Module 65
Updated: 09/01/2005
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Module 65 |
Updated: 09/01/2005 |
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Microwave,
It does little good to have a great news story if you can't get it back to a network or local station for broadcast. To do that we need a knowledge of the things discussed in this module. Although it's being slowly replaced by fiber optics, especially
for transporting TV signals over significant distances, coaxial
cable is still the medium of choice for simple video connections and for
many CATV systems. On the right of this photo you can see a standard coax connector, and to the left a cutaway view of the single copper wire inside. Note that a white insulator surrounds the central copper wire, and that this is surrounded by metal foil. Above this there is electrical shielding consisting of layers of braided wire, and, finally, a rubberized coating. Today, triax, Two other types of coax-based connectors are shown below. On the left is the RCA connector, which is also used for audio, and on the right a professional BNC video connector.
Although coaxial cable has been used for decades to conduct TV signals, it has a number of shortcomings. Topping that list is the need to constantly re-amplify signals with the various problems that introduces.
The medium that has many advantages over coaxial cable is fiber optics (also called optical fiber or OF). The medium of transmission is light. Light waves have an The thickness of an optical fiber is only slightly larger than a human hair. The photo on the right shows a light-conducting OF strand going through the eye of a needle. The tiny, flexible glass or plastic fiber is coated, Fiber optic cables normally carry numerous OF strands within a single enclosure. Compared to a coaxial cable, optical fiber has ten advantages:
As cable and telephone companies continue to move toward optical fiber, eventually home-to-TV-studio video transmissions will become as simple as hooking up your video equipment and dialing the right number.
In much the same way that a flashlight projects a beam of light from one point to another, microwaves can be transmitted along a straight, unobstructed line from a transmitter to a receiver. In the process the microwave beam can carry audio and video information. Microwaves were originally only used in broadcasting for coast-to-coast network television and for studio-to-transmitter links. However, as remote broadcasts became more popular, TV stations saw an advantage in having field production trucks equipped with microwave dishes so that athletic events, parades, civic meetings, etc., could be covered live.
Small, "short hop," solid-state
microwave transmitters and receivers, such as the one shown above, can
be mounted on lightweight tripods to relay TV signals from the field to
a nearby TV production van. Microwave signals must have a straight, line-of-sight path. Solid obstructions, or even heavy rain, sleet, or snow, can degrade or completely obliterate the signal.
The photo on the right shows an extendable mast with a microwave transmitter on the top used with production vans in covering news stories. The microwave signal can be aimed at a local station receiver, or the signal can be beamed to a relay receiver to be re-transmitted one or more times until it reaches its final destination. The inside of a remote production van is pictured below. The van is a mini-production facility with camera control units, audio and video recording equipment, a satellite receiver, and videotape editing equipment.
Vans, Boats and Airplanes Although normal microwave signals go in a straight line, it's possible to use an omnidirectional (nondirectional) microwave transmitter to send a signal over a large receiving area. With this approach signals can then be sent from helicopters, moving cars, motorcycles, boats, etc.
Although not in wide use, there
is a point-to-point transmission system that offers a number Canobeam is multi-channel and bi-directional, and can send audio and video signals more than one mile or about 1.6 kilometers with a quality that exceeds most microwave equipment. It has proven especially useful in countries where delays are common in getting permission to use microwave or fiber optic lines in covering news. The same principle is used in Terabeam, a system being used to exchange e-mail and business data between businesses and buildings in large cities. Although heavy fog, rain and snow can disrupt these beams, the systems have a number of reliability features built in that can take over and compensate for most problems. Satellites hovering about 36,000-kilometers (22,300-miles) above the earth relay most television programming to world viewers. Each satellite or "bird" is composed of a number of transponders, or independent receive-transmit units. Geosynchronous satellites rotate at
the same speed as the earth and end up being stationary in relation to
the earth's surface. This obviously simplifies the job of keeping them
within the range of both the uplink and downlink The reflector dish of a ground station uplink is shaped like a parabola, which is similar to the reflector of a powerful searchlight, the kind that can send a sharp beam of light into the night sky. Signals reflected from the center element (note photo on the left) will hit the dish and then be sent upward on their 36,000-kilometer (22,300-mile) path to the satellite.
The signal from an uplink ground station is aimed along a precise path to the appropriate satellite. As illustrated on the left below, once the signal is received, it's amplified, the frequency changed, and then it is sent back to the earth. The footprint (coverage area) of the returning signal covers many thousands of square kilometers or miles of the earth's surface.
Within the footprint area, receiving dishes work in reverse of the uplink ground stations. The signal from the satellite is collected in a dish and directed toward the receiving element, as shown on the right above. This signal is then amplified thousands of times and fed to a TV receiver. There are two classifications of satellites used in broadcasting:
C-band was the first satellite
frequency range to be widely used in broadcasting. Although dish size is not a major issue with permanently mounted installations, C-band dishes impose limitations for SNG trucks. (Satellite newsgathering trucks or SNG trucks are vans that have been especially outfitted to uplink ENG stories to a satellite.) Compared to Ku-band, C-band is more reliable under adverse conditions-primarily in heavy rain and sleet. At the same time, C-band frequencies are more congested and more vulnerable to terrestrial interference.
Because of the higher frequencies involved, KU-band dishes can be about one-third the size of C-band dishes. Because Ku-band also has fewer technical restrictions, it means that users can quickly set up satellite links and start transmitting. This is obviously an important advantage in electronic newsgathering.
