Studio to transmitter links
Studio-to-transmitter links, and to some degree inter-city relays are familiar elements of most radio stations. They allow stations the flexibility to build their studios in locations that may be miles from the transmitter, allowing certain creature comforts or marketing opportunities. To all but those who must maintain them, they are a silent and sometimes forgotten step in the transmission chain.
The term studio-to-transmitter link (STL) refers to the transmission path that carries the station's program audio. There are variations to this concept, including inter-city relays (ICR) and transmitter-to-studio links (TSL), which are closely related and have essentially the same function: to transport high-quality audio signals between two fixed points.
While TSL can refer to a data-return path used for a transmitter remote control, these links are better described as telemetry return links (TRL) and are limited in frequency response to less than full bandwidth.
With or without wires
The legacy form of an STL is a wired path. Traditionally supplied by the local telephone company, an analog wired circuit was once the most common form of STL. The most basic type of circuit is a dry pair. This is a straight-wire path between two points. There are no active or passive devices between the source and destination. This type of circuit is also sometimes called a burglar-alarm circuit because it is commonly used for signaling by monitoring companies. Because there are no transformers or equalizers, these lines can pass ac and dc voltages. Their frequency response can be unpredictable and will suffer at long distances.
111C repeat coils and equalizers are a common sight for telephone company circuits.
It is possible to compensate for the line losses of a dry pair with external equipment. Transformers, line drivers and equalizer cards can be installed on each end of the circuit to provide a smoother frequency response. Some of this equipment can be found through surplus outlets. Newer versions of this transformer are available, as are quieter and more reliable amplifiers and equalizers. By equalizing a line personally, the ongoing cost of essentially renting the hardware from the telephone provider is saved.
Telephone companies can also supply equalized audio circuits. Such circuits were once the mainstay of radio audio circuits, but it can be difficult to order one of these lines today. These circuits are engineered by the telephone company to deliver an established connection at a specified frequency response. These lines were typically supplied with a frequency of as much as 8kHz for AM use or 15kHz for FM use. Depending on the distance, equalized circuits may have active components throughout the signal path to compensate for losses in the path.
Reliable performance of telephone-company supplied equalized lines can be problematic. The main drawback is that a station must rely on an outside party to provide the service. The established telephone network is far-reaching, but it is susceptible to problems from climate, human error and equipment failure.
As digital data circuits have increased in use and popularity, the telephone companies' analog services have become harder to order. In many cases, the local phone company may not have an installer who can even establish an equalized circuit. Most telephone companies are trying to get out of the analog circuit business as well. Because of this, the cost to install and maintain an analog circuit may be excessive. However, in some cases, these circuits may be the only option.
T-1 multiplexers have proven to be reliable and cost-effective STL systems.
Digital telephone circuits have increased in popularity and availability and in some cases decreased in price. Telephone companies have relied on digital circuits for their own backbone for some time, and they have made these services available to businesses and consumers.
Wired for digital
Dial-up digital circuits, including ISDN and Switched 56, were once expected to replace the analog dial-up services we still use everyday. While this has not happened, these services are readily available and cost efficient. ISDN service is routinely used for remote broadcasts and audio contribution. It can be used for an STL, but ISDN service is typically billed on a timed basis, so unless a flat rate can be contracted, it is not a good choice for permanent use. It is, however, ideal for backup and emergency use.
The high-capacity digital services from the telephone company have found widespread acceptance in radio. T-1 circuits, which are capable of passing 1.544Mb/s of data, are still subject to ongoing monthly costs, as are analog lines. The advantage is that the higher capacity can justify the cost. In addition, T-1 lines are bidirectional. The latest boom in telecommunications services has increased T-1 accessibility, and providers other than the local telephone company can offer service.
T-1 service is offered in North America. Outside North America a similar service, called E-1, is available. E-1 provides a 2.048Mb/s data pipe and uses a slightly different encoding scheme, but the basic principle and service is the same. Equipment designed for one of these circuits can usually function with the other without too much difficulty.
To use a T-1 circuit, some type of multiplexer must be installed on each end of the circuit. The multiplexers offer a variety of options for users to customize the type of signals that can be sent. Linear, PCM-encoded stereo audio with a 44.1kHz sampling rate and 16-bit resolution will occupy 1.411Mb/s of bandwidth. If linear audio is sent to the transmitter site, there is just enough spare bandwidth for remote control telemetry, with a few bits to spare. Most manufacturers offer various encoding cards in addition to those for linear encoding. These encoding options include various forms of MPEG compression, Apt-x and even composite stereo.
Another advantage to T-1 circuits is that most telephone companies can install and maintain T-1 circuits more easily than equalized analog lines.
One additional link in the wired family doesn't even use wire. For our classification purposes, fiber-optic equipment can be considered here because it requires a physical connection. (Instead of wired and wireless, we could have said non-RF and RF.) Fiber-optic encoders provide the same advantage of a T-1 multiplexer system by providing a high-bandwidth, bidirectional pipe. An additional advantage to fiber-optic systems is that they are immune to electromagnetic and electrostatic problems. Lightning and electrical surges are impossible. Ground loops are eliminated.
Digital STLs such as these are a good first step in upgrading to an all-digital air chain.
