CHAPTER 24, Inc., MADISON, WI
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About this Newsletter
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:
Information and/or articles are also accepted by US Mail. Please address them to:
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:
© 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.
Tuesday, September 23, 1997
Practical Choices for the DTV Transition / Guest Night
7952 Tree Lane
(Mineral Point Road at the Beltline)
Dinner at 5:30pm
Meeting and Program at 7:00pm
This month's program will feature a presentation on choices for the upcoming DTV transition. Jeff Conway from Tektronix will present information from an equipment vendor's perspective.
The Chapter is going to provide pizza and soda beginning at 5:30PM for guests that attend the meeting (Members will be asked to contribute $5.00 if partaking of the pizza and soda provided).
Visitors and guests are welcome at all of our SBE meetings!
Tentative Program SubjectsWednesday 10/22/97
Tour of Electronic Theater Controls
Lunch Meeting (Program TBA)
Radio Station Automation
Test and Measurement Equipment
ATM Technology or Related Topic
Elections and NAB Review
Telephone Company Tour
Sullivan NOAA Weather Office Tour
UPCOMING CHAPTER 24 EVENTS
By Fred Sperry
There are a couple of upcoming Chapter 24 events I would like to bring to your attention.
As part of our September meeting on Tuesday the 23rd, the Chapter is sponsoring "guest night." This meeting is going to be held at Rocky Rococo's 7952 Tree Lane (Mineral Point at the Beltline). The Chapter is going to provide pizza and soda beginning at 5:30 PM for guests that attend the meeting (Members will be asked to contribute $5.00 if partaking of the pizza and soda provided). Please spread the word around your workplace and encourage those coworkers who may have an interest in the SBE to attend this event. Please contact me if you would like to have a flyer announcing this event to post at your workplace.
The Chapter's October meeting will take place on Wednesday October 22nd as part of the Broadcast Clinic being held this year at the Holiday Inn West. As has been the case the past several years, SBE Chapter 24 and the Midwest Region of the SBE are programming the Wednesday evening portion of the Clinic. We have lined up David Felland, Director of Engineering for Milwaukee Public Television and the ECB in Madison, to speak on the subject of planning for digital television (DTV) at WMVS/WMVT. Following David's presentation there will be a panel discussion that will address what stations are doing to plan for the transition to DTV. We are looking for participants to be on this panel. Please contact me at (608)264-9806 or via e-mail at firstname.lastname@example.org if you or anyone you know is heavily involved in DTV planning and would be interested in being part of this panel. The success of this panel discussion depends upon finding participants for this portion of the program.
EAS COMMITTEE FILES RULES CHANGE PETITIONBy Leonard Charles, EAS Committee Chairman
The SBE EAS Committee has filed a formal Petition with the FCC for EAS Rules changes and additions. This filing follows an information gathering process from the SBE membership at large which began back in January of this year when the system officially went on line. The Committee used problem case scenarios submitted by Chapters and members across the country to write rules revision suggestions designed to make the EAS system work more reliably and consistently for all.
The submitted petition is published on the SBE Home Page at www.sbe.org under the EAS Committee banner. The Committee asks members to review the suggested changes. Then, when the FCC releases the petition for comment, SBE members and Chapters are urged to file comments in support or in addition to those submitted by the Committee. The Committee feels that the greater the number of comments filed, the better the chance at improving system performance and use.
SBE ANNOUNCES 1997 NATIONAL ELECTION RESULTS
The results of the SBE National Elections were tabulated Thursday, August 28, by volunteers from Chapter 25 in Indianapolis. Our thanks to Chapter Chairman, Stephen Lampen and the eight other members who provided this service.
SBE NATIONAL OFFICERS FOR 1997-98
Elected to Serve One One-Year Term:
President - Ed Miller, CPBE Cleveland, OH
Vice President - Troy Pennington, CSRE Birmingham, AL
Secretary - Thomas Weber, CPBE Indianapolis, IN
Treasurer - James "Andy" Butler, CPBE Alexandria, VA
Elected to the Board of Directors (top six vote-getters)
This chart specifies the cable television channel-numbering scheme currently used in the United States. Column headings are:
• BAND - The band of frequencies, in MHz.
• CABLE CHANNEL - The channel number assigned by the FCC.
• LETTER DESIG - The channel designation commonly used in the 60's and 70's.
• CABLE CARRIER FREQUENCY - The nominal frequency, in MHz, of the visual carrier of an NTSC television signal operating in this cable channel.
• AIR CHANNEL - The broadcast television channel, or other service using this band for over-the-air transmissions.
