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The Advantages of Transmitting

Signals Digitally Over Fiber

Making sense out of complex Pro A/V and Broadcast technologies.

Why Digital Fiber Optics?



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Guide

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Introduction

For the past quarter century, fiber optic communication has been hailed as

the superior method for transmitting video, audio, data and various analog

signals. Fiber offers many well-known advantages over twisted pair and

coaxial cable, including immunity to electrical interference and superior

bandwidth. For these and many other reasons, fiber optic transmission

systems have been increasingly integrated into a wide range of applications

across many industries.

However, while fiber optic transmission has dramatically improved upon the

standards obtained via conventional copper-based transmission mediums,

until recently, fiber optic technology has continued to use the same analog-

based signaling techniques as its predecessors.

Now, a new generation of products that employs pure digital signaling to

transmit analog information offers the opportunity to raise the standard once

again, bringing fiber optic transmission to a whole new level. Digital systems

offer superior performance, flexibility and reliability, and yet don’t cost any

more than the older analog designs they replace. This Education Guide

examines how digital signaling over fiber is accomplished and the resulting

benefits, both from a performance and economic perspective.

Analog Transmission Over Fiber

To appreciate the advantages of digital fiber, let’s first look at the basics

of how traditional fiber optic communications work when transmitting an

analog signal.

Analog fiber optic transmission systems are sold in both AM and FM versions.

In both types of systems, the optical transmitter takes in an analog, baseband

video, audio or data signal and converts it to an optical signal. At this point,

the systems begin to vary.

In an AM (amplitude modulation) system, the optical signal is generated as a

beam of light that varies in intensity with respect to variations in the original,

incoming, electrical analog signal. Either a light emitting diode (LED) or a laser

diode serves as the source of the optical signal. Unfortunately, both LEDs and

laser diodes are nonlinear devices. This means that it is difficult to control the

brightness of their light in a controlled linear continuum, from completely off

to completely on with all variations in between.

An Introduction to Fiber Optics





However, in an AM system, this is exactly how they are used. As a result,

various distortions to the transmitted signal occur, such as:

•

•

•

Degradation in the signal-to-noise ratio, or SNR, as the

length of the fiber optic cable is increased

Nonlinear differential gain and phase errors of video signals

Poor dynamic range of audio signals

Light Passing Through Two Fibers

In AM modulation, variations in the baseband signal are represented by

varying the intensity of the transmitted light.

In FM modulation, variations in the baseband signal are represented by varying the speed, or

frequency, at which light pulses occur.

In an effort to improve upon the performance of AM-based fiber optic trans-

mission systems, FM design techniques were introduced. In these systems,

the signal is conveyed by pulsing the LED or laser diode completely on

and off, with the speed and duration of pulsing varying with respect to the

original incoming signal.

For those familiar with FM transmission over mediums other than fiber,

the term “FM” for this type of system may be a bit confusing. FM stands for

“frequency modulation” and might be interpreted, within the context of fiber

optics, to mean that the signal is transmitted by modulating the frequency

of the light itself. This is not the case. A more accurate name for this type

of system is PPM, which stands Pulse Position Modulation. However, across

industries, “FM” – and not “PPM”– is the name associated with these types of

systems. Just remember that the “F” in “frequency” refers to the frequency of

the pulses instead of to the frequency at which the light itself travels.

While FM transmission systems eliminate many of the problems found in AM

systems, which result from difficulties in controlling the varying brightness

level of light emanating from the diode, FM systems offer their own unique

set of problems. One distortion common in FM systems is called crosstalk.

This occurs when multiple FM carriers are transmitted over a single optical

fiber, such as when using a multiplexer. Crosstalk originates within either the

transmitter or receiver unit and is the result of a drift in alignment of critical



An Introduction to Fiber Optics



filtering circuits designed to keep each carrier separate. When the filters do

not perform properly, one FM carrier may interfere with and distort another

FM carrier. Fiber optic engineers can design FM systems that minimize the

likelihood of crosstalk occurring, but any improvement in design also means

an increase in price.

Another type of distortion is called intermodulation. Like crosstalk, this

problem occurs in systems designed to transmit multiple signals over a single

fiber. Intermodulation originates in the transmitter unit and is most often the

result of a non-linearity present in a circuit common to the FM carriers. The

result is that the two (or more) original incoming signals interfere with each

other before being combined into a single optical signal, causing reduced

fidelity in the transmitted optical signal.

How Digital is Different

As in analog-based fiber systems, transmitters in digital systems take in

baseband 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.

In a pure digital system, the incoming baseband signals are immediately run

through “analog to digital” converters 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 emitting diode 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 run

through digital to analog converters, and the receiver outputs video, audio

and data in the same, analog format in which the signals originated.

