Return-to-zero

When return- to-zero code ( short RZ code, English for return to zero), is a line code, with which it is possible to transmit binary numbers via a medium by the transmitter alternated between three level values ​​( transmit symbols, mostly as 1, 0 and -1 denotes ) can change.

Of the RZ code is a further development of the NRZ code ( non-return- to-zero ), wherein the two level values ​​are equivalent to the dual value of the transmitted digit. The disadvantage of the NRZ code is that there is no level change in the transmission of a long series of zeros or a series of ones. Thus, it is not possible for the receiver during this period, to recover the clock from the signal.

When you return RZ code when transmitting a logical 1 to the level 1 after the half clock to level 0, upon transfer to a logical 0, the level -1 is transmitted for half a period and then returned to level 0. Thus, there is the transmission of a guaranteed bit a level change that can take advantage of the receiver clock timing recovery ( synchronization). Disadvantage compared to the NRZ code is that twice as much bandwidth is needed.

Without additional measures, the RZ code, as opposed to the related Manchester encoding, non- direct component. This means for example that the RZ signal sequence for the purpose of galvanic isolation can not be transmitted via pulse transformers.

Unipolar RZ coding

A special form of the unipolar RZ encoding dar. The advantage is that only two level values ​​( 1, 0 ) are needed as an icon, and coding may be easily implemented using conventional digital circuitry. The disadvantage is that, for the transmission of a logic 0 - long sequence, which is coded with a constant level 0, no signal is being changed, and thus a part of the receiver synchronization is impossible.

A solution of the synchronization problem in unipolar RZ coding is an additional line coding of the transmitted data sequence, for example in the form of bit stuffing or the use of a scrambler. The unipolar RZ coding is generally not direct component.

Common applications for the unipolar RZ coding, with small modifications, such as the IrDA interface in the transmission mode SIR.

Bipolar RZ coding

Bipolar RZ coding is closely related to the unipolar RZ encoding, however, is used as the RZ encoding three levels: The state logic 0 is transmitted as in the unipolar RZ encoding still with level 0. The state of logic 1 is transferred alternately to the levels 1 and -1. The reason lies in the adjustment of the line coding to the physical characteristics of transmission media. The bipolar RZ coding may, by an additional line coding, are held direct component.

In addition, there are further modifications, such as the inverted RZ coding, where the assignment of logic 0 and logic 1 are reversed.

Contrast to the AMI code (Alternate Mark Inversion ): Unlike the AMI coding, which works similarly, returns in the RZ bipolar coding of the level after half a clock back to 0.