The chief drawback of the analogue transmission system I have been describing is that if the amplitude-modulated signal is distorted in any way during its passage through the fiber, and a certain amount of distortion is unavoidable, the distortion will be superimposed on the signal that is extracted and amplified at the receiver. One of the most effective means for transmitting an essentially distortion-free signal is to encode the signal into digital form before transmitting it. This is done by sampling the amplitude, or height, of the continuous signal wave electronically at regular intervals. If the wave is to be represented accurately, it must be sampled at twice the rate of its highest frequency component. Hence a voice signal with a maximum frequency of 4,000 hertz will be accurately represented if it is sampled 8,000 times per second. The individual sample measurements are coded into binary form, represented by a series of 1's and 0's. The binary numbers are now transmitted according to a prearranged code. For example, 1 can be transmitted as a pulse of light and 0 by the absence of a pulse. At the receiver the pulses are detected and used to reconstruct the original wave.
The most important advantage of digital transmission comes in dealing with weak signals. Every detector has an inherent internal noise that corrupts the signal entering the detector to a greater or lesser degree. Thus communication engineers commonly talk about signal-to-noise ratios. The ratios are measured on a logarithmic scale to the base 10 and in units of the decibel. A decibel is defined as 10 times the logarithm of the ratio of two power levels. For example, a signal-to-noise ratio of 20 decibels signifies that the signal level is 100 times higher than the noise level. Since digital pulses are either present or absent, they can be detected with a low probability of error even in the presence of significant noise. For example, with a signal-to-noise ratio of 21 decibels only one pulse in a billion will be lost in the background noise. For analogue signals, on the other hand, any noise tends to distort the message; hence if the signal is to be satisfactorily reproduced, the signal-tonoise ratio must be much higher than 21 decibels. Typically a signal-to-noise ratio of 60 decibels is needed, that is, a signal a million times greater than the noise.
The digital transmission system's greater tolerance of noise means that digital signals can be transmitted farther than analogue signals before amplification is needed. Another great advantage of digital transmission lies in the ease with which digital pulses can be detected and regenerated. Since minor distortions in the shape of the pulse are of little consequence, the weakened pulses can be detected and regenerated without imposing stringent requirements on the amplifiers.
It has become increasingly common in telephony to send voice signals over cable or microwave transmission systems by means of digital pulses. The voice signal is sampled 8,000 times per second, with eight binary digits specifying
the "height" of each sample. Since eight binary digits are capable of specifying 28, or 256, levels of amplitude, they provide an accurate specification of the wave pattern. This means that in order to reproduce the original voice wave, which has a frequency bandwidth of 4,000 hertz, the digital system must be able to transmit 64,000 pulses per second. The large bandwidth of lightwave systems makes it attractive to be somewhat generous in the use of bandwidth in return for a vastly improved signal-to-noise performance, which pays off handsomely in range, or the distance the signal can travel before it must be regenerated.
Thus we see that in a practical lightwave communication system the range depends on the power of the source, the attenuation per unit length of fiber, the noise level of the detector and the kind of modulation or coding that is employed. The capacity of the system in bandwidth, pulses per second or any other measure of information capacity depends on the speed with which the source can be turned on and off, the response speed of the detector and also on the pulse-spreading characteristics of the fiber.