RZ code
The return-to-zero code ( RZ code for short ; English for return to zero ) is a line code with which it is possible to transmit binary numbers via a medium by changing the status of the transmitter between three level values (transmission symbols , usually referred to as +1, 0 and −1) can change.
The RZ code is a further development of the NRZ code ( Non-Return-to-Zero ), in which the two level values are synonymous with the dual value of the digit to be transmitted. The disadvantage of the NRZ code is that there are no level changes when transmitting a longer series of zeros or a series of ones. As a result, it is not possible for the receiver to recover the clock from the signal during this period .
With the RZ code, when a logic 1 with level +1 is transmitted, you return to level 0 after half a clock pulse; when a logic 0 is transmitted, level −1 is transmitted for half a period and then returned to level 0. This guarantees a level change when a bit is transmitted, which the receiver can use for clock recovery (synchronization). The disadvantage compared to the NRZ code is that twice as large a bandwidth is required.
Without additional measures, the RZ code, in contrast to the related Manchester coding , is not free of equal components . This means that positive and negative signal parts do not equalize each other on average over time and that the RZ signal sequence cannot be transmitted via pulse transformers for the purpose of galvanic isolation .
Unipolar RZ coding
The unipolar RZ coding is a special form. The advantage is that only two level values (+1 and 0) are required as symbols and this coding can therefore be easily implemented with conventional digital circuits . The disadvantage is that when a long logical 0 sequence is transmitted , which is coded with a constant level 0, there is no signal change and synchronization on the part of the receiver is therefore impossible.
One solution to the synchronization problem with unipolar RZ coding is additional line coding of the data sequence to be transmitted, for example in the form of bit stuffing or the use of a scrambler . The unipolar RZ coding is basically not free of equal components.
The unipolar RZ coding is used, with small modifications, for example for the IrDA interface in the SIR transmission mode .
Bipolar RZ coding
The bipolar RZ coding is closely related to the unipolar RZ coding, however, like the RZ coding, it uses three levels: As with the unipolar RZ coding, the state logical 0 is always transmitted with level 0. The logical 1 state is transmitted alternately with the +1 and −1 levels. The reason lies in the adaptation of the line coding to the physical properties of the transmission media. The bipolar RZ coding can be kept free of equal components by an additional line coding.
There are also other modifications, such as the inverted RZ coding, where the assignment of logical 0 and logical 1 are swapped.
Difference to the AMI code (Alternate Mark Inversion): unlike the AMI coding, which works similarly, with the RZ bipolar coding the level returns to 0 after only half a clock.
literature
- John B. Anderson: Digital Transmission Engineering . 2nd Edition. Wiley Interscience, Lund, Sweden 2005, ISBN 0-471-69464-9 .