Links Look Beyond Part 74 to Part 101

With the advent of HD Radio and other new technologies, connectivity has become an important issue.

Many transmitter sites are isolated and do not have access to broadband land lines. The need for studio-to-transmitter, transmitter-to-studio and studio-to-studio connectivity in a consolidated, HD Radio environment has brought challenges that present-day FCC rules do not adequately address.

Broadcasters anticipate the FCC will re-examine Part 101 rules with possible favorable amendment to meet the present and future connectivity requirements.

The Thursday morning NAB Radio Show session “High-Bandwidth Capacity RF STL/TSL Connectivity” features Lawrence M. Miller, senior partner of Schwartz, Woods & Miller, and James Moody, senior consultant of James Moody and Associates. They will address the issue of connectivity from a legal and technical standpoint.

Prior to ownership consolidation and IBOC, broadcasters used telco loops or Part 74 STL to deliver programming from the studio to the transmitter. Remote control may have used a subcarrier on the STL for commands and an SCA subcarrier for telemetry return. A composite or dual mono system was used for FM or AM stereo.

With station clusters and IBOC, broadcasters’ needs and requirements have increased exponentially.

FCC Part 74 rules are too restrictive and were originally written prior to ISDN, T1, fiber and other technologies. Cluster markets, HD Radio, transmitter-to-studio video security, telemetry monitoring and control and other demands have strained the number of required frequencies.

A single AES audio pair with two channels at 16 bit resolution sampled at 44.1 kHz requires 4.233 Mb per second with 3x oversampling (1.411 Mbps without oversampling) and a single 950 MHz channel does not have sufficient resolution bandwidth to accommodate this.

The demands of HD Radio require an AES stream sampled at a rate of 44.1 kHz and a 400 bps Ethernet stream. The AES stream will be transformed into time-aligned analog and digital audio to be sent to the analog audio processor and IBOC exciter, respectively. The Program Service Data (PSD) will be separately delivered on the 400 bps Ethernet stream. If a Supplementary Program (SPS) and Advanced Application Services (AAS) data are employed, separate deliveries are also required.

While it is possible to deliver these components individually using a 950 MHz STL, no bi-directional path is provided. It may be possible to use a LAN extension in the 902–928 MHz ISM band; however, this spectrum is quite crowded and no protection from interference from unlicensed devices is provided. A minimum of three times oversampling is important to delivery of HD audio which is very bursty.

Wired options are also available, albeit with significant cost. A DS3 circuit is equal to approximately 672 voice-grade telephone lines and is made up of 28 DS1 or T-1 lines each operating at a total signal rate of 1.544 Mbps. Another equivalence would be seven DS2 or T-2 lines.

A DS3 provides up to 45 Mbps of connectivity. There are two monthly fees associated with DS3: the loop charge and the port charge. The loop charge varies with distance from the network and also with providers. Typical costs range from $4,000 to $16,000 per month. An OC-3 optical circuit offers 155 Mbps, or the equivalent of 100 T-1 circuits and has costs ranging from $20,000 to $45,000 per month. Obviously the costs are not reasonable for typical broadcast use, even for multiple co-located HD stations.

Broadcasters may employ Part 101 frequencies. FCC Part 101 rules were originally written for the common carrier industry; however, broadcasters may use certain bands with restrictions.

Available bands include 17,700–18,580 MHz, and frequencies above 21,200 MHz. The restriction is given in 101.603 (a)(7), where it is stated “Licensees may transmit program material from one location to another, provided that the frequencies do not serve as the final RF link in the chain of distribution of the program material to broadcast stations ...” These restrictions are not implemented to connectivity of studio to studio or intercity relay circuits. They only apply to the final circuits of RF connectivity carrying program content.

Part 101 frequencies are quite high compared to the more familiar 950 MHz Part 74 frequencies. Antennas, transmission line and path considerations are more critical. The “five nines” of reliability must be considered. This means a goal of 99.999 percent reliability is desired.

Antennas for these bands usually are of parabolic construction. Weight and especially windloading of such antennas must be considered when mounting on a tower or other structure. Sufficient Fresnel Zone path clearance must be present. Path reliability calculations must consider rain fade, k factor, fade margins and vegetation growth.

Case in point

Fig. 1 shows a multi-station installation in Boston. This system was in use until interference from other users caused it to be unreliable. A new system, shown in Fig. 2, was designed and implemented, albeit at considerable cost.

Paul Shulins, Greater Media director of engineering in that city, says “This system worked well for a number of years, but as more and more broadcasters got the same idea (especially in larger and more crowded markets), these non-licensed entities started to take interference hits from other users and that made the service unreliable for all of us. Additional bandwidth requirements of HD Radio demanded we explore other options.”

After careful consideration, a new system consisting of Part 101 bands and FCC waivers was constructed. The waivers were required for operation on frequencies where the final link of program delivery was prohibited. The cost factors included research, attorney and consultant fees.

Shulins elaborated, “Although the systems are expensive, and a bit of work to license, the benefits of having a reliable and protected wide pipe between sites can be easily appreciated in today’s HD Radio environment.

“We decided that for our facility where we are running five HD2 signals at the same time and from the same place, getting the network traffic from our studio to our transmitter sites was going to require more bandwidth than our existing spread spectrum radios could offer. In 2007, after doing frequency coordination, we ordered licensed radios installed them in late 2007. I have to say that so far, we have had excellent success in terms of reliability and throughput.”

Another important consideration is computer network configuration at the studio facilities.

“In some cases segregated V-LAN configurations are becoming necessary to isolate traffic requiring a higher quality of service from routine non-critical traffic,” Shulins continued.

“My facility in Boston just went through the process of reorganizing the network to allow for prioritized mission-critical traffic. The process was painful, expensive and involved, because many of the switches and the network infrastructure needed to be modernized. Just about every computer on our network needed to be assigned a new IP address that made sense, and was consistent with our new V-LAN structure.”


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