Find the missing link: Studio to transmitter links

March 1, 2005

In the old days, radio broadcasting meant AM radio and transmitter sites were manned. The studio may have been co-located with the transmitter, or maybe located downtown, so at most there would only be the need to communicate in one direction with one program channel. Even new FM services were commonly located in such as fashion as to share to program source of a sister AM station, with the FM antenna located atop one of the AM towers.

Two things happened in the late 60s and early 70s. First, the FCC approved the use of remote control systems. Second, FM radio just started to come in to its own, and FM transmitters were moved to hilltops, mountaintops or other tall structures so they could be heard by bigger audiences. These changes necessitated several improvements in broadcast technology-- specifically communications pathways that could carry stereo program material (in the case of FM) and remote control information for AM and FM.

The FCC allocated spectrum in the 950MHz band for communications for AM and FM radio stations, and several manufacturers responded with various radio links that could carry monaural programs or composite audio. The same radio links could be used to carry information to the transmitter site for the remote control. The telephone company also provided a means by which transmitters could be remotely located. In the old days, the telephone company would provide a continuous copper connection, often called a loop, from the studio to the transmitter site. With proper equalization it could be used for audio or remote control by the way of its dc continuity.

Not surprisingly, technology changes in the telecom industry--probably been the biggest engine of technological advancement since the beginning of the 20th century--affected the offerings to broadcasters long before changes were seen in broadcast radio STLs. The telephone company converted to digital communications links between facilities long before "digital" became a buzz word. Continuous copper loops became a thing of the past, as did remote controls that used dc continuity. They had to be replaced with slow-speed modems and four-wire data circuits. Audio links left the building as analog, but went from central office to central office via T-carriers.

The first duplex links

I worked in San Francisco during the late 80s when the local telephone company (then known as Pacific Telephone) first offered full T-1 circuits to the end users, thus eliminating the analog last mile. We used a T-1 for KSAN for the first time in 1990.

It was a radical change for several reasons. The first, and most noticeable to ourselves and we hoped our listeners, was that we made use of our own digital audio encoder--a Graham-Patten Systems VAMP 3--and we weren't tied to what the telephone company provided. The second important change was that we could now return audio directly from the transmitter site. Because we didn't have a line-of-sight path, we made use of this link to return an air monitor.

Other broadcasters saw the advantages of having a full T-1 to their transmitter sites and it wasn't long before several manufacturers offered equipment to take advantage of the new telco offering. Graham-Patten Systems (which was a great sounding system, but not modular), QEI, Intraplex and CCS/Musicam USA all became known names for link connectivity. Telos also got in to the game by way of the Zephyr.

Radio links go digital

Radio link manufacturers needed to catch up with the quality provided by digital encoders running over T-1s and they started to do so in the mid 90s. The first digital radio STLs were the existing composite radios that had been in use, except instead of analog, composite audio the radio was fed the output of a high-speed modem that digitally encoded the two program audio channels. Moseley and TFT were two of the early providers of this equipment. About the same time, Dolby appeared with its own entry in the market, a digital radio STL known as DSTL.

There have been rapid advances in the technology of radio systems such as this, and in the last 10 years they have gone from the original analog/digital hybrid to the latest digital radios, some of which can carry three AES data streams without any data compression.

The state of the art

When FM exciters with AES inputs became readily available, there was a new impetus to deliver a station's program audio to the transmitter site in that manner. It has become a trend not only in FM, but AM as well, because newer audio processors can accept AES audio input signals. Now 950MHz radios that can deliver AES audio are commonplace, and in that sense they have caught up with their wire-line brethren.

With the consolidation that has occurred in the industry, it is common for one studio to communicate with multiple transmitters at a given transmitter site, and for that reason radio links that can carry multiple AES data streams have become useful. Whereas their analog ancestors used mux channels to pass control information for the transmitter site remote control, it is now standard for digital radios to have a relatively slow serial data connection for what is essentially the same purpose.