• BROADCAST CARRIER FREQUENCY - The nominal frequency, in MHz, of the visual carrier of a television broadcast transmitter operating in this air channel.
Codes used in the AIR CHANNEL column are:
• 2, 3, 4, ... - Broadcast television channel number assigned by the FCC.
• FM BAND - FM radio broadcast band, 88-108 MHz.
• FAA - Non-broadcast spectrum assigned to the Federal Aviation Administration.
• NON-BCAST - Non-broadcast spectrum assigned to any service other than the FAA.
Source: EIA Interim Standard IS-132: Cable Television Channel Identification Plan. Electronic Industries Association, Washington, DC, 1994. This standard is incorporated by reference into the FCC Rules at 47 CFR 76.605(a)(2).
Note that the broadcast spectrum is divided into three bands. These bands are spaced so that the second harmonic of any visual carrier never falls in any other channel, thus assuring that no television transmitter can interfere with another television signal.
Cable television systems carrying only 12 channels (2-13) utilize the same frequency assignments as the broadcast spectrum. This band-spacing scheme assures that second-order distortion products generated by the trunk amplifiers fall outside the bands of interest.
In short, the same band-spacing technique that the FCC devised to protect television stations from mutual interference also protects cable signals from second-order distortion products.
Figure 1 specifies two channel-numbering systems:
• CABLE CHANNEL - The number assigned to this channel by the FCC. These assignments are specified in EIA Interim Standard IS-132, and are incorporated by reference in the FCC's cable television rules.
• LETTER DESIGNATION - The channel designation commonly used in the 60s and 70s. These designations had evolved over the years in a rather haphazard fashion, with no official FCC sanction. In the late 70s, when converter manufacturers began making converters with digital channel-number displays, letter designations were replaced with numeric designations beginning with 14.
The carrier frequencies specified in these figures are nominal, and do not account for FCC-mandated offsets. We will cover offsets in a future article.
The short answer to this question is historical: the order in which channel numbers were assigned approximately corresponds to the chronological order in which they were first used by the cable television industry.
A more detailed answer follows.
The earliest cable television systems were constructed in the 50s. They were capable of carrying just one or two channels in the VHF low band.
In those days, even using the low band presented a major challenge because of the high signal attenuation of the available coaxial cables. Modern low-loss cables didn't exist, so amplifiers were required every few hundred feet. The amplifiers themselves (typically single-ended vacuum-tube devices) exhibited high cross-modulation distortion levels, severely limiting channel capacity.
Further complicating the situation was the poor selectivity of the television sets of the day, making the use of adjacent channels impossible. Even the most advanced systems could carry only three channels: 2, 4, and either 5 or 6.
In one respect, however, this limited channel capacity was a blessing: all second-order (and most third-order) distortion products fell outside the band of interest. Thus, cross-modulation was the limiting form of distortion.
Carrying Channels 7-13 was out of the question because of the substantially higher cable loss at the VHF high band.
Making the jump to the VHF high band was an even bigger challenge: the visual carrier frequency at Channel 7 (175.25 MHz) is over twice the frequency of the Channel 6 carrier (83.25 MHz). Hence, cable loss was higher, so more amplifiers were required; this led to higher distortion levels. And, of course, the increased channel loading itself also led to higher distortion levels.
The first attempts to carry the high band incorporated an ingenious trick for getting around these problems: Channels 7-13 were carried in the so-called "subband" below Channel 2 (Figure 3). Channels in this band were designated T7 through T13. "Upconverters" installed at various points in the distribution network converted the T-channels back to their proper positions. In some systems, upconverters were installed outdoors, each serving a relatively small area, such as a city block. In other systems, an upconverter was installed at each subscriber's home.
|T7||5.75 - 11.75||7.00|
|T8||11.75 - 17.75||13.00|
|T9||17.75 - 23.75||19.00|
|T10||23.75 - 29.75||25.00|
These channels were originally used to carry Channels 7-13 in the unused bandwidth below Channel 2. Although they are no longer used for this purpose today, these original channel assignments are still in use.
This system allowed cable operators to carry as many as seven channels: 2, 4, 5 (or 6), 7, 9, 11, and 13. But they still couldn't carry adjacent channels because of the poor selectivity of the subscribers' receivers.
But even this restriction had its bright side: second-order distortion products never fell in occupied channels. The four new visual carriers (at 7, 19, 31, and 43 MHz) produce five second-order products at 26, 38, 50, 62, and 74 MHz. These products all fall in unused channels (T10, T12, 3) or unused guard bands.