Digital Transmission System for Analog Baseband Signals

Video

Video

Data



Audio

Baseband

Analog

to Digital

Converter

Digital

to Analog

Converter

Digital Combiner

Analog

to Digital

Converter

Analog

to Digital

Converter

Digital Separator

Baseband

Audio

Light Pulses On and Off

Corresponding to

1’s and 0’s

10101010101010

Optical

Transmitter Fiber Optic Cable

Optical

Receiver

Digital

to Analog

Converter

Digital

to Analog

Converter

Data

An Introduction to Fiber Optics



Pure digital processing offers many advantages over traditional AM and FM

systems, ranging from improved performance and system flexibility to lower

installation costs. Let’s examine some of these in detail and discuss their

direct benefits both to the system installer and to the user of the system.

Performance Consistency

Unlike in AM and FM systems, pure digital transmission guarantees that the

fidelity of the baseband video, audio and data signals will remain constant

throughout the system’s entire available optical budget. 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 systems using AM signaling techniques exhibit a linear

degradation in signal quality over the entire transmission path. This charac-

teristic, combined with the fact that AM systems can only transmit over

multimode fiber, limits the use of these systems to applications in which a

relatively short transmission distance must be covered. FM systems fare a

bit better, with signal quality remaining relatively constant over short

transmission distances, but decreasing dramatically at longer distances. And

even at short distances, there is some signal degradation. 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.

Consistent signal quality, or lack thereof, is also an issue when considering

systems designed to transmit more than one signal over a single fiber (multi-

plexers). For example, a multichannel analog system designed to carry four

channels of video or audio might restrict the bandwidth allocated to each

signal in order to accommodate all the desired signals to be transmitted.

Systems using digital transmission need not make this compromise. Whether

sending one, four, or even ten signals over a single fiber, the fidelity of each

signal remains the same.

Comparison of Signal Degradation in AM, FM and Digital Systems

System’s Maximum

Transmission Distance

System’s Maximum

Transmission Distance

System’s Maximum

Transmission Distance

Signal-to-Noise Ratio

1

30



60

7

7

60



30

1

Transmission Distance

Transmission Distance

1

Transmission Distance



An Introduction to Fiber Optics

30



60

7

AM System

FM System

Digital System



Transmits Higher Quality Baseband Signals Relative to Price

Due to the inherent design of digital transmission systems, baseband analog

signals maintain a higher fidelity when transmitted using d igital processing

techniques. This is because the only signal distortion that occurs in a digital

system is that resulting from the analog to digital conversion and back again.

While no conversion is perfect, the state of today’s A/D and D/A converter

technology is so advanced that even the lowest cost converters can produce

video and audio quality that far exceeds that attainable by conventional AM

and FM based systems. Evidence of this can be seen by comparing signal-to-

noise ratios and measurements of nonlinear distortions (differential phase

and differential gain) between digital and analog products. Comparing

systems designed to transmit similar signal types over the same fiber and at

the same wave-length, digital systems will consistently offer higher signal-to-

noise ratios and lower levels of nonlinear distortion.

In addition, digital design offers tremendous flexibility to fiber optic engineers

who wish to scale the performance level of a product to meet the needs of

different markets, applications and budgets. By changing the number of

sampling bits used in the analog to digital conversion, engineers control the

transmission bandwidth of a system – affecting the product’s ultimate

performance and price. However, regardless of the number of bits used,

the other digital properties of the system remain constant, including lack

of distortion and consistent performance up to the maximum supported

transmission distance. This makes it easy for engineers to anticipate how

different variations of a given system design will work if scaled for high-

end and lower-end performance requirements. When designing analog

systems, engineers are constantly struggling with ways in which price and

performance may be balanced without compromising critical qualities of the

transmitted signals.

Infinite Signal Extension

Another advantage of digital systems over their analog predecessors is their

ability to regenerate a transmitted signal without incurring any additional

degradation to the original baseband video, audio or data signal. This is

accomplished by using a device called a repeater.

As light travels through a length of fiber, its optical power gradually

attenuates. Eventually, not enough light remains to be detected by a

receiver. By placing a repeater on the fiber at a point prior to where the light

diminishes too much to be detected, the repeater can regenerate and restore

the digital signal to its original form. This regenerated signal is then launched

again on the same wavelength, with the same optical power found at the

original transmission point.

An Introduction to Fiber Optics

7



It is important to note that the baseband video, audio or data signal is

never actually recovered in a repeater; only the data stream representing

the original signal or signals is processed. In this manner, the quality of the

original baseband 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 this application.

Lower Overall Cost

Given the many advantages that digital systems offer, one might think that

they cost more than traditional analog systems. This should not be the case.