Even with ever increasing types of STL systems, the 950MHz band is often crowded in the larger markets; frequencies are often shared twice and three times. In a market such as Seattle every fraction of the 950MHz band is in use, even though many of the stations are using high-speed data circuits for STLs. Many stations use wireline STLs for the main path and radio links as backups. Prior coordination notice is now required before an application for use of a 950MHz band channel will be accepted by the commission; and so, in many cases, the informal nature of frequency coordination in a given market has gone by the wayside.

Wireline STLs

Products made as wireline STLs have evolved during the last 10 years. Intraplex has been in the market for some time, having originally made equipment that was sold to the telephone companies. Its form factor has been that of a shelf that accepts plug-in modules that accomplish various functions. QEI also made a modular device called the Catlink, which had an interesting feature: a composite signal could be connected directly to its input and a composite signal would come out on the other end. In addition, it could be configured with other modules that provided lower bandwidth audio. There were other manufacturers that made codecs as stand-alone boxes; CCS (Musicam USA) and Telos come to mind.

Later generations of these systems still are in production today, and probably the most common change is the ability to use the full T-1, a fraction of a T-1 or ISDN (in a backup, lower-data rate mode). The coming of IBOC has made the capacity to handle 20kHz of audio bandwidth sampled at 44.1kHz more and more critical. The ability to extend a LAN to the transmitter site has become a hot feature as well.

With many stations operating as part of larger organizations, it is not uncommon for a station to be connected to a WAN, and thus an ever-present data pipeline exists for TCP/IP communications between two or more locations. Depending on the traffic load, some of the data capacity can be used by audio codecs that are connected via Ethernet. This is a relatively new development for wireline STL products. The ubiquity of the Internet has driven their development as well; Internet connections at two or more locations can be loaded with such devices for applications that are not necessarily mission-critical.

Other bands and full-duplex

A relatively recent development now becoming common in radio broadcasting is that of unlicensed radio spectrum. This spectrum is widely shared by different types of services. There are three industrial, scientific and medical (ISM) bands in use by broadcast equipment manufacturers; the 900MHz band, the 2.4GHz band and the 5.8GHz band. The Unlicensed National Information Infrastructure (UNII) band is around 5.3GHz. These combine the idea of free radio spectrum and thus no bills from the telephone company with the functionality of a full-duplex system that was heretofore only available from telephone providers.

There is a major drawback: with more users on the ISM bands, the systems become more likely to be interfered with at some point in time. Fortunately, the use of the spread-spectrum modulation scheme and highly directional antennas diminishes the likelihood of interference. There are also real distance limitations on radio links such as these, and power is limited. See 47 C.F.R. 15.247 for details.

Where are we going with this?

With Ethernet being so common around radio station facilities, it is no surprise that broadcast engineers are taking pains to extend such networks to the transmitter sites themselves. Over the years I have seen many SCA systems that used some sort of computer at the transmitter. That was in the old days though, and most of those communicated via a slow, old modem. Now they can be connected directly with a LAN extension operating over a T-1 or a radio link. Having a computer at the transmitter site is handy for picking up e-mail and looking at manuals on-line.

More importantly, though, we need to be concerned with the ability of an STL system to deliver multiple data streams over and above what we consider to be the station's program audio to the transmitter site. These data streams could be anything from station-specific SCA data, such as now playing info sent over RBDS, to other more specific SCA data and even additional audio channels for NPR's Tomorrow Radio project.

Irwin is director of engineering for Clear Channel Radio in Seattle.

Resource Guide-A sampling of STL systems

A new method of creating point-to-point links, the Broadcast Electronics Big Pipe provides a bi-directional link with up 45Mb/s of data throughput depending on the model. Modular interfaces allow carriage of analog and digital audio, HD Radio data, RBDS data, Ethernet, serial data, video and telephony. Audio can be encoded at 44.1kHz or 48kHz sampling rates. License-free systems operate at 5.3GHz UNII band or the 5.8GHz ISM band. Using 16QAM, 12 radios can be located without interference.

