Russell Brown /
01.04.2009
Originally featured on BroadcastEngineering.com
Getting from here to there digitally

Studio/transmitter links (STL) have been incorporated into microwave radios since almost the beginning of TV broadcasting, mainly because they are cost-effective and reliable. For added reliability, redundant microwave transmitters and receivers also could be installed. Years ago, an audio/video circuit could be run up to the transmitter from the local phone company, but these could be unreliable because patches could be pulled or amplifiers could fail. Depending on a transmitter’s location, there is more than one way to get the transport stream (TS) to its destination.

Current options

Currently, more options than ever exist to get a station’s TS from the studio to the transmitter. Although all of these options might not address every station’s needs, it could be a good idea to explore some or all of them in case an emergency arises that requires broadcasting from a different location or if a current STL link becomes unavailable for some reason.

Microwave

Digital microwave radios are the most common link used by stations; they are reliable and they place the responsibility for maintaining the link in the station’s hands, but the initial cost can be high — especially when redundant receivers and transmitters are added to the system. For digital transmissions, modems are used before the transmitter to convert the digital signal to one that can be transmitted.

BAS (Broadcast Auxiliary Service) is made up of microwave bands authorized by the FCC to transport video and/or audio for internal use by broadcast stations. These are remote pickups from news events as well as STLs and TSLs (transmitter/studio links). BAS has been allocated frequencies in the 2GHz, 2.5GHz, 6.5 GHz, 7 GHz, 13 GHz and 18GHz bands, and all of them are authorized for digital modulation. BAS licenses are tied to a station’s license, so when the station’s broadcast license is renewed so are all of its BAS licenses. This is not the case with private operational fixed service (POFS) licenses, which some stations use for their STL and TSL. POFS encompasses the 23GHz band, and while there are many more bands in the POFS list, they are designated for use by cable systems and other industries. POFS licenses are not tied to a station’s license and must be handled separately; many times, a station forgets to renew a POFS license, which can have disastrous results.

There is a choice of modulation schemes to choose from including FM, VSB, QPSK, COFDM and 16- to 128-QAM. There are also many different interfaces for connecting to the microwave transmitter as well, such as DVB-ASI, SMPTE-310M, DS3/E3, T1/E1 and RS-422 data channels.

Fiber optics

Fiber-optic links have become popular with many stations. Phone companies are willing to forgo an installation charge when the stations agree to a multiyear contract, thus eliminating the upfront costs associated with microwave systems. The drawback is the monthly bill, which over the course of several years would more than pay for a microwave system. Also, there is the chance of “backhoe fade,” in which some of the construction crew winds up cutting the fiber-optic cable out in the field with its backhoe. Once a fiber-optic cable is cut, it can take the phone company anywhere from a day or more to repair the cable and get the signal back on the air. Some stations use both a microwave and a fiber-optic feed to get their signal to the transmitter for even more redundancy.

When fiber optics are installed, a cable with at least 12 fiber-optic strands will be terminated at a junction box, where they are terminated at fiber-optic connectors. From here, jumper cables or patch cords are used to connect to the transmit or receive equipment. Typically, the phone company provides what it calls an “AVS 270” link; this refers to an audio/video service at 270Mb/s, which is the data rate for SD-SDI video with embedded audio. The equipment provided will usually accept either SD-SDI or DVB-ASI inputs and outputs; although, some phone companies will disavow any knowledge of ASI signals and report that they cannot monitor it to check quality. This is not a problem because all broadcasters want is a stream of bits to make it cleanly from point A to point B; it’s not like the old days where distortion in a phone company circuit could cause diff gain, phase or low sync levels.

Besides the phone company, there are many others out there that can provide fiber-optic lines. Many data companies or ISPs have purchased fiber-optic cables or strands, and they may be sitting dark (unused). And although it may take a few connections to make the complete run, there may be multiple fiber-optic paths from the studio to the transmitter. Because many companies are investing in wireless data networks, there could be fiber going directly to a transmitter site. That telecom rack in the transmitter shack may be connect to a fiber-optic circuit that could run right next to the studio. A little research could really pay off for either a backup or even the main link for a station.

As a side note, Comcast cable in the San Francisco Bay Area has been using a fiber-optic ring — two rings, in fact, to connect all the local cities together and provide a single pickup point for local TV stations and national feeds. The ring starts in San Francisco, travels across the Golden Gate Bridge and into Marin County, loops around and comes back across the Bay Bridge. The second ring travels down the peninsula around the bottom of the bay and back up and across the Bay Bridge back to San Francisco. Being a fiber-optic ring precludes any signal lost due to any single break in the fiber-optic cable.

Data circuits

Another option is data circuits, in which the same equipment that is used to connect businesses and the Internet is used to transport the station’s signal. A point to remember is that some data circuits are not real time; they send packets of information that can be stored and reconstructed. In fact, some packets may even take different paths to the destination, thus causing them to arrive out of order and delaying them even more. A dedicated circuit can be used, when available, that provides a point-to-point service and does not go through a switched network.

Radio has used integrated services digital network (ISDN) circuits for years to transport audio over standard telephone wires. ISDN has a bandwidth of 144KB, and this low bit rate is what allows it to be used on standard telephone wiring and switches. But this is not enough for video, and although high-speed data circuits can transport it, the cost of these data paths can be prohibitive.

The most common circuit to use for getting an ASI TS to the transmitter is a DS3 circuit; this will carry 45Mb/s and can be dedicated so it avoids the delays found in other data circuits. DS3 uses either coax or fiber-optic interface (typically send and receive have their own path and cable). These types of circuits can be quite expensive, but they would provide a two-way high data rate link between the transmitter and studio. On a typical DS3 link, there would be enough bandwidth to carry two 19.4Mb/s streams up to and two 19.4Mb/s streams back from the transmitter, in addition to data. If this fit a station’s needs, then it could be a viable alternative to traditional STL/TSL circuits and may be the way TV stations’ data (TS) is delivered to the transmit site in the future as part of a larger data stream.

Acknowledgments

Tim Posnar of StreamQ and Dane Ericksen of Hammett & Edision contributed to this tutorial.

Have comments or questions about this article? Leave a comment or visit our Forum and start a discussion.

You can also find a complete list of past “Transition to Digital” tutorials here.


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