Unfortunately, end-to-end fiber connectivity is not available in most places. College campuses and business parks may have fiber capability, and a fiber-optic STL may be a possible solution. Another use would be for a long cable run, perhaps to a rooftop from a ground-floor studio.
Along this line, there are systems that use CAT5 cable as the wire medium. Again, distribution within a campus or other controlled space lends itself to these systems. CAT5 and fiber systems are ideal for covering spans between a studio and a tower or antenna farm installation. The send path can carry program audio, IFB audio and transmitter remote control. The return path can carry transmitter telemetry, RPU audio and satellite feeds.
The wired STLs mentioned so far all work with private connections. The connections may exist through a wider network, but the connection is still isolated in some way, and constant bandwidth or frequency response is guaranteed. There is one system that uses the Internet as its medium. The Energy-Onix Tele-Link uses a 126kb/s or greater Internet connection to provide a 22kHz audio path that can be received by as many as five online decoders.
Breaking the tie
The trend in STLs has come full circle for wired systems. The effort to eliminate the telephone company from the transmission path resulted in the introduction and subsequent extensive use of wireless systems. STLs in the 950MHz spectrum are commonly used for mono, discrete-stereo and composite-stereo applications. This small chunk of RF real estate has become so crowded that some cities are unable to coordinate new users. This congestion, coupled with the increased cost efficiencies of wired systems, has pushed some telco-based systems back into popular use, but RF STLs are still practical when the spectrum is available.
Short distances can be covered with CAT5 and fiber-optic multiplexers.
Analog RF STL systems have not seen much change since they were first introduced. Like most technology, improvements in hardware and electronic design have made them more efficient, cost-effective and reliable.
The transition to digital has also been introduced to RF STLs. With this transition come the usual advantages of digital technology. The audio path can be less prone to noise and interference.
There are a variety of digital systems from which to choose. Digital paths can be linear or data-reduced with a popular coding algorithm. There are also digital codecs to transmit digital audio over a conventional composite STL system.
An efficient modulation technique, quadrature amplitude modulation (QAM), provides a way to encode audio into a limited carrier frequency. By adding a level of data reduction, as many as six channels of audio can be transmitted on a single carrier. With linear encoding, two or four audio channels can be sent.
Transmitting multiple channels in the space where one was previously sent has helped relieve some of the RF congestion, but with increased capability comes increased demand. Digital STLs offer additional noise rejection from adjacent carriers, so it may be possible to add a few extra channels into a congested area. Some stations use the additional capacity for backup links to sister stations or as one-to-many links when the receive sites lie in a common path.
In areas of RF congestion, spread-spectrum links may offer a workable solution.
One caveat to digital STLs in the 950MHz band is that the FCC is currently not licensing digital modulation. Stations can apply for a waiver and STA to use a digital STL, but this STA must be renewed regularly until the rules are changed to allow digital modulation. Keep in mind that the emission designation for a digital STL is different from an analog STL. The FCC has a pending rulemaking that will likely change this, but that is still several weeks away.
Converting to a digital STL offers advantages, but it is best to coordinate its use. You may be licensed for the frequency, but the digital transmission occupies almost the entire RF mask, whereas the analog transmission is concentrated at the center of the carrier. A digital STL may interfere with analog STLs on adjacent channels.
While 950MHz has been the popular space for aural STLs, the FCC also permits aural STLs in the 18GHz band. Analog transmission on 18GHz is not practical because of the excessive phase noise, so digital modulation is preferred — and allowed by the FCC. This band is not extensively used by radio broadcasters.
In addition, aural STLs can be used in the 23GHz business band by licensing their use for a private business. Both 18GHz and 23GHz STLs work well for distances up to five miles with few problems. Distances up to 10 miles can be covered with larger antennas. Longer distances push the limits of the spectrum, and the entire frequency band can be disrupted by heavy rain.
When RF congestion in an area is too great, spread-spectrum transmitters may offer a suitable alternative. Operating in ranges from 2GHz to 6GHz, these systems fall under FCC Part 15 rules and do not require a license. The transmitter power is limited to less than one watt, but high-gain, directional antennas can provide a useable distance of several miles.
There are a variety of choices for an STL that go beyond the traditional conceptions. As with any equipment decision, it pays to evaluate the available choices and choose the system that offers the greatest flexibility while still meeting the demands and restrictions of the installation.
Thanks to Dane Ericksen of Hammet and Edison for providing some of the information on RF systems.
A schematic and pinout diagram for the 111C is available at in the Engineer's Notebook.
Substitutions on the fly
Stations strive to have complete redundancy in on-air systems to minimize or even completely eliminate lost air time. The STL path deserves the same attention to system backup, making the transition automatic if possible.
Some digital exciters have the built-in capability to accept more than one source and automatically switch to a backup source when the primary one fails. If this is not available, a station can build its own switching device using squelch contacts from receivers, loss-of-data alarms from digital systems or silence sense alarms. Commercially produced products such as the SBS Guardian II are available to handle this automatic switching.
With the convenience and affordability of digital storage systems, an auxiliary audio source can be included in the STL backup chain. Minidiscs, CD players or devices such as the Digigram Hitplayer work well in this application.
An inexpensive, stand-alone automation computer system can be pressed into service. To reduce wear on the system, it could be configured to boot and await a play command when the last link is switched to air.
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