IP-based STLs

May 1, 2009

Goodbye analog (and some digital) STLs, hello IP

By now you know I'm a big fan of new technology. However, I also know there is still an important role in a radio station's technical facility for legacy equipment. Like many, I find it difficult to retire a piece of equipment that has served its purpose reliably over a 10- or 20-year (or maybe even longer) period of time. Some equipment is truly classic. It seemed very expensive when new, but easily proved its worth over the years. However, time and technology move on, and those classics' original design specs — good as they were then — are in many cases simply inadequate today.

Analog STL systems fit into this category well. While we've moved on to the current state-of-the-art in our STL systems here in New York, we still hold on to our old reliable STL systems as backups.

Perhaps you find the time has come to relegate your old analog STLs to backup status as well. There are too many advantages to ignore in a digital STL, such as constant audio performance over a varying receive level, or the ability to extend a local area network from the studio to the transmitter site. There are various radio systems available for any need in a broadcast facility.

Basic change from analog to digital

The digital STL systems for our 950MHz band will of course work on a channel that you currently have licensed, but because they use more bandwidth, you'll have to essentially relicense the channel to reflect the change. For example, a Moseley PCL 6010 has an FCC emission designator of 300KF8E (which essentially means it has analog modulation with 300kHz of bandwidth) while a Moseley Starlink (using 64 QAM) has an emission designator of 500KD7W (digital modulation with 500kHz of bandwidth). You will have to recoordinate with local users in the same band. The good news is that a digital STL transmitter is less likely to interfere with co-channel or first adjacent channel analog receivers (since whatever happens to be heard by the other receiver pretty much sounds like incoherent noise). Conversely, a digital receiver is more likely to be interfered with by an analog transmitter, because any analog modulation heard by the digital receiver has the potential to degrade its BER since it is essentially incoherent noise.

My experience is that digital STL systems are very good at ignoring other co-channel digital systems; some experiments showed that co-channel systems would ignore each other as long as the desired-to-undesired ratio exceeded 15dB. Your mileage may vary, but don't forget to consider fade margins if you put more than one system locally on the same channel.

The well-known STL manufacturers of course have mature digital STL products available.

TFT Model 46

TFT Model 46

TFT makes the Model 460 digital STL. This radio system will carry up to six channels of uncompressed digital audio, with the modulation scheme (and bandwidth) depending upon the number of channels actually used. For example: six channels of audio (at a 32kHz sample rate) calls for the 256 QAM modulation scheme. In that case the receiver (according to their published specs) requires, at minimum, a -84dBm signal to operate at a BER of 10-6. This system has both analog inputs and outputs, along with AES ins and outs. Configuration is done by way of a GUI that runs on a PC (serial connection).

Goodbye analog (and some digital) STLs, hello IP

For those who remain unconvinced that there are advantages to using a digital STL, let's consider those specs for just a moment. This type of radio system is carrying the equivalent of what three analog radio systems would carry; at the same time, its audio specs are way beyond what the best analog radio could ever do, and the receiver needs less signal to do it.

Moseley Lanlink and Starlink SL9003Q

Moseley Lanlink and Starlink SL9003Q

In a nutshell, the digital radio systems are way more efficient in terms of the bandwidth they need, along with the amount of power they need, and oh by the way — the audio is better too.

Moseley offers the Starlink SL9003Q, which will also carry up to six channels of digital audio; the bandwidth and modulation scheme depend upon the number of audio channels specified by the end-user. The Starlink has both analog inputs and outputs along with AES inputs (built-in SRC) and outputs. As you would expect from a digital radio, the audio specs are great with a dynamic range of 90dB and a THD spec of less than 0.01 percent. The Starlink can also be configured to use up some of its native bandwidth for a simplex LAN extension which will deliver UDP data streams (up to a total of 544kb/s).

Other configurations

The trend in technology for audio transport (along with most other forms of communication) is certainly more and more based on IP. So, though you may simply need one AES data stream for your analog transmitter, there may very well be many other pieces of gear at the transmitter site that want to communicate with you by means of a LAN. These would include RBDS encoders, remote controls, computers, remote cameras, VoIP phones, satellite receivers, and on and on.

As mentioned above, the Starlink can send UDP data streams to a transmitter site, but most of the items you would place at the transmitter site are going to require a duplex link so they can communicate via TCP (as opposed to UDP). What's the practical way to accomplish that, assuming there are no other options at your transmitter site?

Fortunately for us broadcasters there is the 900MHz ISM band (which actually extends from 902 to 928MHz). Unlicensed channels can be used in this band (subject to FCC parts 18 and 15) for the purpose of carrying digital data, to and from the transmitter site. The relative proximity to the 950 band we're accustomed to using means that we can re-use the antennas already in place easily enough. This is done by adding duplexers to an already established link. (When you fill out the 601 for your new digital STL system make sure you add in the additional loss incurred due to the duplexer on the transmit side.)

Moseley makes a system called Lanlink that works in exactly this fashion. The ISM band transceivers that are the heart of the system can deliver 512kb/s of data, 10base-T Ethernet. (In addition to the Ethernet connection, it also provides two RS-232 serial links that can be configured for a data rate between 1200 and 115,200 baud.)

With the explosion in networking, though, there may be other options for you to check. A quick search online is all that is needed to find other manufacturers of 900MHz ISM band radios that transport Ethernet. You could even avoid the cost of the 900MHz duplexers by purchasing separate antennas for the ISM band. And, obviously with this method, there is no effect upon your current 950MHz system.

