STL Redundancy: Radio's Forgotten Infrastructure Crisis
The STL, or studio to transmitter link, is the unglamorous connection that keeps a radio station on the air, and when it fails everything goes dead silent even if every other piece of gear is working perfectly. This episode covers why most stations never built backup STL systems, how broadcast consolidation pulled out the engineers who used to catch these failures fast, and what real redundancy looks like using fiber, cellular bonding, and gear like Comrex and Peplink. It's a case study in how cost cutting quietly makes critical infrastructure more fragile, which is basically the whole tech accountability story in miniature.
Before anything else, I gotta give you the usual spiel. This is my own opinion, my own point of view, and none of it speaks for anybody I work for currently or have ever worked for. It's just me. So, a while back, I'm standing in a transmitter building, and everything in that room appears to be perfect. Listening to the sounds from the magic tile transmitter it's happy power is good the antenna is fine every light that should be green is green and yet the station was off the
air not because anything in the room failed because well the audio stopped showing up the thing that carries the sound from the studio to the building had quietly died somewhere out there and the transmitter was sitting there doing its job perfectly it was just transmitting nothing but the carrier that link it sort of has a boring name it's called the STL And that stands for the studio to transmitter
link. And it is at most stations in this country, the single most important thing. Nobody has a backup for. This is the Tyler Woodward project where people like tell me how wrong I am on everything in the comments section. And well, today I'm talking about that one cable. That one radio RF path or that one circuit that can take your entire station off the air while every other piece of expensive gear is Sitting there working fine All right, let me set the scene your studio
it's in one place Your transmitter is usually somewhere else out there on a hill, on a tower, out on the edge of town, where the coverage usually works out a little better. Sometimes that's a couple of miles. Sometimes it's a lot more. So you need a way to get the audio from here to there, from point A to point B, all day, every single day, without ever taking a break. That's the STL. That's its entire job in the problem with the STL is structural it Sits in the middle
of the chain. It is single. It is the single most important piece everything downstream of it can be absolutely flawless and It really doesn't matter because if the audio doesn't make it across that link your broadcasting silence. There's a guy named Doug Irwin who, you know, writes about this stuff in radio world. If you haven't seen his articles on radio world, go check it out. He's got a couple of good ones on here. I shouldn't say a couple. He's got way more than a couple. But, you know, he's a serious engineer. This guy knows his stuff and he put it about as plainly as you can put this. An STL system.
I mean it's just as crucial in a radio station's air chain as the transmitter or the antenna. If there's no audio to transmit, even the best transmitter and antennas are of little use. It's kind of useless. Nobody tunes in to hear dead air. Boom right out the gate right there at the top of this article from back in 2018 That's the whole thing right there Nobody tunes in to hear dead air So why does almost everybody run just one of these STL systems? Money mostly a
real backup basically doubles what you spend and there's Sort of this inertia this it's you know, it's worked for 15 years. So Why would I touch it? If it ain't broke don't fix it. How many times have I heard that saying? and honestly Fewer and fewer stations. I mean have an engineer around To even build out the backup because those people get consolidated out years ago. And the gear is old. There's a guy out there who does this stuff for a living, runs a company called Max Connect, and he walked into a station recently
that was still running an STL transmitter built in 1988. That radio is older than, you know, a chunk of my audience. It's still on the air. Until the day it isn't So let's talk about you know the actual ways you can get audio across because there's more than one option nowadays and This is where it's going to get very very interesting an old Classic is a licensed radio link in the 950 megahertz band You point a dish from the studio at a dish on the tower. It's all line of sight and you basically beam the
audio across that link. It's point to point. When the path is clear, it's great. Low latency, it's your own license frequency, no monthly bills. The catch is there's barely any room left in the band, especially if you're in a busy market. The gear is also very expensive. And if a building goes up in your path in your way or you get interference You're kind of stuck Doug Irwin counted it out once and figured there's something like 16 usable
channels in that whole band and you know in a big city where everybody's antennas you know converge on the same hill you're gonna run out fast then there's IP audio over the internet basically it's sort of over oversimplifying it but this is where most new STL installs are going you take the audio you packetize it you send it over a circuit could be fiber could be a regular internet connection could be your own private microwave link and this is the part i actually you know really want to push on because not all
internet is the same not all internet is the same internet if you can get fiber get fiber and if you can get two fiber circuits that don't ride on the same backhaul that don't funnel back through the same path somewhere upstream. That's the best thing going. That's the dream. Because the whole point of a backup is that the two paths don't die together. That's the part I think some people miss. You can have two circuits that look separate on paper and both of them run back through the same junction. you know, five miles away.
