Contents for this Newsletter
 About this Newsletter
Meeting Announcement
Upcoming Meeting Schedule
Auction Language in Budget Bill
SBE National Awards
Meeting Minutes
The New FCC
FCC Rulemakings
Amateur Radio News
EAS Firsthand
Telecom Industry News
Broadband Networks (Part 13)
SBE Short Circuits
Sustaining Members
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ABOUT THIS NEWSLETTER
The Chapter 24 Newsletter is published monthly by Chapter 24 of the Society of Broadcast Engineers; Madison, Wisconsin. Original hard copy edited by Mike Norton on Pagemaker 5.0. Submissions of interest to the broadcast technical community are welcome. You can make your submissions by e-mail to:
Mike_Norton@went.pbs.org
Information and/or articles are also accepted by US Mail. Please address them to:
SBE Chapter 24 Newsletter Editor
46 Trillium Court
Madison, WI 53719-2308
Please submit text file on DOS or Windows 3.5" floppy diskette if possible.
Leonard Charles is the editor for the Electronic Version of this Newsletter uploaded monthly onto SBE Chapter 24's web page.
Thanks to Chris Cain for his work on the Chapter 24 WWW page and electronic newsletter.
Contributors this month:
Neal McLain
Tom Smith
Fred Sperry
Paul Stoffel
Tom Weeden
© 1997 by SBE Chapter 24. Views expressed herein do not necessarily reflect the official positions of the Society, its officers, or its members. SBE Chapter 24 regrets, but is not liable for, any omissions or errors. The Chapter 24 Newsletter is published twelve times per year. Other SBE Chapters are permitted to use excerpts if attributed to the original author, sources, and SBE Chapter 24.
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MEETING ANNOUNCEMENT
Wednesday, August 27, 1997
Video Server/Automation Technology
J.T. Whitney's
674 S. Whitney Way
Dutch Treat Dinner at 5:30pm
Meeting and Program at 7:00pm
Visitors and guests are welcome at all of our SBE meetings!
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UPCOMING MEETINGS
Tentative Program Subjects
Thursday 9/18/97
Video Servers/New Member Night
Wednesday 10/22/97
Broadcast Clinic
Tuesday 11/18/97
Tour of Electronic Theater Controls
Wednesday 12/17/97
Lunch Meeting (Program TBA)
Thursday 1/22/98
Radio Station Automation
Tuesday 2/17/98
Test and Measurement Equipment
Wednesday 3/18/98
ATM Technology or Related Topic
Thursday 4/16/98
Elections and NAB Review
Tuesday 5/19/98
Telephone Company Tour
Wednesday 6/17/98
Sullivan NOAA Weather Office Tour
If you have any suggestions for program topics you'd like to see, please contact one of the Chapter 24 Program Committee Members.
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AUCTION LANGUAGE IN BUDGET BILL AFFECTS BROADCASTERS
By Tom Smith
There were a number of items in the new budget bill that caused either relief or concern among broadcasters, depending upon who they are.
Television broadcasters were relieved that they will not be required to pay a spectrum fee for their digital channels, and that Congress did not make the 2006 deadline for the conversion to DTV a law. The bill will require the FCC to reclaim the analog TV spectrum in a market in 2006 only if more then 85% of the households are able to receive digital TV signals and all of the stations affiliated with the four major networks have converted to digital. The bill directs the FCC to make sure that TV stations make an honest effort to get their digital signal on.
The FCC is to auction the analog channels by September 30, 2002, with reclamation of the signals at the end of 2006 or when the FCC determines the analog shutdown date.
Channels 60 to 69 are to be reallocated with four channels going to public safety and the rest auctioned off for commercial use, including for the use of broadcasting.
The budget bill also allows stations in cities of greater than 400,000 to own two TV stations and for TV/newspaper cross ownership. This allows existing TV stations and newspapers to bid on returned analog channels.