In fact, users of digital systems save money on many levels. For starters, the

cost of digital components has decreased greatly in recent years, resulting in

the ability of fiber optic manufacturers to design and manufacture products

that are priced similarly or even less than earlier generation analog products.

Of course some manufacturers would like the public to believe that the

superior performance of digital systems can only be obtained for a premium

price, but in reality, they are simply choosing not to pass their own savings

on to their customers. By competitive shopping, users of fiber optic trans-

mission systems should be able to find products offering digital performance

at the same price as analog systems.

Other factors also contribute to the economics of owning and operating

a system. Most obvious is the expense of fiber optic cable. Digital systems

allow for the transmission of more information over a single fiber, thus

minimizing the required quantity of cable. This advantage is particularly

evident in applications requiring the transmission of disparate signal types,

such as video with audio, or audio with data. Fiber optic engineers can easily

design affordable digital systems that accommodate a mixing and matching

of signals types, such as two channels of video with four channels of audio,

all over a single fiber. Using analog technology, this same signal transmission

would most likely require two systems, or at least two fibers, for separate

transmission of video and audio signals.

Even in situations where multiple channels of the same signal type are

transmitted over a single fiber, digital designs offer more reliable, consistent

performance. For example, a ten channel video multiplexer that uses digital

processing and transmission can provide uniform transmission of all ten

signals over the entire transmission path – something impossible in an analog

system. Another cost factor to consider is the amount of maintenance and



An Introduction to Fiber Optics



repair that a system is likely to require throughout its years of use. Once again,

digital systems provide a clear advantage. First of all, digital systems require

no adjustments in the installation process. Transmitter, cable and receiver are

simply connected together and the system is ready to go. By contrast, analog

systems often need tuning and adjustments to accommodate for trans-

mission distance and signal strength. Additional setup time translates to

additional costs. Then, once a system is fully operational, digital systems are

far more likely to perform consistently and reliably over time. This is because

by design, digital systems have far fewer components that may break or

malfunction. There is no possibility for the system to require retuning or

adjustments. And, the user is likely to spend less time troubleshooting

because the digital design makes it immune to interference, crosstalk,

drifting, and other performance issues that plague traditional analog systems.

Lower transmitter and receiver costs; lower cable costs; lower maintenance

costs. Digital fiber optic transmission systems provide a clear economic

advantage at every level.

An Introduction to Fiber Optics





Summary

Just as fiber optic transmission offers many advantages over traditional coax

or copper wire, digital fiber optic transmission brings the medium of fiber

several steps further, offering users a whole new set of benefits.

Characteristics unique to digital systems include consistent signal quality

over the entire transmission path, absolute minimal signal distortion

(including immunity from crosstalk and intermodulation), and the potential to

regenerate and extend the distance of a transmitted signal indefinitely with

no degradation to the original baseband signal. Because of these qualities,

baseband analog signals that are transmitted digitally over fiber maintain a

level of fidelity not possible if the transmission were to occur via an analog

system.

In addition, digital fiber optic transmission systems are available at prices

similar to analog systems, and actually cost less overall to own and operate

when considered within the context of an entire installation, including fiber,

setup and maintenance costs.

For your next fiber optic transmission project, don’t spend valuable time

weighing the advantages of digital versus analog systems. That choice is

clear. Digital systems will serve you better on every level. Instead, limit your

search to digital systems and evaluate carefully which line of digital fiber optic

products best meets your needs. Even among digital systems, you will find

tremendous variation. Some questions you might consider are:

1.

2.

.

.

.

.

7.

Is the system easy to install?

If the transmitter and receiver units are user-configurable,

is this process simple and trouble-free?

Is the design compact, sturdy and mechanically reliable?

Are units available as both box units and for use in a rack?

Are systems available for use with both single mode

and multimode fiber?

Does the manufacturer have a history and reputation for

designing quality products?

Is the price competitive with traditional analog systems?

(Digital designs cost no more to manufacture than analog

systems and should not cost a premium.)

With a little investigation and comparison shopping, you are bound to find a

digital fiber optic transmission system that will serve you well now and for

many years to come.

10

An Introduction to Fiber Optics



Other issues in the eduGuide Series

Introduction to Fiber Optics

Undisputably, fiber is the future. Learn all about the benefits

of fiber optic technology in this easy-to-read guide.

Advantages of Digital Fiber Optics

Examine how digital signals over fiber are accomplished,

the phenomenal results they achieve, and how cost-effective it is.

Fiber Optic Cables, Connectors and Integration

Learn how easy it is to terminate and fabricate your own fiber optic cables,

what types of fiber and fiber jackets are available and how to design

and integrate a fiber optic system.

