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Decades ago, the idea of sending audio, video, voice and data through a glass fiber instead of a copper wire was the dream of every communications engineer. It was obvious to those who understood fiber optic technology at the time that fiber had many technical and potential economic advantages over copper wire for transmitting these signals. Although the advantages were obvious at the time, the technology was not equal to the requirements of the A/V market and the costs were prohibitive for most all applications.
Today, partially as a result of the telecom boom of the late 90s, fiber optic technology is now equal to the technical challenges demanded by modern A/V system design. Equally as important, it also can meet these technical requirements economically and be cost competitive with traditional copper based systems. This is especially true in complex system designs. Until now, A/V system designers and integrators usually thought of fiber optic communications as the technology of last alternative. That is, when copper transmission would no longer work due to excessive transmission distance only then would the designer think of fiber as a solution. But now fiber can, and should, be considered as the communications medium of choice from the start of the project regardless of the transmission distances involved.
The Very Basics of Fiber Communication
The early days of fiber optic communications were very simple. Thirty years ago the technology consisted of little more than just a composite video or audio signal that would vary the brightness of an LED emitter inside the transmitter unit. This first fiber optic transmission technology was called Amplitude Modulation or Intensity Modulation (AM or IM). It would produce an invisible light where the brightness would vary in direct proportion with the amplitude of the video or audio signal. The light from the LED would go directly into a fiber optic cable. At the other end of the fiber, inside the receiver unit, the light would shine on a photo diode commonly called a PIN, to convert the light back into an electrical current. This current would then be amplified to recover the signal level of the original video or audio signal that was sent into the transmitter input.
All this sounds very simple, and it was, but it didn't work very well except under ideal conditions. As the length of the fiber optic cable increases, the brightness of the light diminishes when it arrives at the PIN and the amplitude of the current at the output of the PIN is smaller. The fiber optic receiver has to provide more amplification to compensate for this loss of light. As a result, noise is added to the video and audio together with other undesirable distortions.
AM fiber technology is still in use today but it is used mostly for Security and Transportation systems where picture quality is not as important as it is in A/V. In almost all situations, the quality of the resulting video and audio signals when AM fiber technology is used is considered unacceptable for most A/V applications.
The Advantages of Digital Fiber Compared to Copper
As in AM fiber systems, transmitters in digital systems accept baseband video and audio signals and the receivers output these signals in their original format. The "digital difference" occurs in how the signals are processed and transmitted between transmitter and receiver. In a pure digital system, the incoming baseband signals are immediately passed through analog to digital converter circuits within the transmitter. This converts the incoming signal or signals to a series of 1's and 0's, called "digital streams." Then, if more than one signal has been processed, the transmitter combines all the resulting digital streams into a single digital stream. This combined stream is used to turn on and off the LED (or laser) emitter at a very high speed, corresponding to the 1's and 0's to be transmitted. At the receiving end, the process performed by the transmitter is reversed. The combined digital bit stream is separated into multiple bit streams, representing each of the unique, transmitted signals. These are then passed through digital to analog converters, and the receiver outputs video and audio in the same, analog format in which the signals originated. Figure (multiplex.jpg) illustrates this concept.
Unlike AM fiber systems and copper systems used today, pure digital transmission guarantees that the quality of the baseband video and audio signals will remain constant throughout the system. This is true whether you are transmitting one or multiple signals through the fiber, over short or long distances (up to the longest distance allowed by the system). By contrast, analog fiber systems using AM signaling techniques and copper transmission systems exhibit a linear degradation in signal quality over the entire transmission path. This characteristic limits the use of these systems to applications in which a relatively short transmission distance must be covered. Only systems that use pure digital transmission techniques can claim to have an absolutely uniform signal quality over the entire transmission path, from transmitter to receiver.
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