The budget bill requires the FCC to auction off 20 MHz of the 2 GHz ENG spectrum with the possibility of auctioning off another 15 MHz This would reduce the 2 GHz ENG band to 85 MHz with the possibility of going to 70 MHz. Broadcasters had already lost 15 MHz to the Mobile Satellite Service, but had been promised some spectrum above 2110 MHz. Congress ordered that auctioned off as well. The NAB let that spectrum go in return for no spectrum fees on digital TV.
The final item that Congress placed in the budget bill was the authority for the FCC to hold auctions to settle contested application proceedings for awarding of construction permits for new broadcast stations. The FCC has about 1500 contested applications for both radio and TV stations. The FCC has been trying to figure out a method of awarding licenses after an appeals court threw out the FCC criteria for selecting winning applicants in comparative hearings. The FCC would hold auctions after giving the various parties six months to work out a settlement.
It will take some time for the FCC to enact these provisions as they will have to first go through the rule making process to determine the final methods of carrying out these laws.
Compiled from BROADCASTING and CABLE
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SBE NATIONAL AWARDS ANNOUNCED
By Fred Sperry
I am pleased to announce that once again Chapter 24 is the recipient of several awards in the SBE’s Annual Awards Program.
Chapter 24 received four awards in the following categories for 1996:
Best Chapter Newsletter (tied with Chapter 70, Northeast Ohio) - Editors Mike Norton and Mark Croom, and contributing authors.
Best Chapter Frequency Coordination Effort - Tom Smith
Best Technical Article or Program - Neal McLain for his series on broadband networks.
Best Article, Paper or Program by a Student Member - Vicki Way for her article entitled “Television in the U.K. - London, England”
These awards will be presented at the Society’s Annual Awards Banquet on Friday, September 26th, in conjunction with the Central New York SBE Regional Convention and SBE National Meeting in Syracuse, New York.
Congratulations to the award recipients and to the membership at large for making Chapter 24 so successful.
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JULY BUSINESS MEETING MINUTES
Minutes submitted by Fred Sperry, Chair
Chapter 24 of the Society of Broadcast Engineers met on Thursday July 24, 1997, during its annual summer picnic for an abbreviated meeting. The total attendance was nine all of whom were SBE members. Of the nine, seven were certified.
The meeting was called to order at 6:58 PM by Chair Fred Sperry. Fred thanked Denise Maney for her work in putting the picnic together. The minutes from the June meeting were then approved as published in the July newsletter.
In Newsletter Editor Mike Norton’s absence, Fred reported the deadline for submissions to the August newsletter and the date of the folding session.
There was no old business, new business, or professional announcements to report.
The meeting was adjourned at 7:03 PM.
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THE NEW FCC HAS TAKEN SHAPE
by Tom Smith
All the nominations have been made to fill the four vacant, or soon to be vacant, seats on the FCC. President Clinton has designated previously nominated William Kennard as Chairman to replace Reed Hundt. Kennard is FCC general counsel and a former broadcast attorney and counsel to the NAB. President Clinton also nominated Michael Powell and Gloria Tristani to the FCC. Powell is chief of staff at the Justice Department’s anti-trust division and son to General Colin Powell. Senator John McCain pushed for Powell’s nomination. Tristani is Commissioner of the New Mexico State Corporation Commission.
President Clinton previously nominated House Commerce Committee Chief Economist Harold Furchtgott-Roth to the Commission. Kennard and Tristana will join Commissioner Susan Ness on the Democratic side and Powell and Furchtgott-Roth will fill the Republican seats.
From BROADCASTING and CABLE and WISCONSIN STATE JOURNAL
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FCC RULEMAKINGS
Compiled by Tom Smith
PUBLIC NOTICE
Docket No. WT 97-150
Commission Opens Inquiry on Competitive Bidding Process for Report to Congress
The FCC held an inquiry on competitive bidding to make a report to Congress, as required by the Omnibus Budget Act of 1993. Comments were due on August 1st. The FCC released this notice on July 2nd and published it in the FEDERAL REGISTER on July 9th, pages 36,752-36,756.
