Biphase coding

 

Why coding?

The data from the pseudo-random generator or from the data generator has the following form:

Zero logic for example is represented by a voltage level equal to zero volt, whereas the one logical is represented by a voltage level equal to plus five volts. Such a signal is called Non Return to Zero (NRZ).

One could imagine to directly present this NRZ signal to the modulation input of the transmitter. This would have the advantage to be extremely simple.

There are however two major disadvantages with this manner of making:
    - First of all the spectrum of the signal is not satisfactory,
    - in addition one would be unable at the reception end to recover the rate/rhythm of transmission (the original clock signal).

Indeed, the spectrum of signal NRZ 2Mbit/s has the following form:

It is observed that there is  DC and very low frequency components, related to the contents of the message, and that it is of course necessary to transmit under penalty of making the signal received not exploitable. However the ATV transmitters cannot transmit these components very well. In ATV, the DC component which transmits the luminance information is recovered with the clamp or alignment of the sync signals. However those do not exist in data transmission.

In addition, we need at the reception end to know with precision the rate/rhythm of transmission and to regenerate the clock signal emission. After a long succession of zeros, or ones, the rate/rhythm ,which can be known only by a sufficient number of transitions, is definitively lost.

We thus should carry out a complementary operation called coding.

 

Biphase coding

There are several manners of solving the difficulties we have seen above. A rustic manner, but nevertheless very effective is to use biphase coding.

The code is extremely simple: One shares the duration of the binary bit in two periods of equal durations. Symbol zero is coded zero-one, the one symbol is coded one-zero.

Construction of biphase coding:

The two quoted disadvantages of NRZ coding disappear:

- we obtain by construction exactly the same number of zeros and ones. There is not thus more DC component, or rather this one is constant and does not carry any more information.

- we are ensured to have at least a transition by transmitted binary bit. It is observed that a rectification of the received signal reveals a beautiful energy ray at the double of the rate/rhythm frequency. It is then easy to divide the frequency of this signal by two to retrieve the clock signal.

The operations of transformation of NRZ into biphase and vice versa are carried out very simply by combining the signal to be transformed with the clock signal in an exclusive-or gate.

The spectrum of the 2Mbit/s biphase signal  is perfect for a transmission in an ATV channel:

Of course, the simplicity and the effectiveness of the process are paid. In particular the necessary bandwidth is practically doubled compared to that of a NRZ signal. A 2 Mbit/s signal coded into biphase presents the essence of its energy in a band which is spread out of 0.5 MHz with 3 MHz. We have a channel of television able to transmit 6 or 7 MHz, this disadvantage thus does not have for us any importance.

 

B5+ et 73 de Jean-François Fourcadier, F4DAY

back to the welcome page

© 2000-2004  J.F. Fourcadier F4DAY