The APT Worldnet Oslo is a 3RU modular frame system that can transmit five stereo channels of audio via a T-1 or E1 line. A failsafe option allows the unit to use ISDN as a back-up connection. It also supports X.21 and Ethernet data interfaces. Audio is encoded with Apt-x at 24-bit resolution and 48kHz sampling frequency. Audio latency through the system is less than 5ms. Each audio channel includes an RS-232 data path. Dual power supplies can be fitted for redundancy. The unit can be controlled via IP or RS-232. A smaller, 1RU version, the Baby Oslo, is also available.

MDO UK offers hardware and software systems to transport audio over IP networks. The Audio TX STL-IP is a 1RU interface to deliver 24-bit, 96kHz audio over existing private networks, such as a LAN, WAN, telco network, T1/E1, wireless networks, satellite and the Internet. The system can accept an external wordclock signal or sync to an external digital signal. It also provides ancillary serial data and contact closures. Each unit can simultaneously transmit audio to six connections and receive audio from one location.

Audio TX Communicator is a software-based audio transport system via an IP or ISDN connection. The software runs on Windows 98/NT/2000/XP/ME. A single stream can be received by multiple users.
+44 121 256 0200

The Energy Onix Tele-Link is a full-duplex, 22kHz stereo transmission system for IP or Internet delivery. One stereo source can feed multiple destinations. The software is loaded on a computer to provide an encoder and decoder at each end of the path. A minimum 128kb/s path is required for operation. If audio is lost, the software can be configured to automatically play recorded audio from the receiver computer. An option is available to provide eight relay and five metering channels. A spread spectrum version is available to encode audio at 16-bit MPEG4 rates from 32kb/s to 96kb/s, 24-bit MPEG4 rates from 128kb/s to 384kb/s and a non-compressed rate. Systems operate at 2.4GHz or 5.8GHz. As many as four stereo channels can be transmitted through the system.

Bext offers three STL systems. All are frequency-agile systems, and the operating frequency can be set from the front panel. Other common features include direct FM to frequency modulation, mono and composite audio inputs and SCA/RBDS inputs and telemetry/remote control connections. The LC series has three SCA inputs and occupies 3RU. Metering displays modulation in peak and peak hold, forward and reflected power and LED status indication for all functions. The unit can operate directly from a 12Vdc supply.

The LD series includes three SCA inputs and occupies 2RU. Metering is provided on an LCD screen for operating frequency, modulation, forward and reflected power, voltage, current, operating temperature, and alarms and status info.

The LJ series features two SCA inputs and occupies 2RU. Metering of modulation as peak or peak hold, forward and reflected power is provided, as are LED status indicators for all functions.

The LD and LJ have an optional stereo generator/endcoder module.

The CDX encoder/decoder provides two or four channels of digital audio via a composite STL link.

Marti manufactures the STL-20 in a mono (STL-20M) and composite (STL-20C) version. Both are capable of transmitting 20W and are frequency-agile. The front panel features a power switch, power adjustment and a test meter switch. The test meter can display forward and reflected power, PA current, subcarrier level and the power supply. A peak hold bar graph meter shows the modulation level. A 15-pin DB connector provides remote control and remote power metering. Modulation is by direct FM synthesis.

Long-time STL manufacturer Moseley offers several options for STL connections. The PCL6000 series is a frequency-agile, analog 950MHz system for discrete or composite operation. The series includes the PCL6010 transmitter and PCL 6020 and PCL6030 receivers.

One of the earliest offerings for a digital STL link was the DSP6000, which provides a digital path on an analog composite link. Using Layer II or ADPCM encoding, audio channels and a data channel can be sent via an analog composite STL system. The V.35/RS-422 interface allows it to be used with digital circuits as well.

The Starlink is the digital platform for several digital STL configurations for 950MHz or T-1/E1 use. It transmits linear encoded audio via QAM. The SL9003Q accepts a stereo analog composite input or six discrete audio inputs. The SL9003 Composite provides a direct digital replacement for analog composite STL systems. The Starlink SL9003T1 can be configured using a variety of plug-in modules to transmit audio, RS-422, Ethernet and E&M signals.

The Lanlink 900 is a long-range, high-speed wireless IP/Ethernet/IP facility controller that can use an existing 950MHz STL antenna infrastructure for LAN and RS-232 connectivity. It uses 902MHz to 928MHz FHSS spectrum to provide a 512kb/s data rate.