The FCC asked for comments on revenues from competitive bidding, methods on bidding (multiple round, seal bids, etc.), service to rural areas, and effect on small businesses.
This inquiry was not for the purpose of rulemaking, but for a report to Congress.
FINAL RULE
MD Docket No. 96-186; FCC 97-215
Assessment and Collection of Regulatory Fees for Fiscal Year 1997
The FCC has issued it’s revised Schedule of Regulatory Fees for fiscal 1997. Congress sets the amount of fees that the FCC is to collect each fiscal year. The FCC then bases the fees for each service on the cost of regulating that service.
For Broadcasters, there are some changes this year. Radio is under a new fee structure based on class and population coverage. Large markets pay more than small markets, high power stations pay more than lower powered stations. Radio stations will be mailed a fee notice and can check their fees on the FCC web site. For other broadcast services, check the notice for the new fees.
This notice was adopted on June 16, 1997 and released on June 26, 1997. The notice becomes effective as of September 15, 1997. The notice was published in the FEDERAL REGISTER on July 11, 1997 on pages 37,408-37,446
FINAL RULES, PETITIONS FOR RECONSIDERATION
MM Docket No. 87-268; DA 97-1377
Advanced Television System and Their Impact on the Existing Television Service
The FCC extended to August 22, 1997 the date that supplemental filings relating to reconsideration of the Sixth Report and Order that request changes to DTV allotments are due. This extension was due to requests that OET Bulletin No. 69 be release before petitions for reconsideration petitions were filed. The FCC released OET 69 at the same day that this notice was released.
This notice was released on July 2, 1997 and published in the FEDERAL REGISTER on July 11, 1997 on pages 37,145-37,146.
MM Docket No. 87-268; DA 97-1481
Advanced Television System and Their Impact on the Existing Television Service
The FCC rejected a petition requesting an extension to the due date for petitions for reconsideration of Sixth Report and Order. That date was July 18, 1997. The petitioners wished to avoid multiple filings. This action does not affect the August 22nd due date for petitions for reconsideration of DTV allotments.
This action was published in The FEDERAL REGISTER on July 22, 1997 on pages 39,128-39,129
PROPOSED RULE
ET Docket No. 97-157; FCC 97-245
Reallocation of TV Channels 60-69, the 746-806 Mhz Band
As part of the DTV actions, the FCC is proposing to allocate TV channels 63, 64, 68, and 69 for fixed and mobile public safety services. Channels 60, 61, 62, 65, 66, and 67 would be allocated to fixed, mobile and broadcasting services with the anticipation of assigning the licenses in these channels by auction.
Full power TV stations will be protected during the transition to DTV and low power TV and translator stations will retain their current secondary status. These changes in allocations would occur as soon as the FCC completes it’s rule making process and before the rest of the DTV transition occurs.
Comments are due on September 15, 1997 and replies are due on October 14, 1997. This notice was adopted on July 9, 1997 and released on July 10, 1997. Published in the FEDERAL REGISTER on July 31, 1997 on pages 41,012-41,015.
Compiled from the FEDERAL REGISTER. These notices are available on the FCC Web Site (www.fcc.gov) or from the Government Printing Office WEB Site (www.access.gpo.gov)
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AMATEUR RADIO NEWS
By Tom Weeden, WJ9H
• A downturn in the amateur radio economy has forced several radio stores out of business in the last year. The latest victim is Amateur Radio Supply in Seattle. ARS’s parts and inventory have been purchased by another area ham radio store, Radio Depot of Bremerton, Washington.
• The FCC has announced that it will open “vanity gate 3” on August 6th. This “gate” will permit Advanced class amateurs to request a vanity call sign for a $30 fee. The previous two gates were for Extra class operators and for those wishing to claim the call of a deceased relative. General, Technician and Novice class operators are not yet eligible for vanity call signs. The FCC has not indicated when it plans to open “gate 4” to make those classes eligible.
