Reliable Studio Transmitter Link (STL) solutions


Radio station priority number 1. ensure your content reliably gets to your listener.

For every radio station, one of the most important links in the chain from studio to the listener is the link that takes audio from the studio and sends it to the transmitter. Over the years most FM stations have opted for a link from BT (called X21 link) which is a serial connection, with reasonable levels of repair time. This is usually backed up by ISDN EG ISDN2, which can dialup the studio if the X21 link goes down.

BT are currently deciding on the End Of Life (EOL) date for X21 as its doesn’t fit in with their 21CN network strategy, and the cost to maintain the equipment, coupled with dwindling customer numbers means the service is no longer viable. The current date BT are working to is March 2018 “at which point they (kilostream services), unless otherwise stated in the product specific sections below, will be fully withdrawn from service” link here

The WideFM Solution – WideFM Sure-STL

WideFM have been working on solutions to provide reliable transmission of audio from studio  to transmitter without the high costs associated with the current alternatives to X21 such as fibre ethernet over the past 10 years, the result of this work, is what we call WideFM Sure-STL.


The technology borrows ideas from a disc technology called R.A.I.D. Redundant Array of Inexpensive Discs.  The idea being with the disc technology that data is striped over multiple discs, so if one failed the others could make up the information to allow the data to be read as normal.

Our solution:

To start with we encode the audio into IP packets, using APT-X technology. To ensure we maintain broadcast quality audio and signal to noise ratio, we use high end hardware DSP encoders designed for broadcast applications. The audio now appears as a number of Realtime Transport Protocol packets (RTP) basically IP packets with fragments of the audio encapsulated. We call this a stream, or IP stream.

Because the audio is now an IP stream this opens a number of possibilities up for us. You could, for example transmit the IP stream over the internet, install an  inexpensive ADSL connection at the studio and one at the transmitter site, and it would work.

However we all know, that while the internet is fairly reliable there are no guarantees of any quality of service. If your ADSL provider is having a bad day, you might notice google load a bit slower than usual, or maybe an outage for a few minutes. However because our audio is realtime, we cannot have any delays, and so these glitches would cause audio break up. A bit like a mobile phone in a bad coverage area, and definitely not broadcast quality!

So rather than send (and rely on) one stream of audio, we duplicate the IP stream over multiple inexpensive links (ideally you need at least 2, it works well with 3, we recommend 4), in this example perhaps an ADSL, a cable connection, and a license free microwave link.

At the transmitter site, we have the other end of the 3 connections, and our hardware audio decoder listens to all 3 streams at once.

Here is the clever bit – the decoder looks at the 3 streams of audio and puts the packets into a queue. The first link to produce the next packet in the sequence, is the one that is used. If the ADSL decides to dropout, and misses a few packets, the other 2 links will still be producing the packets and the decoder will use the remaining links to product the next packet.


All this happens in milliseconds, and as we use different providers or link types for each one, the chances of them all failing at the same time is minuscule, hence we call the technology ‘hitless’


Our Sure-STL technology can be a big benefit to radio stations that have multiple transmitters, but all of them broadcasting a single output, due to the high cost of running a traditional STL to each one.

Now with Sure-STL you can inexpensively run a Sure-STL to each transmitter,  allowing your ad’s to split over the different transmitters. This opens the door to potential new advertising revenue, at the same time future proofing your STL solution and potentially saving you money in the process.

If you would like more information on this solution  please get in touch, or call us on 01202332331