The following is an edited perspective by John Lopinto, president, CEO and co-founder of Communications Specialties, Inc., Hauppauge, N.Y.
As in analog-based fiber systems, transmitters in digital systems take in analog or digital video, audio and data 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.
Think Digital StreamingIn a digital system, the incoming baseband signals immediately run through "analog to digital" converters within the transmitter. This converts the incoming signal or signals to a series of 1s and 0s, 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 emitting diode at a very high speed, corresponding to the 1s and 0s 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 run through digital to analog converters, and the receiver outputs video, audio and data in the same, analog or digital format in which the signals originated.
Digital processing offers many advantages over traditional analog AM and FM systems, ranging from improved performance and system flexibility to lower installation costs.
For example: In digital transmission, fidelity of the baseband video, audio and data signals remain constant throughout the system's entire available optical budget, no matter if transmitting one or multiple signals and no matter over short or long distances (up to the longest distance allowed by the system). This is an attribute that cannot be matched by analog-based systems.
Similarly, multiple channel analog, carrying four channels of video or audio, for example, may restrict bandwidth allocated to each signal to accommodate all the desired signals. Or, the multiplexer may add an additional optical carrier to support all of the channels, increasing system cost. By comparison, in a pure digital system, neither of these compromises are necessary.
Less Problems with DistortionDigital also addresses analog AM and FM signal distortion problems. In a digital system, there is no opportunity for distortion to occur in the transmission process or as a result of the method of modulation. The only signal distortion is a function of the conversion process from analog to digital and back again. Still, while no conversion is perfect, today's A/D and D/A converters can produce video and audio quality that far exceeds conventional systems.
There also are cost-and-design timesaving advantages of digital. When evaluating the cost of digital transmission systems, the design issues that most impact performance level are the quality of A/D and D/A conversion, and the speed at which the diode is able to operate. Better converters and faster diodes increase the cost of the transmission units.
In a digital system, all incoming signals- video, audio or data-are converted to the same basic units of 1s and 0s. Therefore, the system processes them all in the same manner, providing a high level of design flexibility. For instance, bits representing an audio signal can be substituted for the bits representing a data signal. And bits can be easily combined either from multiple same-type signals, such as 4 video channels, or from different-type signals, like combined audio and data. In conventional AM and FM systems, multiplexing of many video, audio and data signals becomes expensive and taxes the limit of the technology.
Repeating the Data StreamOne more advantage of digital fiber: its ability to regenerate a transmitted signal without incurring any additional degradation to the original analog or digital video, audio or data signal. A device called a repeater accomplishes this. By placing a repeater on the fiber at a point prior to where the light diminishes too much to be detected, the repeater regenerates and restores the digital signal to its original form. This regenerated signal is then launched again on the same wavelength, with the same optical power. It is important to note that the video, audio or data signal is never actually recovered in a repeater; only the data stream representing the original signal. So quality of the original signal is not degraded, regardless of how many times the signal is repeated and over how great a distance.
The advantages this offers to the system designer are obvious. Not only can tremendous distances be covered that far exceed the capability of any AM or FM system, but the designer can also be assured that the quality of the received signal will always be consistent and meet the performance requirements of his application.