code. This code associates 1 and 0 logical values respectively to the high and low level.

In transmission lines such, as TELEPHONE DUPLEX CABLES, which arrives directly at user’s house, or COAXIAL CABLES or OPTICAL FIBRES, we cannot use this code. This is because the carrier cannot be regenerated and at the same time because it has a continuous component that does not let it cross DIFFERENTIAL TRANSLATORS, located in switching exchanges.

We can solve sometimes the problem of regenerating the carrier by using RZ CODE, just making a transition at a half period; so in fact we have the carrier’s spectrum inside the signal, but a continuous component is yet present.

When differential translators are present along transmission channel, as the continuous component cannot cross them, we have to modify the signal and transform it to avoid continuous components.

We can do it by using AMI CODE or HDB3 CODE 

WIDENING:

NRZ – RZ - AMI – HDB3 CODES