This is one in a series about trends in how signals get to the transmitter.
Ted Nahil is U.S. sales representative of 2wcom Systems GmbH. He has served as chief engineer or director of engineering at facilities in Boston, Detroit and Denver and worked for ERI, GatesAir and Nautel.
The shift to IP-based studio-to-transmitter links represents the most significant change in the way radio stations deliver their audio and data to their transmitter sites.
Telco-provided services like T1 are either no longer available, or in markets where they are, the services are prohibitively expensive. Licensed Part 74 RF links are still in widespread use, but in most markets, the 950 MHz band is saturated.
Stations that are not broadcasting HD Radio programming, or those broadcasting a basic HD1-only HD Radio program, might still be able to get by with an STL channel in the 950 band, but with few exceptions, what you’ve used for years as a primary means of getting audio, and eventually some data, to your transmitters has become unsuitable because of the sheer amount of information you need to move to your site.
Consolidation of transmitter facilities, and of radio stations, has resulted in many engineers having to move audio and data for multiple stations to a single location. The need for bandwidth has increased almost exponentially, and today’s chief engineer can easily be responsible for getting audio and data for five, six, seven or more radio stations to the same physical transmitter site.
- You can use a modified conventional 950 STL system, converted to all IP, and with a very narrow back-haul channel.
- You can use a Part 101 licensed, duplex radio link with megabytes or even gigabytes of bandwidth.
- You can use an unlicensed, IP-based, duplex radio link in the 2.4 or 5.8 GHz band, but these links are subject to interference from a growing number of sources and as a result, they are not as reliable today as they were when they became an option a decade or so ago.
- Or you can use an IP “pipe” provided by a local ISP if you have that service available for you at your transmitter site.
Regardless of your choice, the common element here is that these are IP-based solutions, and they give you the ability to move a lot of audio and data to your transmitter site.
Codecs that can transmit and receive µMPX streams are becoming more prevalent as well, as a lot of stations deal with links that don’t have the bandwidth necessary to transport uncompressed audio or full MPX streams, and the data that goes with those signals.
We are also seeing a focus on resilient links utilizing transport mechanisms like Secure Reliable Transport (SRT) and Reliable Internet Stream Transport (RIST). These tools are some of the latest available to users of IP-based STL systems that help mitigate network issues inherent in every IP path like latency, jitter, dropped packets and missing packets.
Error mitigation is beyond the scope of this article but suffice it to say that most manufacturers of IP-based STL systems have come a long way in supplying tools to broadcasters to overcome many network connectivity issues.
The primary delivery methods of the recent past — RF and even satellite delivery systems — are increasingly becoming the backup delivery methods as IP-based STLs become more reliable.
Scalability
Along with these changing trends in delivery methods we are also seeing the beginnings of a shift toward centralized, cloud-based air chains. CBS News has been broadcasting its newscasts to affiliates entirely from the cloud since the pandemic struck.
At 2wcom, we see broadcasters beginning to embrace MoIN — Multimedia Over IP Network, which is an all-in-one audio software solution. In its simplest form, MoIN is a multi-channel virtual audio over IP encoder and decoder platform that supports all common standards, protocols and audio qualities. MoIN installations can be implemented on user-supplied hardware, a virtual machine, or totally in the cloud as a Kubernetes cluster for hardware redundancy.
[Related: “Kubernetes Brings Broadcast to Next Level”]
One main advantage of a cloud-based infrastructure is that it is completely scalable. The system grows as the station’s needs and requirements grow. Regardless of where the actual hardware is located, software designed to run on cloud-based systems needs to operate reliably in both small- and large-scale environments, and it needs to do this on off-the-shelf hardware.
A versatile system like MoIN allows the station to deliver audio that is scaled via the coding choice to accommodate the bandwidth available at the transmitter site. For example, the path to a main site may have much more bandwidth than the path to a backup site. A versatile distribution system can easily account for this and generate streams that fit in the available bandwidth. One path may be linear; a backup path may be compressed. Algorithms like Opus give the station the ability to deliver high-quality audio over narrow-bandwidth paths for backup while a linear/PCM algorithm is used for the main transmitter path.