• Ham radio volunteers stand to benefit from “The Volunteer Protection Act of 1997,” signed into law June 18th by President Clinton. The measure aims to provide Federal protection from “frivolous, arbitrary, or capricious” lawsuits filed against individuals affiliated with nonprofit organizations and government entities and acting in their volunteer capacities. This would include members of the Amateur Auxiliary and volunteer examiners, as well as other volunteers. The law becomes effective September 16, 1997.
(Excerpted from August 1997 “QST” and “Badger State Smoke Signals”)
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EMERGENCY ALERT SYSTEM FIRSTHAND
By Paul Stoffel
Compiled from excerpts of the Chapter 24 email list server
• As proposed at the WBA EAS Summit, future RMTs will be conducted on the first Wednesday of each month. For the months of January, March, May, July, September, and November, RMTs will originate from Weather Radio at 8:50 AM. For the months of February, April, June, August, October, and December, RMTs will originate from the State Relay at 11:50 PM. LP-1 and LP-2 stations will no longer originate RMTs.
• The legislature will decide the fate of nine additional Wisconsin NOAA Weather Radio transmitters to minimize gaps in coverage. According to Wisconsin Emergency Management, “Approximately 65% of the Wisconsin population is covered by NWS radios. WEM is presently working with several agencies to obtain funding to increase that coverage.”
• Doug Schwartz at Marcus Cable, Madison, has been appointed Cable Chair on the Wisconsin State Emergency Communications Committee. Marcus has installed EAS equipment in cable systems with 5,000 or more subscribers.
• For incoming RMTs, add CIV (or ALL) to the filter’s ORIGINATOR parameter. Future RMTs will be originated from the State EOC.
• Check www.hamtronics.com for a VHF low band receiver, model #R100-50. TFT’s model number is #7100-4290.
• National Weather Specific Area Message Decoder by Morris Softronics is available at www.htcnet.com/msftrncs/products/NWSamd/
• Check out “EAS and NOAA Weather Radio” at www.crh.noaa.gov/mkx/eas.htm. The article states, “...The new Weather Radios with SAME capabilities will be available at electronics stores in August, 1997.”
• Wisconsin Emergency Management has a section devoted to EAS: http://badger.state.wi.us/agencies/dma/wem/eas.htm.
• At the WBA EAS Summit, broadcasters asked Rusty Kapela of NOAA’s Sullivan office to consider shortening the Weather Radio “Warning” scripts within an EAS alert, in an attempt to keep alerts under one minute.
• Current software versions: Gorman-Redlich, version 6; TFT, version 0.79; Sage, version 5.88.
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TELECOM INDUSTRY NEWS
By Neal McLain
APPEALS COURT REVERSES FCC “FEE ON FEE” DECISION
Federal laws governing cable television systems allow local franchising authorities to impose a “franchise fee” on cable operators. This fee is intended to reimburse the franchising authority for the use of public rights-of-way and for the costs of regulating cable systems. This fee also supports government- and public-access programming activities.
Federal law also restricts the franchise fee to 5 percent of gross revenues: “For any twelve-month period, the franchise fees paid by a cable operator with respect to any cable system shall not exceed 5 percent of such cable operator’s gross revenue derived in such period from the operation of the cable system.” (1) Most franchising authorities impose the full 5 percent, although some authorities impose lesser amounts.
Some time ago, a dispute arose between cable operators and franchise authorities over the method for calculating the 5 percent fee. Cable operators claimed that the 5 percent fee was like a sales tax; accordingly, it should be based only on revenue received for the provision of cable television service. However, a number of franchising asserted that the fee should be based on all revenue collected by the cable operator, including funds collected to pay the franchise fee. This came to be known as “fee on fee” revenue.
The case began as a dispute between the City of Baltimore and a local cable provider, United Artists Cable of Baltimore (UACB). UACB had originally agreed to pay the city a 5 percent franchise fee. In calculating its gross revenue, UACB treated the fee like sales tax: it calculated the fee based on its charges for cable television service. If a customer’s monthly bill was $30.00, UACB divided the bill into two portions: $28.56 allocated to cable services, and $1.44 (5% of $28.56) allocated to pay the franchise fee.