In the day, a backhoe finds that junction. Both of your circuits are going down at the same time. Backhoe fade. Most radio engineers know that term. It's a real term because it's a real problem. Always, always have a plan for the backhaul dying. Not just the last mile to your site. And please, for the love of God, don't build your station's lifeline on a residential internet plan. Business class, at the absolute minimum. Enterprise, if you can get it, if you can afford it, that's the good stuff. That's where you get an actual
service level agreement. And somebody who picks up the phone, I mean, seriously, the difference, between residential and enterprise is the difference between we'll get to it when we get to it and we're already on it. DSL and cable, I'd stay away from both of those. DSL because it runs on copper and we know the phone companies are actively trying to walk away from copper. Let's consider that a dead technology at this point. and cable because, well, of latency. Coax just isn't built to be consistent the way you need
a station feed to be consistent. I mean, it'll mostly work right up until the moment it doesn't. And then there's the third option. This one has gotten really good over the last few years. Cellier. 4G and 5G. You can run a modem as a backup that kicks in within seconds when your main path drops. The nice ones, I mean they can bond multiple carriers together so you're not betting everything on one cell network having a good day. Peplink. They sell many different routers where you can do Bonding, you can bond multiple cell networks
together. You can bond other networks that aren't cellular, your fiber, your cable, your whatever other circuit you've got out there. If you've got a WISP, a wireless internet service provider, you can run that through a Peplink and bond multiple connections together either used in their cloud service or a piece of gear that sits in your own rack. A simple 5G backup behind a Good fiber primary circuit is a I mean, that's a totally legitimate setup now. I Mean, it wasn't a few years ago, but it is now You know and the other thing is when people hear audio over a cell connection and they think that eats into a fortune of data
and yeah it used to especially if you were doing raw PCM audio at like a meg and a half just for one audio path that's one way but there's this awesome thing now called micro mpx it lets you send the whole composite signal the full FM stereo picture the RDS everything in about 300 to 400 kilobits a second that's super tiny i mean that's that's literally nothing You can run that over the most modest connection and it sounds like the real thing. And for the gear itself, the range is wild. At the bottom of the pile, you've got those little barracks boxes. An encoder on
one end, a decoder on the other end. I mean, a pair might run you five, six hundred bucks. They're not fancy, but they work. Just put them behind a VPN. Don't put them on the open internet and if you don't know what I'm talking about go Google it I'm sure you'll find plenty of radio insight radio world other articles about barracks boxes being on the open web, you know and looking at Broadcast general store not sponsored by the way Just saying, you know, it looks like well, I guess the the extreamer 500 is about 900 bucks. So I mean that's still that's not too terribly bad in the middle is the workhorse something
like a comrex you know brick link three around i think these go for about i don't know 2200 bucks it's basically built to be an stl it does the automatic failover for you it'll jump to a cell modem on its own. The guys at Vermont Public run something like 30 of them. And the word on these? I mean, they just plain work. And you forget they're there. That's a high compliment. That's the highest compliment you can pay a piece of broadcast gear, especially nowadays. If you can forget it's there, that's what you want.
Then you've got options like Tie Line. They make great little stuff in the same range. They've got the mpx1 and the mpx2 and At the top at the very tippy top. You've got the full licensed radio links You've got the the old standard the moseley starlink sl -900 3q then you've got things like the the gates air Intraplex We run these at work as our primary STL path to get audio to our sites. Think of this, by the time you've bought the radios and the dishes and you paid a crew to climb the tower, you're easily into
tens of thousands. In a real, a real backup, like the man said, roughly doubles that. Here's where I actually land on all of this The fix it's obviously not a mystery Nobody has to invent anything. We know exactly how to do this to pass that don't share a single failure point things that make the switch over automatic make sure also that something Pages a human when it happens That's it That's the whole playbook man, and it's been published over and over and most stations
still They don't do it Not because it's hard Because there's no money allocated to do it and no engineer left in the building to fight for it the same consolidation that Pulled the people out is the same reason there's nobody around to notice the second that audio drops. That's the part that actually really bothers me. It used to be when the audio stopped, somebody working in the building noticed it. Somebody there heard it. And within a minute or two, they were out
there fixing it. Now, You've got, you know, one engineer covering a dozen or so stations across three or four different states. And the silence detector, when it kicks in, it'll start probably looping back some backup audio. In this station, technically, I mean, it's alive, but it's really dead. And it can sit like that for a while before anybody notices. I think back to the transmitter building that I started this story with. Everything green, everything looking... I mean, perfect.
But, still, silent. Because the whole thing, you know, the one thing nobody backed up was the one thing that really mattered that day. It's the cheapest... Insurance in the whole building. It's it's the thing that gets Skipped time and time again a few hundred bucks in an afternoon would probably cover most stations and They rolled the dice instead Because it's worked so far and so far is doing a lot of work in that sentence Get a second path make it real make it diverse That's the whole episode Subscribe on Apple Podcasts
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