Nicom offers the TSL 910 and RSL 900 950MHz system. At 1RU for each unit, this system offers 12W of transmitter power, remote control via RS-232, frequency agility in 25kHz steps, three SCA inputs, an RF monitor jack on the transmitter front panel and an IF monitor on the receiver front panel. All functions are accessed via the LCD screen, and access is password protected. The system includes monitoring software. The unit can be run from a 15Vdc supply.

The MT/MR Platinum STL from OMB features a frequency-agile, microprocessor-controlled transmitter. An LCD shows operating parameters such as frequency, forward and reflected power, modulation level and pilot signal. The system accepts balanced mono and composite audio and three SCA inputs. It includes SWR fold back protection. The operating frequency is adjustable in 10kHz increments. The unit can be remotely controlled via an RS-232 port for remote control of frequency and power.

The Superior Broadcast SBP1200M is for 950MHz use. The transmitter and receiver each occupy 3RU. A selectable multimeter displays all operating parameters of the 10W system.

The Xlink is Armstrong's new generation of STL systems. This microprocessor-controlled system is frequency agile and requires no tuning across the entire STL band. It has improved sensitivity and better selectivity than previous units from the company. The front-panel LCD display provides control and monitoring of system parameters, plus the Xlink is remote-control ready. It includes composite and mono inputs.

The DTX and DRX make a digital encoder/decoder system for use with a composite STL. The system provides a two-channel 15kHz path and an RS-232 path. Audio is sampled at 32kHz and encoded with sub-band ADPCM at 256kb/s. Each unit occupies 1RU.

Harris offers two STL systems. Using a 950MHz path, the CD Link is a digital STL that can deliver a 16-bit, AES-3 signal and two RS-232 signals. As an option, it can support an additional two 6kHz audio channels or one 12kHz audio channel. The self-contained unit does not require an external modem of interfaces.

The Intraplex STL HD is a T-1 multiplexer that offers a bidirectional connection via various types of T-1 circuits, including private, public leased, microwave, spread spectrum radio and T-1 subcarrier over video microwave. Audio can be sampled at 32kHz, 44.1kHz or 48kHz. Various modules can be installed to provide several types of audio paths with variable quality, telephone signaling, Ethernet and data connections. Harris also offers the Aurora spread spectrum radio for wireless T-1 links.

For IP applications, the Intralink IP connects via IP networks.

The TFT 460/467 offers six uncompressed program channels and a data channel. The transmitter provides a 2W output and accepts audio at sample rates of 32-, 44.1- and 48kHz. The RF system is frequency agile and set by software in the transmitter and receiver. The transmitter and receiver can be remotely controlled over an Internet or LAN connection. The receiver offers a threshold sensitivity of 84dBm for 256 QAM.

The 9100 series and 9200 series STL systems are similar in form. The 9100 is a composite system, while the 9200 is a mono system. Both are frequency-agile systems. The transmitters and receivers feature a multifunction test meter to verify proper operation. An optional SCA/MUX encoder or decoder can be installed into the system. An automatic switchover is built-in to the 9107 receiver.

The DMM92 is used to transmit digital audio via a composite STL. It is available in two versions: the DMM92-75 and the DMM92-100, which can transmit two or four audio channels respectively. Both can accept an optional capability to support a 32kb/s data channel.

Musicam USA provides two units for wired connections. The Team is a T-1 or E1 audio multiplex unit that features a modular frame to support several modules. Each unit has a control processor with Ethernet port and an ac power supply. An optional 60V dc power supply or second ac supply may also be installed to provide redundancy for the primary power supply. The seven user slots can accept stereo codec, stereo encoder, stereo decoder, X.21, V.35 or ISDN modules to customize each system.

The Superlink can connect via LAN, WAN, DSL, ATM, ISDN, T-1 or E1. Interchangeable function modules support many applications, while user configurable hardware and software control the DSP-based audio codec that supports all the popular compression algorithms. The 2RU unit accepts three of the same modules that are used in the Team.


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