The city contended that this method of calculation was incorrect. Instead, the city argued that under the franchise agreement, UACB was required to pay 5 percent of the full sum collected from subscribers. Therefore, if a customer’s bill was $30.00, the franchise fee would be $1.50.
UACB appealed the matter to the FCC’s Cable Services Bureau, which ruled in favor of UACB. The full Commission upheld the Bureau.
At that point, the cities of Dallas and Laredo, Texas, among others, appealed to the federal courts, where the case eventually made its way to the Fifth Circuit Court of Appeals. The central issue in the case turned on the question of Congress’ use of the term “gross revenues.” The cities claimed that the term should be interpreted broadly, to include all revenues received by the cable operator. Cable interests relied on the FCC’s statement that “nothing in the statutory provisions of the Cable Act states that franchise fees are to be included in calculating an operator’s gross revenues.” (2)
The Court issued its decision on August 8, 1997, reversing the FCC’s interpretation and holding that cable operators may be required by local franchise to pay a franchise fee on the revenue collected to pay that fee.
Cable operators now may be required to pay the full “fee on fee” amount. Using standard compound-interest formulas, that works out to about 5.26 percent.
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(1) 47 U.S.C. 542(b).
(2) FCC Memorandum Opinion and Order at 14.
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BROADBAND NETWORKS
PART 13 - Cascade depth and murphy's law
By Neal McLain,
nmclain@compuserve.com
This is Part 13 in a series of articles about coaxial broadband networks. In this article, we’ll continue with the discussion of trunk cascades — and the reasons for breaking up long cascades.
INTRODUCTION
In previous articles in this series, we have discussed two factors which degrade the performance of a broadband network: noise and distortion. We noted that these factors are mathematically related to cascade depth: they increase as functions of cascade depth according to well-defined mathematical relationships.
Because noise and distortion increase with cascade depth, they impose a limit on the number of amplifiers which can be connected in cascade. This in turn limits the size of the broadband distribution system which can be fed from one headend.
This limitation is rooted in the fundamental laws of physics. We can’t eliminate noise and distortion by any kind of “management” techniques; we simply have to accept the limitations they impose and design our networks accordingly.
But noise and distortion aren’t the only factors which limit cascade depth: many other factors impose limits as well.
Unlike noise and distortion, these other factors aren’t rooted in any fundamental laws of physics. Consequently, we can minimize them (or, theoretically, even eliminate them altogether) by proper management techniques: conservative design, careful installation, rigorous testing, and diligent maintenance.
But in spite of our best efforts, we still run afoul of another fundamental law: Murphy’s Law. This law tell tells us that the longer the cascade, the higher the probability that something will go wrong, and the greater the number of subscribers that will be affected.
In this article, we will look at some of these other limiting factors.
RELIABILITY
Like all electronic devices, the components which make up a broadband network (amplifiers, cable, splitters, connectors, etc.) are subject to failure. Such failures can occur for many reasons: corrosion, water intrusion, power surges, lightning, temperature cycling, semiconductor failure.
This leads to the concept of reliability: the probability that a network will not fail. Reliability is usually expressed as percentage; for example, a device having a rated reliability of 0.9999 is expected to operate satisfactorily 99.99% of the time. This translates into a failure rate of less than one hour per year.
When devices such as amplifiers are cascaded, the reliability of the cascade is severely degraded. Probability theory can be used to estimate the severity of the degradation: according to this theory, a cascade of n devices, each having a reliability p, has a net reliability pn. To illustrate this, consider the 32-amplifier cascade we have used in previous examples, assuming that the reliability of each amplifier is 99.99%. Thus:
For one amplifier:
n=1
pn=(0.9999)1=0.9999
For a 32-amplifier cascade:
n=32
pn=(0.9999)32=0.9968
Thus, the reliability of the signal at the output of the last amplifier is 99.68 percent. This translates into a failure rate of about 28 hours per year.
Of course, this analysis considers only the amplifiers. Other network components, such as cable and connectors, are also subject to failure, further degrading the reliability of the network.
