Spread Spectrum, T1/E1 Circuits, the Internet and Lasers Accommodate a Redefined STL Criteria
Many broadcasters use some sort of studio-transmitter link, or STL, to get program material from one studio to the other. This connection traditionally was made with 15 kHz equalized lines provided by the phone company; 950 MHz microwave links provided an attractive alternative, particularly for remote mountaintop locations, although line-of-sight has always been a consideration.
The digital revolution has changed the requirements for STLs. Existing STL microwave systems lack the capacity to transfer program and data for HD Radio. Also, traditional microwave gear does not answer the need for bi-directional communications.
Alternate technologies – such as spread spectrum, T1/E1 circuits, the Internet and fiber and optical connections – are being deployed to meet these demands.
Numerous vendors are active in the area of transporting program material; many offer unconventional or new-technology approaches. They include Broadcast Electronics, Harris/Intraplex, Moseley, TFT, Musicam USA and Energy-Onix. Talk to your favored vendor to learn about the options. We sampled opinion for this article.
For stations with line of sight to their transmitter sites, spread-spectrum communication may be an attractive option, particularly when the 950 MHz band is overcrowded.
Bernie Wise, president of Energy-Onix, lists the advantages: “Spread spectrum on the 2.4 and 5.8 GHz bands is unlicensed, so there are no FCC charges, and no monthly bills from an ISP. The service is full-duplex, with the potential for multiple T1 capacity.”
Each link of the Energy-Onix Tele-Link system can provide four uncompressed AES/EBU, or balanced 22 kHz analog stereo channels. Such systems have a theoretical range of 40 miles, although eight to 10 miles is a more realistic figure for reliable service.
Wise adds that products such as the Tele-Link can select non-compressed or several compression rates from 32 to 384 kbps. A remote control option can provide control of eight relays and transmission of five analog meter readings.
In the event of a loss of connection between studio and transmitter, or prolonged silence, the system can provide program fill from a hard drive at the transmitter site, and automatically page station personnel.
Wise said the Tele-Link system is popular where signals must cross the Mexican or Canadian borders. In the case of crossing the Canadian boundary, microwave links are illegal, and connecting with both U.S. and Canadian phone companies can cost up to $2,500 per month. To date, 100 Tele-Link units are in the field.
One disadvantage to systems such as the Tele-Link and other spread spectrum devices is the delay incurred, which can be four to seven seconds, although Wise said efforts are under way to reduce this figure.
Where line-of-sight connections are not feasible, a dedicated T1/E1 circuit is a popular option.
Art Constantine, vice president of business development for Musicam USA noted several advantages.
“T1/E1 service enables users to send bi-directional CD-quality uncompressed stereo audio over a 1..536 Mbps pipe.”
He adds that rates for full T1 service traditionally have been high, but are becoming more affordable from some providers. Because T1/E1 circuits are synchronous, with a clock at each end, the amount of delay is minimal.
Constantine said that products such as Musicam USA’s TEAM enable broadcasters to connect to several T1 lines for multicasting. Open slots are available for a variety of user-selected modules, including encoders, decoders and T1 and E1 multiplexer modules, as well as ISDN and RS-422 modules. Compression algorithms can be assigned to different cards for maximum flexibility.
T1/E1 solutions typically will auto-connect in the event of an outage. Should the dedicated data line be disconnected, the device can dial an ISDN connection automatically. Constantine said this system still leaves broadcasters vulnerable to a power supply failure in the card cage. A more thorough solution would be to have two separate T1/E1 units connected to data lines with different paths to the transmitter.
Codecs, IP and climate-proof lasers
When the expense and bandwidth of a full T1 is not necessary, a 128 kbps synchronous data circuit with codecs at each end provides an attractive option. Such a system can provide bi-directional stereo audio and eight contact closures, along with an RS-232 connection for ancillary data. Dave Chancey of Moseley said the downside for these types of connections is the need to compress audio.
While there are standards for data compression, a certain subjective sound characteristic exists regarding the way different manufacturers program the compression algorithms. Constantine said that if a codec with a superior compression algorithm is on the transmitting end, an overall improvement in audio quality may be noted.
As a backup, should the dedicated data line fail, most devices can automatically dial an ISDN connection.
IP, or Internet Protocol-based systems, are gaining in popularity.
“The world is moving towards IP systems for audio transmission. The only operating cost in the charge for a wideband Internet connection,” said Constantine. Products such as Musicam USA’s NetStar can send and receive full-fidelity, real-time stereo audio, contact closures and ancillary data, not only via ISDN and dedicated data lines, but also via IP. The system can use a host of compression algorithms, or send uncompressed linear audio. An on-board Web server enables control of NetStar from any Web browser.
Dropouts and poor fidelity have given the Internet a bad reputation for sending audio, but the medium can be broadcast-quality and reliable. Wise cites a one-month test of Tele-Link in which a signal was sent from Fort Collins, Colo., to the Energy-Onix factory in Valatie, N.Y. The signal traveled over ,3000 miles through 25 local servers with no degredation in signal quality or interruptions of service, he said.
“We believe that the reliability of an IP-based Tele-Link system is better than 99.99 percent, but we cannot be responsible for broadband servers with the system.”
The success of any wired data transmission scheme hinges on having a guaranteed bandwidth available 24/7. Wise urges users to proceed with caution when signing up with a provider for these services.
“Some of the biggest telco providers in the business will give you written guarantees for bandwidth, but not live up to them.”
Constantine agreed, adding that many of his customers have had good luck with providers such as Amerinet or Qwest. In any event, when choosing a provider, all agree it is important to get customer lists and investigate thoroughly.
For short hops of 900 feet or less, lasers are an alternative. Whirlwind’s E BEAM sends Ethernet or CobraNet data over a modulated Class One IR laser beam. 100 Mb full-duplex bandwidth is available for data or 128 channels of CobraNet audio in real time. A 1 Gb version of E BEAM is to be released shortly.
The laser beam is unaffected by the effects of rain, fog, heat or visible light. Heating elements in the antenna are powered by a copper wire embedded in the fiber optic cable to keep the lenses free from condensation. Laser connections also are immune to RFI, radiated noise from nearby electronics or ground loops.
Telecommunications experts cite hundreds of miles of dark fiber that were pulled during the Internet’s boom years when discussing the surplus capacity of the digital infrastructure. It is unlikely however, that fiber optic links will be major players in the STL for the forseeable future. Constantine said the effort to pull fiber that last mile to customer sites and install terminal equipment might make the costs prohibitive, and adds that it is unlikely there is fiber running anywhere near rural transmitter sites.
All of the interest and activity surrounding alternate STL schemes raises the question of whether the traditional 950 MHz STL will go the way of the dinosaur.
Darryl Parker, senior vice president for TFT Inc., thinks not.
“When stations started using these alternate products, we had some concerns about the future of microwave STLs. But broadcasters discovered that no system is 100 percent reliable. While more stations may use these ‘alternate’ devices as their main link to the transmitter, we’ve noticed that they tend to keep their RF systems as a backup.”
Constantine agrees, adding that the 950 MHz service will probably be around until the FCC has a more pressing need for the spectrum space.