IAN LANG ELECTRONICS
Aspects of Optical Transmission.
In this section so far we have looked at methods and problems of optical transmission but so far neglected the system itself. We shall now redress the balance. Below is a high level diagram of a complete system:
In this part of the website we are concerned with the input signal, transmitter, cables and splice. Let us then study each in turn.
Beginning with the input signal, this may be either analogue or digital as a system like this can carry both. For audio it would most likely be analogue, for television it could be either and for data it would be digital. Digital signals have very fast rise and fall time pulses and the requirement is for very fast transfer. We can do this at high frequencies and achieve mega- or even giga- bit transfers. This can be achieved by transmitting binary numbers. Let us say that each piece of information is represented by a binary number consisting of eight digits. Going from right to left each digit represents a power of two, the zeroth power being on the right, and any number from 0 to 255 can be made from this; for instance decimal 112 is equal in binary to 01110000: the sense of this is shown below.
In binary wherever 1 is present then the bit is true, where 0 it is false. in this way we can transmit any number simply by having a standard eight bits. We can feed this to the transmitter and have a start bit, a stop bit and inbetween either a pulse or no pulse to indicate 0 or 1. This necessitates a precise timing. It is more complex but more accurate to have a low pulse for zero, a high for 1. In this way zero is actively rather than passively marked. This gives our transmitter modulation, and on the next page we break it down to constituent parts.