Higher ISM bands

So far we've covered the 900MHz ISM band mainly because it was close to our familiar 950MHz STL band. You're probably aware that there are quite a few other ISM bands (2.4, 5.3 and 5.8GHz) and there are multiple manufacturers that make gear for use thereupon. Again, the explosion of networking has had a positive effect for end-users in the sense that there are more and more players making the gear and antennas, since the potential market is so big.

Goodbye analog (and some digital) STLs, hello IP

BE Big Pipe LT

BE Big Pipe LT

Of course the down side to that same explosion is that the potential for interference, even while using the spread-spectrum (frequency-hopping) radios, is real and substantial. Proper engineering of the link will be necessary to minimize the effects of interference from other unlicensed users: highly directional antennas are a must. (Be sure to adjust the TPO of each transmitter so that the ERP remains within legal limits.) Obviously you can also change the polarity of the link to minimize the effects of interference.

Broadcast Electronics makes the Big Pipe LT, which is a radio system that works in the ISM bands mentioned. The entire system is made up of eight components: the program interface (single rack unit that goes in the technical center); a managed Ethernet switch; the IDU (indoor unit) which also goes in the tech center; and finally the ODU (outdoor unit) which mounts near or upon the antenna used on either end. The far end is a duplicate of the near end.

The interface unit has both analog inputs (A/D conversion of up to 24-bit, 96kHz sample rate) and AES inputs (with clock source selectable between its internal clock, AES derived, or word-clock). Additionally, the unit has provision for four GPI/O ins/outs and status, along with an RS-232 data path. By means of the managed Ethernet switch, all the TCP connections necessary for HD Radio (including one representing the analog audio and MPS), the SPS (from a local importer), and the PAD information will be aggregated and trunked over to the IDU. The IDU and ODU are connected by way of a coaxial cable; and finally the ODU communicates with the far end via the selected ISM band RF link. The radio link itself can be managed by way of SNMP, RS-232, HTTP or Telnet.

Adtran 4205

Adtran 4205

Perhaps you just want to build a high-bandwidth LAN connection to your transmitter site. After all, you have quite a few choices today in choosing boxes that take audio (analog or digital) in, and put TCP out. As I wrote earlier, almost everything communicates in that fashion today.

Adtran 5045

Adtran 5045

Take, for example, the Adtran Tracer series 4205. This radio operates in the 5.8GHz ISM band, and the system consists of only two units — one transceiver on either end. (No IDU/ODU combo.) It has a 50ohm female N output (so plan on using some very good transmission line). The interface for DS-3 data is a 75-ohm BNC so, if you were to elect this type of radio system, you would probably opt to aggregate all of your host TCP sources by means of a managed switch that has a 75-ohm BNC DS-3 interface. The Adtran 5045 is a similar radio system, but includes a built-in Layer 2 switch on both ends, and so obviously the data interface is strictly Ethernet. Both systems are specified to have a maximum TPO of +20dBm, and will achieve a BER of 10-6 with a -78dBm receive level signal.

The options are nearly boundless but for a system that is more economical than the Adtran you may want to consider the Airmux 200 system. Like the other radios I've mentioned, it operates in the high ISM bands. One feature of interest in the Airmux 200 is that in addition to the 100base-T Ethernet port, it has up to two separate T1 inputs; so in the event that T1 must be transported for a legacy TDM system, it can be sent along with the Ethernet transport. The total data throughput of the 200 system is specified at 48Mb/s.

Goodbye analog (and some digital) STLs, hello IP

Dragonwave Air Pair 100

Dragonwave Air Pair 100

Another option available to broadcasters is a portion of the 18GHz band that can be licensed for STL use under Part 101. If this makes more sense for you then there are at least two options out there right now. Radio Systems offers a microwave radio link called IP Connect. Hardware configuration is ODU and IDU (single rack unit with all interfaces). It's very configurable, giving the following options: Ethernet up to 250Mb/s, or up to 32 T1s, or 2 DS3s, or 2 STM1 (155Mb/s) plus 2 T1s. Management of the system is done via a Web browser, SNMP or Telnet.

Axia users have made use of various Ethernet radios such as the Dragonwave Air Pair 100. This system also makes use of the ODU/IDU configuration. The native interface of the system is gigabit Ethernet, and it provides for full-duplex 100baseT (200Mb/s of bandwidth). It can also be configured to transport T1s if so desired — that way TDM systems can be kept during a transition period (or maybe forever). Management of the system is done via SNMP.

Radio Systems IPConnect

Radio Systems IPConnect

So there you have it. The original digital STLs are themselves practically in the class of legacy equipment today. Carriage of Ethernet is the primary function of all the radios mentioned herein. So many of the devices we find in a broadcast plant today communicate by way of IP; what was once kind of a luxury (LAN at the transmitter site) has now pretty much become a necessity. If you haven't already jumped on the bandwagon, then in my (humble) opinion, you need to do so as soon as you can. Fortunately there are many, many options today that'll make the job a little easier.

Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.

Resource Guide

Airmux 200

Airmux 200


Armstrong Transmitter



Broadcast Electronics




Marti Electronics

Moseley Associates


OMB America

Radio Systems

Superior Broadcast


Trango Systems

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