By careful management techniques, we can minimize such failures. We could, for example, specify amplifiers having lower theoretical failure rates.
But the most direct approach is simply to limit cascade depth. Again using our previous example, let’s see what happens if we break it into three 11-amplifier cascades:
For an 11-amplifier cascade:
n=11
pn=(0.9999)11=0.9989.
Thus, the reliability of the signal at the output of the last amplifier is 99.89 percent, which corresponds to a failure rate of less than 10 hours per year.
AMPLIFIER GAIN ANOMALIES: "SIGNATURES"
Theoretically, the frequency response of each amplifier in a cascade of amplifiers precisely compensates for the attenuation in the previous span of cable. Or, stated another way: at every frequency, the gain of the amplifier precisely equals the loss in the previous span of cable. Ideally, the resulting frequency response should be a smooth curve, with no “bumps” or “dips.”
In fact, most real-world amplifiers generally don’t meet this ideal. Most amplifiers exhibit minor gain anomalies: a slight bump at one frequency; a slight dip at another frequency. These bumps and dips result from minor variations in component values (primarily inductors and capacitors) and their placement within the amplifier housing.
These anomalies can be readily seen on a “sweep” display (a sort of spectrum-analyzer display which plots the frequency response of the network). Cable TV technicians call this display the “signature” of the amplifier. An experienced cable tech can identify the manufacturer of an amplifier just by its signature.
When amplifiers are cascaded, the bumps and dips of the signature are magnified, and can reach several decibels at the end of a cascade of 20 or 30 amplifiers. Such anomalies can severely distort some types of signals. I will leave it to the reader to consider the result of a 5-dB level differential between the visual carrier and the chroma subcarrier of an NTSC video signal.
By careful management techniques, we can minimize signature anomalies. Obviously, amplifier manufacturers are in the best position to address the problem. It’s also possible (although inconvenient) to mix amplifiers having different signatures (I once knew a cable engineer who insisted that the ideal cascade consisted of an equal number of Jerrold, Scientific-Atlanta, and C-COR amplifiers installed in A-B-C-A-B-C sequence).
But again, the most direct approach is simply to limit cascade depth.
HUM
“Hum” is a catchall term used to describe what the FCC calls “low-frequency disturbances.” Hum can arise from several sources; the most common source is AC ripple in an amplifier’s DC power supply. If the ripple is not filtered properly, the amplifier’s gain varies as a function of the ripple voltage; this, in turn, literally amplitude-modulates the entire broadband spectrum. This has an obvious detrimental effect on any amplitude-modulated signals within the broadband spectrum.
In the case of an NTSC video signal, hum appears as a series of light-and-dark bands in the demodulated video. Hum resulting from power-supply ripple occurs at a 120-Hz rate, and produces two dark horizontal bands which slowly drift upwardly through the picture.
Hum usually arises at a single point in the network. Once introduced, however, it affects all downstream subscribers. Obviously, the longer the cascade, the greater the number of affected subscribers.
Breaking the cascade into shorter pieces reduces the probability that hum will occur within any given cascade; moreover, it reduces the number of subscribers affected when hum does occur.
MICROREFLECTIONS
Back in Part 2 of this series (May 1996), we noted that broadband networks operate at a constant impedance of 75 ohms. The characteristic impedance of the cable is 75 ohms, and all network devices (amplifiers, splitters, connectors, etc.) are designed to operate at 75 ohms. Ideally, everything in the entire network operates at a perfect 75-ohm impedance.
Real-world networks do not always meet this ideal. All sorts of conditions introduce impedance discontinuities: a slight mismatch at the input or output of an amplifier; a “leak” in the cable shield; an improperly-installed splice; a waterlogged tap; a dent in the cable sheath; a defective feeder termination resistor. Over the course of a long cascade, hundreds of such discontinuities can exist.
Each discontinuity produces so-called “microreflections”: some of the broadband signal is reflected back toward the source. If the reflected signal then encounters another discontinuity, it may be reflected again, thus producing a ghost image of the original signal.