Networks and TV production facilities
routinely distribute their programming via satellite. This is how TV productions
originating in the Los Angeles-Hollywood area Once they arrive on the East Coast they are recorded, scheduled into the network agenda, commercials are typically added, and then the programs are beamed back up to satellites for distribution across North America. When the network-to-affiliate link is not being used to relay regular programming, it's used to send news stories, program promotion segments, and other broadcast-related segments to affiliated stations. Stations not affiliated with a network can receive news and information from satellite services such as CNN. Cable (CATV) companies also receive most of their programming
from satellites. This includes both TV and audio services. Many
TV and audio services (satellite "stations") are not broadcast over the
airwaves, but are only available directly from satellites. Satellite-to-Home Services For people living in rural areas out of the range of local TV and CATV services, a satellite receiver may be the only way they can get TV programming. The basic satellite-to-home services are available on C-band and Ku-bands. Other satellite-to-home services, such as the Dish Network and Direct-TV, use their own satellites and frequencies. Although much of the programming on C-band is free, there are many scrambled channels, which means they are encrypted and only available by paid subscription. Pay-per-view movie channels and digital satellite-to-home
services work in this way. Once fees are paid, the unique identifying
number in your satellite receiver is uplinked along with the satellite
TV signal. When this signal is received by your home satellite receiver
this identifying number unlocks Satellite-to-home services, such as the Dish and Direct-TV, have their own satellites and home receiver systems. These services have a capacity of at least 50 simultaneous digital TV channels. Services such as Dish-TV may require two or even three receiving dishes aimed at different satellites. Unlike the C-band and Ku-band services, once you install this type of receiving equipment, you are locked into that particular service-and the monthly subscription fees that go along with it. Although many satellite services are scrambled (subscription based), there are several hundred free TV services ("stations") available on C and Ku bands. These include:
C-band satellites, which typically carry 24 TV channels, have names such as Galaxy 9, Satcom C3 and Morelos 2. For example, the Florida Sunshine Network is on Satcom C1, Channel 24. Because of the limited life of satellites (not to mention their occasional malfunctions), C-band and Ku-band satellite assignments occasionally change without notice. Several newsstand publications are available which represent a type of "TV Guide" for home satellite viewing. Although most satellite TV programming is in English, Spanish or French, satellite programming is also available in dozens of other languages. A single C-band or Ku-band satellite channel is capable of carrying both a TV signal and one or more separate audio channels. Taking advantage of this fact are more than 100 free audio services, most in stereo and many without commercials. Some are standard broadcast stations that distribute their signal by satellite. Examples are CBM-AM in Quebec and WQXR-FM in New York. In recent years, many C-band and Ku-band satellite services have moved from analog to digital signals. This has made it necessary for many home viewers to upgrade their satellite receivers. In late 2001, satellite radio was launched in the United
States. More information on this can be found
From the large commercial satellite services we now turn to point-to-point satellite applications used in electronic newsgathering. News agencies have been using satellite phone links to send audio and video from remote locations-generally from third-world countries where standard satellite services are not readily available. Although satellite phone links were originally just intended for voice transmission, it was found that a highly compressed video signal could also be sent on the audio channel. Even though the quality of satellite phone links leaves much
to be desired, a satellite phone system is small enough to be put in the
overhead bin of an airplane and, once in the field, it can be set
up quickly. This is not the case with -
In the late '80s portable, freestanding satellite uplinks referred to as flyaway units were introduced for electronic newsgathering (ENG) work. (Note photo on the right.) These units can be disassembled and transported in packing cases to the scene of a news story. Flyaway units are used in remote regions, including offshore areas and third-world countries. Unlike satellite phone links, the flyaway units provide full quality video and audio signals..
Once Cybercafes or Readily-available solid-state drives called USB drives, jump drives, or sometimes thumb drives can be used to store compressed video before it's sent. In fact, a complete and relatively high-quality news segment can be stored in the two-gigabyte storage device shown in the photo. Using the Internet in this way not only saves time and is of higher quality than the satellite phones, it eliminates the need to rent expensive satellite time. |
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three-conductor video cable, is often used to meet extra video needs, instead of
the coax (two-conductor) video cable shown above. A cutaway version of triax
is shown on the left.
extremely high frequency and travel at 186,000-miles (300,000km) per second. A single
OF cable can theoretically carry trillions of bits of information every second.
both
for protection and to enhance its characteristics as a reflective lightwave guide.
The
information carrying capacity of OF is thousands of times greater than a normal
copper wire. Note on the right the comparison between an optical fiber link and
a telephone cable with its hundreds of wires. Both have the same information carrying
capacity.
The
van then sends the signal to one of the city's relay points - generally
on top of a tower or tall building - and the signal is then sent to the
studio or production center.
of
advantages over microwave. Since the Canobeam system relies on a
high-powered light or laser beam to send its signal, no FCC license or
special permits are necessary. 
dishes
on the earth.
Compared
to Ku-band, C-band requires relatively large receiver and transmitting
dishes.
are
sent to the East Coast for network distribution.
the
signal so it can be displayed on your TV set.
the
story is on a tape, disk, or in a solid-state memory card, it can be sent
through the camera's Firewire or USB-2 connection to its destination
via a high-speed Internet connection. 