Ingenious though it was, this system didn't work very well. The old vacuum-tube upconverters weren't stable, and they must have been very expensive to operate.
The T-channel distribution scheme didn't last long. But its legacy lives on to this day: the T-channel numbering scheme is still used to identify channels in the subband. Although these channels are no longer used for subscriber distribution, they are frequently used for closed-circuit applications such as distribution of instructional programming to schools or to carry locally-originated signals back to the headend.
By the mid-60s, distribution equipment was improving dramatically. Cable manufacturers had introduced low-loss cables with solid aluminum sheaths — the same types of cable still in use today. Amplifier manufacturers had designed new products utilizing transistors in push-pull circuits, greatly improving distortion performance. These improvements made it possible for cable operators to carry Channels 7-13 at their proper frequencies.
TV receiver manufacturers were improving their products as well. The introduction of varactor tuners and solid-state IF strips made it possible to utilize adjacent channels. By the late 60s, virtually all new cable television systems were capable of carrying all 12 VHF channels.
A typical amplifier of the day was Jerrold's "Starline" series. This product utilized a strand-mounted housing with plug-in solid-state modules. Besides greatly-improved performance, this arrangement simplified field maintenance: a defective module could be replaced quickly. The original cable systems constructed in Madison (Complete Channel TV) and Fitchburg (Fitchburg Cable Communications) utilized Starline equipment.
As always, distortion control remained a significant design constraint. Fortunately, even on a fully-loaded 12-channel cable system, all second-order distortion products fall outside the bands of interest: either below Channel 2, above Channel 13, or in the unused space between Channels 6 and 7. This fortunate situation is, of course, a direct result of the FCC's original channel allocation plan for VHF broadcasting.
By the mid-70s, the cable television industry was growing rapidly. Program suppliers, eager to cash in on this new market, began creating programming services for distribution via satellite. HBO was the first to make the leap to satellite; two others quickly followed: Ted Turner's Atlanta TV station WTCG (now WTBS), and Pat Robertson's Christian Broadcasting Network (now The Family Channel). By the end of the 70s, dozens of programming services were available by satellite.
With all these new services available, it wasn't long before cable systems were filled to capacity and needed more channels. The obvious place to find space for new channels was in the unused "midband" between the FM Band and Channel 7.
Theoretically, there's enough space in this band for eleven 6-MHz television channels:
(174-108)/6 = 11
However, only nine channels were assigned; the two channels in the 108-120 MHz band were skipped. The reason: as you will note in Figure 1, this band is assigned to the FAA for a service called VHF Omnidirectional Range, or VOR. VOR transmitters serve as location beacons, enabling aircraft pilots to navigate from one VOR station to the next by following VOR "radials." It doesn't take much imagination to dream up all sorts of nightmare scenarios which might result if leakage from a cable system caused interference to a VOR transmitter.
The nine remaining midband channels (120-174 MHz) were adopted as cable television channels, raising the total channel capacity to 21. These channels were originally designated A through I; however, with the advent of settop converters and cable-ready TV sets equipped with digital channel-number displays, they were redesignated 14 through 22.
Using the midband imposed a whole new set of constraints on amplifier design. As always, adding more carriers resulted in higher distortion levels. But an even greater problem was second-order distortion: designers could no longer ignore it on the assumption that it would fall outside the bands of interest. Indeed, in a fully-loaded 21-channel cable system, hundreds of second-order distortion products are generated, and many of them fall in active channels. In short, second-order distortion replaced cross-modulation as the dominant form of distortion.
Amplifier manufacturers adopted many techniques to control second-order distortion:
• Pair-matching: hand-selected matched pairs of transistors operating in push-pull circuits.
• Power doubling: four matched transistors in a push-pull parallel configuration.
• Feedforward: a circuit in which a separate transistor circuit is used to amplify the distortion. This distortion signal is then inverted and used to cancel the distortion added by the main amplifier.
And, as we've noted in previous articles, second-order distortion forced a reduction in the number of amplifiers which could be connected in cascade. A maximum of 20 amplifiers came to be accepted as the limit.
The insatiable demand for more channel capacity continued unabated, and 21-channel cable systems soon became obsolete. By the late 70s, the 216-300 MHz band was being used. This band contains 84 MHz, enough space for 14 channels.
These channels were originally designated J through W, and were later redesignated 23 through 36.
Of course, adding yet another 14 channels made distortion control even more difficult. As we noted in a previous article (November 1996) third-order distortion becomes significant as channel loading increases, and becomes dominant when the number of carriers exceeds 30 or so.
Some manufacturers tried to get around the distortion problem by using various frequency-control and frequency-offset techniques. We'll discuss these techniques in a future article.