Scan Converters Buyer’s Guide

Everything you need to evaluate and decide on the perfect Scan Converter.

Video Scaling

A comprehensive overview of the technology, how it works

and when to use this technology effectively.

Advanced Video Scaling

Easy explanations of Inverse :2 Pulldown, Anamorphic Scaling and Other

Confusing Concepts.

Other Educational Resources

commspecial.com

Visit our website for online education resources, product literature and more!

Pure Digital Fiberlink® Application Brief – Everyday Pro A/V

This application brief illustrates the benefits of using Pure Digital Fiberlink®

in any Pro A/V installation. Regardless of your Pro A/V market specialty,

this Application Brief is a valuable guide to integrating and specifying

Pure Digital Fiberlink® products.

Pure Digital Fiberlink® Intelligence Brief – Government/Military

Detailed analysis of how Pure Digital Fiberlink® has become the solution of

choice for mission critical government, intelligence and military applications.

If you are involved in this market segment, this document will prove to be

valuable during your next project specification.

An Introduction to Fiber Optics

11



12

An Introduction to Fiber Optics



An Introduction to Fiber Optics

1



About Communications Specialties, Inc.

Communications Specialties, Inc. (CSI) is an award-winning, Long Island

based company that manufactures and sells a variety of products for the

distribution, conversion or transmission of television and computer video

signals, including fiber optic transmission systems, scan converters and video

scalers.

The company was founded in 1 by veterans of the broadcast industry.

Since then, CSI has managed to consistently design innovative products that

are used worldwide by Fortune 500 Companies and Government Agencies in

a variety of markets such as Broadcast, Professional A/V, Videoconferencing,

Education, Home Theater, Security, ITS, Industrial Monitoring, Digital Signage,

Government/Military and more!

The

Pure Digital Fiberlink®

line offers an extensive and affordable family

of fiber optic transmission systems for the Professional A/V marketplace

and includes several ground-breaking products for the transmission of

high-resolution RGB signals. Systems for point-to-point and

point-to-multipoint signal distribution make these products highly

desirable for any Pro A/V applications.

Our premier product line, the

Scan Do®

family of computer to video scan

converters, has redefined industry standards in computer video to NTSC/PAL

technology with unsurpassed performance in its price range. All models

support high resolutions and refresh rates and are VGA and Mac® compatible.

The feature-rich and versatile Scan Do family offers the widest range of scan

converters on the market.

The award-winning,

Deuce®

video scalers convert NTSC and PAL to high-

resolution, non-interlaced video and offer a far superior and affordable

alternative to line doubling and quadrupling. The new generation of Deuce

products offer a wide range of non-interlaced resolutions and refresh rates

for every application, from professional A/V installations to home theater,

including a model designed especially for use with HDTV displays.

In addition, CSI manufactures a comprehensive selection of distribution

amplifiers, VGA monitor, keyboard and mouse extenders and accessories for

our entire product line.

1

An Introduction to Fiber Optics



Communications Specialties and its products have been the recipient of

numerous industry awards. In 2005, the Pure Digital Fiberlink® 7220 Series

for high-resolution RGB and Stereo Audio was honored as one of the AV

industry’s best technological innovations of the year by receiving a “rAVe

Radical Product of the Year” award as “Best New Analog Signal Processing

Product”. The rAVe email newsletter is published by professional audiovisual

industry veterans and is read industry-wide.

Among CSI’s many other awards are AV Video Magazine’s Platinum Award

(given to Scan Do® Ultra and Deuce®) and the Video Systems’ Vanguard

Award (given to Deuce).

The company is headquartered in the United States on Long Island,

New York, with Sales Offices in Florida, Indiana and Virginia. Research,

development, design, engineering, manufacturing and customer support

operations are performed at the New York headquarters. Other locations

include Communications Specialties Pte Ltd (CSPL) - a wholly owned

subsidiary office in Singapore that provides support to distributors in the Far

East and Pacific Rim.

Our in-house sales department handles complete product-line sales directly

to end-users as well as to an international network of representatives and

resellers. All of our products are backed by an exceptional warranty.

©2007 Communications Specialties, Inc. All Rights Reserved.

Specifications, claims or other product information contained in this document are subject to change without notice.

This document may not be reproduced, in whole or in part, without the express written consent of

Communications Specialties, Inc.,

Fiberlink, Pure Digital Fiberlink, the starburst logo, Scan Do and Deuce are registered trademarks of

Communications Specialties, Inc. CSI and the triangle designs are trademarks of Communications Specialties, Inc.

October 19, 2007

An Introduction to Fiber Optics

1



Learn

An educational resource published by Communications Specialties, Inc..

edu

Guide

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