HOWTO: Use Icecast as a backup STL for Nautel transmitters (Part 1)

Anyone who’s done any broadcast work can tell you that STLs aren’t perfect. Whether you’re using microwave or T-1, at some point, you’ll have an outage. I’ve run into this recently with one of our stations, where we apparently have a water intrusion issue on our link somewhere. Get the right amount of rain, and you’re off the air (or at least, broadcasting The Sound of Silence…)

One of the nice things about Nautel transmitters is their ability to pull audio from an Icecast or SHOUTcast server. I’d looked at this before, but never was able to figure out exactly how to make it work from the documentation provided. I recently sat down and started toying with it again, and successfully made it work.

This post will be the first in a two-part series. We’ll first set up a Nautel transmitter to pull its audio from an Icecast stream. In the second part, we’ll discuss how to make this work in a backup/failover configuration.

Before we start, you’ll already need to have a working Icecast stream of your station. Setup of the streaming server and encoder are beyond the scope of these posts, but I’ll recommend you use butt as your encoding software and Digital Ocean to host your Icecast server. (Sign up using my link and you’ll receive a $10 credit!) Once you have a working Icecast stream, return here…

Next, you will need to log in to your Nautel AUI over the web. Once logged in, click the Menu button at the bottom of your screen, and then on Audio Player within the AUI menu. Once the Audio Player opens, click on the Streams tab. Then, at the bottom left, click Add.

Once the “Add Audio Stream” dialog box comes up, you’ll want to fill it out with your stream information, like this:

You’ll first want to select the stream type. Once you select the stream type, the field for the URL will appear. You can give a descriptive name. Of course, the URL should be the URL of your Icecast stream. Then click OK.

At this point, the stream (and its information) should show up in the Audio Player screen. Click Menu at the bottom again, then click Presets. This will bring you to the current operating parameters of the transmitter. (At this point, I am assuming you have a working transmitter, on-air, with your presets defined for correct frequency, output, etc.)

While in “Current Settings”, click on Save New at the left. Specify a name for your new streaming preset, and click OK. You have now just duplicated your current operating parameters into the new Preset, but you still must load that preset into the editor. To do so, click Load on the left side, then select your newly-created preset, and click OK. The new preset name should show at the top of the editor, and you can begin making your changes.

First, on the “General” tab, verify your frequency, output power, and mode are correct. Then move to the “Main Audio” tab.

Once in the “Main Audio” tab, verify your Audio Source is set to “AES/EBU 2” , then set your secondary source to the stream you just created. Set Audio Mode appropriately (stereo or mono, depending on your station and stream. If you’re a stereo station but only streaming in mono, you can safely use mono here.) Enable the low-pass filter as desired, and set your Preemphasis appropriately (75us in the USA.) Failure to set this will result in less than optimal audio quality, to put it lightly.

On the left, click the Save button to save this preset.

You should now be able to select the preset from the drop-down at the top of your AUI and activae it. After a few seconds of dead air, you should hear your stream on-air. If not, verify you’ve entered the correct URL, etc.

Note that you may need to edit this new preset and adjust the Digital Level and Audio Mod Adjustment to get your modulation where you need it to be. In my case, Digital Level is set to -5.0dBFS and Audio Mod Adjustment is at 0.0dB which renders audio that is barely distinguishable with what our composite STL is feeding. For reference: My setup is feeding composite out of an Optimod FM 8200 through a two-hop 900MHz link to the transmitter site, and the streaming encoder is fed from the XLR outputs of the same Optimod unit.  My streaming preset’s audio mode is set to stereo (to enable the Nautel’s internal stereo generator when streaming), and preemphasis is set to 75us.

As always, your mileage may vary, but if you’re messing with this kind of thing, chances are you know what you’re doing and can make the proper adjustments.

In part 2, I’ll explain how to failover from the composite STL to streaming, stay tuned!

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