HOWTO: Use Icecast as a backup STL for Nautel transmitters (Part 2)

In the last post on this subject, I walked you through how to setup your Nautel transmitter to pull audio from an Icecast server. This can be used on a permanent basis if your station does not have a terrestrial microwave or T-1 link normally used as your STL. However, since the quality of Internet connectivity can vary, some stations do prefer to have a non-Internet link for STL purposes.

That said, you can still use the Icecast server as a backup to your normal STL. This works for us because our normal streaming service (for our website and various apps) uses Icecast, at a high bitrate (192Kbps MP3.) We simply tie the Nautel to that stream as an additional listener.

Once you have two presets (STL and Internet) configured, you can switch between them at-will, or as needs arise. However, it is also possible to configure your Nautel to switch automatically in the event of an outage.

In this HOWTO, I’m working on the assumption that you have a terrestrial microwave link for your STL (although satellite links can work as well), which you wish to use as a primary feed. Your Icecast server will serve as a backup feed. (Of course, it wouldn’t take much to adapt this HOWTO to use Icecast as your primary, and your regular STL as a backup instead.)

These directions make one very important assumption — and requirement. When there is a failure of your normal STL, it must fail silent. If it fails to static, your transmitter will sense modulation and never switch over. If you have an analog STL that’s failing to something other than static, you may have to adjust the squelch of your receiver(s). Again, you want any failure in the link to fail to silence, not static, tone, or anything else.

To configure automatic failover to your backup source: Log in to your Nautel AUI. Click the Menu button at the bottom of the AUI, and then Presets. Click Load and then select the preset corresponding to your primary STL’s audio input, and click OK. This will load that preset into the editor.

Click on the Other Settings tab. Set the Mod Loss Timeout to enabled.  This will expose a few other options which you will need to configure.

In the Action field, select “Change Preset.” For Mod Loss Preset you will want to select the preset you created for your Icecast server. (If you do not have one, read my previous post for information on how to do this. Set it up and verify that you can manually switch to it and have audio on-air, then come back here.) For Timeout Minutes, I recommend leaving it at zero. (More discussion on this later.) For Timeout Seconds, put a reasonable value in (I use 30.) Set the Threshold to something reasonable (I use 60, but more discussion on this later.)

Once you have these values set, click Save on the left to save the preset. Then use the preset dropdown at the top of the AUI (below your TPO reading) and select the preset, and click Activate to make it active. Note that even if the preset was already active when you started editing (and saved) it, you will still need to activate it again for your changes to take effect. (This holds true anytime you make a change to the preset that’s currently active — and not the “Current Settings.”)

As I mentioned earlier, you’ll need to set an appropriate timeout and threshold:

The timeout you specify is the amount of time the transmitter’s modulation must be below your configured threshold before changing to the Icecast preset. Keep in mind that a failing STL may drop in and out. I originally used a two-minute timeout, but immediately changed that when I heard rain issues causing the station to be silent for 20-30 seconds at a time, with the occasional 2-3 seconds of audio popping in before dropping out again. Obviously, the station was unlistenable for most, but it wasn’t hitting that two-minute threshold. A two-minute timeout will ensure you don’t switch to the Icecast server unless your STL is truly dead, but you risk (depending on the issue) having a period of time where the signal is unusable. I decreased my timeout to 30 seconds, and left it at that. The drawback being a board-op who misses a break during your local high school football game because he/she went to the restroom may cause your transmitter to switch to Icecast. You’ll have to decide what value is acceptable here.

The modulation threshold you specify is also important. Keep in mind that a dead STL may not drop to 0% modulation. For example, in testing I simply stopped our automation system’s audio output, and found that the transmitter was still showing 40-50% modulation. Mind you, the STL was still up, and perhaps muting during a signal loss on the STL would bring it below this, but you may want to set some kind of value here in case you don’t end up with a totally “silent” link. The modulation will have to drop below this threshold for the timeout duration before the preset will switch. I am set to 60%, but again, this is a decision you will need to make for your use case.

One other important item to note: If your transmitter switches to Icecast, it will not switch back to your STL automatically upon return of audio on that link! At first, that seems a bit annoying. However, if you’ve failed over to Icecast and your STL starts coming back intermittently, you really don’t want to flip flop between two feeds. That’d just sound horrible! So, once your STL is restored, you’ll have to manually log in and switch presets. This also applies if a board op misses a break cue in the restroom during a high school game, as well. (You may be able to fudge the modulation threshold if you can determine enough difference between a sleeping board op and a dead STL, but this is left as an exercise for the reader.)

On a side note: If you use the “RF Inhibit” action for modulation loss, it turns out the transmitter will bring up the RF stage whenever audio is restored.

As I mentioned in the last post, I spent a while tinkering with all this and figuring out how to make it work, without finding a whole lot of documentation on the subject (and some of it was just simply out-of-date, which led to me not finding things.) Hopefully this helps someone out there who’s setting out to do the same thing I did.

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