Each discontinuity affects different frequencies in different ways; some discontinuities may reflect some frequencies and not others. The longer the cascade, the more microreflections occur. Over the course of a long cascade, hundreds of microreflections can occur, and hundreds of ghost images can build up.
The result “muddies” the signal. A video signal looks “out of focus,” and a data signal can be completely destroyed.
By careful management techniques, we can minimize impedance discontinuities. But, as before, the most direct approach is simply to limit cascade depth.
INGRESS
Theoretically, a broadband network is a “closed” network: the entire network is completely contained inside a grounded sheath in which the only openings are carefully-shielded impedance-matched input and output ports.
In fact, broadband networks are subject to “leakage.” A leak is any break in the continuity of the sheath which allows RF signals to pass. Leaks can be caused by many factors: improper installation, animal damage, corrosion, wind-induced cracks, water intrusion, vehicle accidents, vandalism.
Leaks degrade the performance of a broadband network in two ways: they’re a major source of microreflections, and they allow “ingress”: the passage of RF signals from the outside airspace into the network. (1)
RF signals entering a broadband network from the outside airspace directly interfere with the RF signals carried on the network. This interference can manifest itself in many ways depending on the relative amplitudes and frequencies. A strong interfering signal (as, for example, the broadcast signal from an FM or TV station) can completely obliterate co-channel signals carried on the network.
Interference into an NTSC video signal can produce beat frequencies which appear as alternating light and dark bands in the picture. If the interfering signal is another NTSC video signal, both images may be visible. A common example of this form of interference is called “3 on 3,” in which the off-air signal of WISC-TV interferes with the signal carried on cable channel 3.
This form of interference occurs even if the two signals carry the same modulation. Because the off-air broadcast signal always arrives at the point of ingress several microseconds before the signal carried on the network, the modulating video signals are not in phase. Depending on the relative amplitudes of the two carriers, the one signal may appear as a ghost in the other, or both signals may obliterate each other.
By careful management techniques (including a diligent monitoring-and-repair program), we can minimize ingress. But once again, the most direct approach is simply to limit cascade depth.
AMPLIFIER BEHAVIOR
The entire theory of noise and distortion is based on the assumption that amplifiers actually operate as designed; i.e., that they are “well behaved.” In a cascade of well-behaved amplifiers, the noise and distortion levels at the output of any given amplifier do not exceed the levels which would be predicted mathematically.
Real-world amplifiers aren’t always well-behaved. Occasionally, the noise or distortion level of an amplifier will “go out of spec”: it may jump abruptly, or it may build slowly over time.
The phenomenon usually results from damage to one of the transistors in the amplifier. An abrupt increase in distortion levels usually indicates that one of the two transistors in a push-pull circuit has failed.
This sort of damage can be caused by many factors. Lightning and power surges can damage a transistor without destroying it. Temperature cycling and corrosion can cause long-term degradation. Transistors sometimes fail for no apparent reason.
Like hum, out-of-spec behavior usually occurs at a single point in the network. Once introduced, however, it affects all downstream subscribers. Obviously, the longer the cascade, the greater the number of affected subscribers.
Breaking the cascade into shorter pieces reduces the probability that out-of-spec behavior will occur within any given cascade; moreover, it reduces the number of subscribers affected if it does occur.
CONCLUSION
There should be no doubt by now that there are many advantages to keeping amplifier cascades short. Modern broadband networks are designed to accomplish this: indeed, many fiber-based networks restrict cascades to just one or two amplifiers.
But we should not leave this subject without first noting a tradeoff: in order to keep cascades short, some sort of transmission medium is needed to get the broadband signals from the headend out to the various nodes where cascades originate. In previous articles, we have discussed two such transmission media: fiber optics and AML microwave.
But fiber and AML impose their own restrictions: they, too, are subject to many of the same degradations which affect amplifier cascade performance: noise, distortion, gain anomalies, hum, reflections, and unpredictable behavior. Moreover, they are subject to their own peculiar problems: AML signals can be degraded by heavy rain and interference from other microwave transmitters, and fiber signals are subject to degradation from a whole range of circumstances unique to fiber.