Another 18 channels, in the 300-402 MHz band. These channels were originally designated AA through RR, and were later redesignated 37 through 54. Note that X, Y, and Z were never used.
At this point, the boundaries of the various "bands" became somewhat indefinite. Many authors identify this group of channels as part of the superband.
Some authors also include channels above 54 in this band, although there is no consensus on how far it extends. The EIA standard defines channel numbers all the way up to Channel 158, topping out at 1002 MHz, but makes no attempt to break the spectrum into bands.
A memory device for those who still like to use letter designations: Channel SS = Channel 55.
Remember those two channels in the 108-120 MHz band that were skipped when the midband was first used?
In the years since, the cable industry has been able to recover the use of these channels. Two developments made this possible:
• Improved technology allowed equipment manufacturers to build headend modulators capable of very precise frequency control, typically on the order of ±1 Khz.
• The FCC, working in cooperation with the FAA, established frequency-assignment rules for the cable industry. These rules specify cable frequency offsets so that the cable frequencies don't conflict with VOR frequencies. For example, a visual carrier nominally assigned to 109.25 MHz is actually offset +0.025 MHz, to 109.275 MHz, where it falls halfway between VOR frequencies at 109.25 and 109.30 MHz.
These two channels have had several designations. At the outset, they were called A-1 ("A minus one") and A-2 ("A minus two") because they fall just below Channel A.
For obvious reasons, converter and TV-set manufacturers didn't want to use such klunky channel numbers. But there was no accepted numeric identification plan at the time, so manufacturers pretty much did what they pleased:
• Some manufacturers called these channels 0 and 1 (or 00 and 01).
• Some manufacturers used the next two available numbers at the top of the band; however, what was considered the "top" of the band varied from manufacturer to manufacturer. This led to such designations as 54/55, 57/58, 60/61, and probably others.
When the EIA standard was drafted, these channels were officially designated 98 and 99.
Many cable systems use the FM band for FM audio services. TCI, for example, carries all Madison-area FM broadcast stations, plus WXJ-87 and stereo simulcasts of several video channels.
But some cable systems use the FM band for video services. There's enough space for three television channels, extending from 90 to 108 MHz.
These channels were originally called (in order of increasing frequency) A-5, A-4, and A-3. The EIA Standard identifies them as 95, 96, and 97.
In a standard NTSC television channel, the nominal frequency of the visual carrier is 1.25 MHz above the lower edge of the channel. However, many cable television channels are operated at frequencies slightly offset from nominal. There are two reasons for this:
One reason is aviation safety. As Figures 1 and 2 make clear, many conflicts exist between the cable television frequency spectrum and the frequency allocations used in the open airspace. Conflicts with FAA frequencies are particularly significant due to the FAA's role in aviation safety. In an effort to mitigate these conflicts, the FCC has imposed a number of frequency-offset requirements on the cable television industry.
A second reason is distortion control. As we noted earlier, some equipment manufacturers have tried to get around distortion problems by using various frequency-control and frequency-offset techniques.
Next month, we'll discuss offsets in detail.
"At the Forefront of Technology" will be the theme at the Kentuckiana SBE Regional Convention in Louisville, Saturday, October 18. The Convention will feature three 90 minute Ennes Workshops.
The Kentuckiana SBE Regional Convention will be at the Holiday Inn-Hurstbourne. Cost of the Workshops is $29 for members of SBE, SMPTE and the Broadcast Section of IEEE. and $39 for others.
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 presentation by our keynote speaker, Bernard Wise, President of Energy-Onix, comments from new SBE National President, Ed Miller, and the presentation of the SBE National Awards and Fellowship honor. 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.
The SBE National BBS ended service this past July, after the computer which houses the system went down. Because the BBS has received little use in the last year, it was determined that replacing the computer in order to continue BBS service would not be cost-effective. The SBE Web Site, www.sbe.org, contains a wealth of information about SBE, including the full SBE Job Line text. Members are encouraged to use the Web Page as their 24 hour-a-day source for information about the Society.
The deadline for the November Certification Exam period in local chapters is September 26. Why not plan now to become SBE Certified and receive the recognition you have earned in your profession. Register today to take a SBE Certification Exam.
Exam dates for 1998 have been released. See the SBE website and determine when you would like to take an exam.
For more information about SBE Certification, see your Chapter Certification Chairman or contact Linda Godby-Emerick, Certification Director at the SBE National Office at (317) 253-1640.
Welcome to our new sustaining member:
Thanks to all our Sustaining Members:
Clark Wire and Cable