Even for stations not in the cloud, those same coding choices — Opus and linear, for example — allow a tremendous amount of flexibility for delivering discrete audio.
MPX
Many stations using IP-based systems choose MPX or µMPX codecs to deliver baseband audio to the transmitter site. While IP-based MPX systems can require a lot of bandwidth — three to four megabits before any error correction schemes are employed — µMPX systems can deliver baseband audio in as little as 380 kilobits before error correction. µMPX saves considerable bandwidth on a backup path, for example. In a single-frequency network (SFN) application, using MPX or µMPX over IP allows you to keep the processing at a single, central point (usually the studio).
This is important because it means that there is only one setting in the system for some critical variables like pilot injection and pilot phase, and the audio is processed the same for both (or more) transmitter feeds. SFN networking success is dependent on having the modulation indices of all the transmitter signals as identical as possible. MPX SFN systems have a very good track record. Some transmitter manufacturers have recently been testing SFN operation using µMPX with great success.
In any IP-based system, redundancy is critical because of the nature of IP networks. Codec manufacturers offer various forms of redundancy, some for free, some at additional cost.
Look for systems that allow you to send the same stream across two different paths for path redundancy. Usually, the requirement is that both streams be encoded using the same algorithm and at the receive end (the transmitter site), the codec uses one path as the primary and the second path as the backup. If packets are missing or dropped from the primary path, the receiver fills those gaps with packets from the other path.
The result of this kind of redundancy is “seamless” switching between main and backup paths and no audio glitching discernible to the listener. Some codecs have the ability to deal with streams that are encoded differently. The results are similar in that there are no audio “holes” discernible by the listener but there may be a glitch or a tonal change that is detectable.
Similar to path redundancy, some codec manufacturers use the same physical path to send two identical streams to the transmitter site, with the second stream delayed by some amount of time. This implementation allows the receiver to deal with packet loss by comparing the two streams as they arrive separated by some period of time. This time diversity functionality adds yet another method to deal with network issues.
In addition to these redundancy methods, in an STL system any redundancy is better than none. You may look at multiple ISPs to guard against a failure of one ISP network, or you may use an ISP in conjunction with a private IP-based microwave network. Wherever possible, any redundancy is better than no redundancy.
Safety
One last point to consider is that any STL system that uses IP-based devices is subject to the whim of hackers who can turn a beautiful Friday afternoon into a nightmare. How do you keep your network safe?
Having a separate Ethernet port on your IP codec that is used solely for management adds a layer of security to your system. Look for codecs with three ports: two for streaming, one just for management. Use a VPN if possible. Router and switch prices have come way down, so use a router or layer 3 switch that supports port-mapping and VPN functionality whenever you can.
Change the default IP addresses of any device on your network — routers, switches, codecs, processors — and use non-routable (i.e., private) IP addresses other than the defaults.
Almost every network device you can buy has a default IP address of 192.168.1.1. Change it!
HD Radio devices from some major transmitter manufacturers — the importer and exporter, and HD inputs to exciters — have default IP addresses that start with 10.x.x.x for example. Change default user names and passwords if you are not forced to do this when you initialize your equipment.
Set up accounts with different permission levels. Your air talent certainly doesn’t need permission to add users to a network device’s configuration menus! When you are setting up new devices for your IP-based STL, don’t skip important steps. “I’ll get to that part later” almost never works!
Conduct penetration testing on your equipment using tools available on the internet. Most codec manufacturers continually test their equipment. At 2wcom, for example, there are multiple procedures in place to ensure firmware quality and security that include automatic dependency checks for known vulnerabilities, static code analysis using AI-assisted detection, and vulnerability scanning conducted by third-party security specialists. Much like anti-virus software used on a computer or smart device, manufacturers are motivated to ensure that their products remain secure and reliable.
As our industry shifts to IP-based STL systems and cloud-based content delivery systems, you can take full advantage of the scalability and flexibility of this equipment. A little planning and regular, thorough testing of your system and its components will deliver the reliability you need in an STL link, and keep your Friday afternoons from becoming a nightmare!
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