What happened to the days where getting audio to the transmitter site was just a matter of ordering one or two equalized audio loops? Simple point-to-point audio paths have become complex networks joining multiple studio facilities and transmitter sites, sometimes spread over a large area. Local telephone companies have all but eliminated departments that specialized in the creation and maintenance of dedicated analog audio loops. They now have specialized products that support high-bandwidth digital data communications, such as DS-1, DS-3 or DSL. As the price of digital audio encoding/decoding equipment drops, the transmission of digital audio content from studio (or satellite provider) to the transmitter site is becoming a popular trend.
This interconnection of facilities can be extended beyond transporting digital program material. Properly designed networks can also extend access to a central Local Area Network (LAN) from connected remote facilities, thus creating a Wide Area Network (WAN). Many digital telephone switches can also be extended to remote facilities.
Wired or wireless?
Delivering digital audio between two points can be accomplished in either of two methods: wired or wireless.
Most local telephone companies have invested in upgrading their infrastructure to increase their data communications abilities. The problem is that, while high bandwidth data communications products are widely available in most major cites and, to some extent, the suburbs surrounding those cities, they can be expensive. The recurring monthly expenses for these services vary by region depending on the specific transport method (DS-1, DS-3, etc), distance, Inter/Intra LATA issues, length of contract and the Quality of Service (QOS) agreement chosen (i.e. how much downtime is permitted.) In addition, you may be charged for rental of the terminal equipment on one or both ends. The real cost that you might incur, however, is that which may be required to bring service into a building that does not have suitable copper cable pairs or fiber nearby. In that situation, costs for utility construction may run into the tens or hundreds of thousands of dollars, not to mention such possible problems as the need to get municipal approvals and obtain easement rights with other landlords. In data communication terms this is called the last mile problem.
Wireless radio systems eliminate many of these problems with only line of sight and a place to mount external antennas. While wideband wireless microwave systems have been in use for about 60 years, they were expensive and difficult to license. Most of the non-broadcast licensed microwave spectrum used by private and common carriers operated above 3GHz and was regulated by the FCC. Common Carrier microwave operators were regulated by Part 21 and private operators by Part 94. In 1996, both were consolidated into a single Part 101.
Although the use of these frequencies was not intended for broadcast STL/TSL applications, the commission may grant such use providing a proper application and technical documentation in support of the non-conforming use.
Unlicensed microwave radio systems became possible in 1997 when the FCC revised certain rules in Part 15 to allow point-to-point and point-to-multipoint communications using a variety of modulation schemes, including spread spectrum. The FCC permits this operation within three bands: 902 MHz to 928MHz, 2400MHz to 2483.5MHz and 5725 MHz to 5758MHz. These frequencies were used exclusively for Industrial Medical and Scientific applications, also called the ISM bands.
The ISM bands currently host a variety of wireless applications that are gaining in popularity, such as 802.11 and Bluetooth, which permit short-range wireless Ethernet connectivity. Several manufacturers are producing high quality radio systems that permit the reliable transmission of wide-band duplex data for several miles over a properly engineered link.
Kevin McNamara, BE Radio's consultant on computer technology, is president of Applied Wireless Inc., New Market, MD.
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