These restrictions notwithstanding, the tradeoff is worth making: all modern broadband networks are designed with short amplifier cascades.
---------
(1) A leak also causes “egress,” the passage of RF signals from the network to the outside airspace. Egress generally does not interfere with the operation of the network; however, it can cause severe interference to signals using the airspace. We will discuss egress in more detail in a future article.
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SBE's SHORT CIRCUITS, AUGUST 1997
By John L. Poray, Executive Director
NATIONAL AWARDS DINNER TICKETS AVAILABLE
If you are planning to attend the Central New York SBE Regional Convention and SBE National Meeting September 26, you’ll want to take in the SBE National Awards Dinner that night. Tickets are just $10, with the remaining cost picked up by our sponsor, Leitch, Inc. The program will include a keynote speaker, comments from the new SBE National President and the presentation of the SBE National Awards and Fellowships. You can order your ticket(s) with your MasterCard, Visa or American Express by calling or faxing the SBE National Office at (317)253-1640 (voice) or (317)253-0418 (fax). Tickets are limited and are available on a first come, first served basis.
SBE REGIONAL CONVENTIONS OFFER EDUCATION OPPORTUNITY
A number of SBE Chapters sponsor regional conventions each year. This fall, there are six to choose from. Why not check the list below for the one nearest you and plan to participate. Each one is a little different, offering a unique mix of educational sessions, equipment exhibits and social events:
• September 26, Central New York Regional SBE Convention and SBE National Meeting
• October 1, Pittsburgh Chapter 20 Regional SBE Convention
• October 18, Kentuckiana SBE Regional Convention
• October 21-24,1997 Broadcasters Clinic Holiday Inn West, Middleton (Madison), Wisconsin. Contact: Charles Lukens, WBA (608) 255-2600
• October 22-23, Electronic Media Expo, Bellevue (Seattle), Washington
• October 29-31, Texas Association of Broadcasters/SBE Regional Annual Convention
11 STUDENTS COMPLETE BROADCAST ENGINEERING LEADER-SKILLS COURSE
Eleven students from across the US successfully completed the first SBE sponsored, and 29th overall, Leader-Skills Course for broadcast engineers. The course, held June 9 to 13 in Indianapolis, was conducted by veteran management trainer, Richard Cupka. Students learned about their own communication and management styles, and how to identify the styles of others. Solutions to different personnel problem situations were explored. Students were ready to put in practice what they had learned upon returning home.
Details are being worked out to offer the program again next year. Watch the SIGNAL and Short Circuits for more information.
ELECTION BALLOTS MAILED TO MEMBER
Ballots were mailed to voting members of SBE the last week of June. Completed ballots must be returned to the SBE National Office no later than 5:00 pm, Thursday, August 28 to be included in the count. Members of Chapter 25 in Indianapolis will tabulate the votes. Due to an error at the mailing house, the ballot was mailed without the special return envelope. Members were mailed this envelope to use when returning their ballots several days after the first mailing with an explanation. Because they made the error, the mailing house paid for the second mailing.
A list of candidates for SBE National Officer and Board seats can be found at the SBE Web Site: www.sbe.org. Candidates who have personal Home Pages are providing information about themselves and their candidacy by linking them to the SBE site.
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CHAPTER 24 SUSTAINING MEMBERS
RECENT RENEWAL:
BCS Wireless (formerly Broadcast Communications)
Skyline Communications
Thanks to all our Sustaining Members:
Alpha Video
CTI
Clark Wire and Cable
Comark Communications
Fuji Film
Harris Corporation
Louth Automation
maney-logic
Niall Enterprises
Norlight Telecommunications
Panasonic Broadcast
Richardson Electronics
Roscor Wisconsin
Scharch Electronics
Sony Broadcast
Tektronix
Teleport Minnesota
Video Images
WISC-TV 3
WKOW-TV 27
WMSN-TV 47
WMTV-TV 15
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