When it comes to studio-to-transmitter audio links, bigger markets typically have a microwave option, free from the woes of a provider, and an alternative (T1, MPLS, 15k Audio Circuit).
There are a lot of devices out there that offer solutions to STL failures, able to make decisions that will keep the modulation going and the 3 a.m. phone calls down to a minimum.
DEVA and BDI, to name a few, make devices to detect when a source goes away based upon word clock loss and audio threshold silence. I find the best monitoring solutions almost always involve having fewer devices in the air chain, thus keeping maintenance down.
Upon loss of RF lock, the Starlink drops its word clock output�a good stimulus for STL switching.
It�s important to consider just how the various stimuli provided by manufacturers will actually work in your particular environment. Let�s consider the typical ones.
Word clock loss: In the AES domain, word clock can be an excellent indicator of when a source is no longer present. For example, with the Moseley Starlink family of products, word clock will be lost when RF lock is lost. One may think this is not favored; however, this provides a status to the next device in line, without adding extra monitoring in the middle.
The Omnia 6EX has provisions for switching its input from analog to digital, which are best controlled by external relays.
Audio silence threshold: Silence is pretty much the only indicator in the analog domain, if a source is no longer there � with one big problem. How do you know this is an STL failure and not a studio silence? Processors such as the Omnia-6EXi do provide a failsafe option in software, but switch to the alternate without checking to see if there is audio there first. Every studio silence would cause a flip. This is not ideal, clearly.
External GPIO: Devices like the GatesAir Intraplex STL HD offer contact closures on the back for when the device is in alarm. It is always important to test these alarm four-wire circuit. If the TX side at the studio (on-air audio) is having issues reaching the transmitter, it does not necessarily mean the studio device will be in alarm. If the TX to studio (receive) portion of the circuit is still intact, no alarm will generate on the studio end, just the transmitter end. Any remote control logic only on the studio end would not work in that circumstance.
TUNE YOUR SOLUTION TO THE SITUATION AT HAND
To tackle the STL switch problem in our market of San Diego, we created a system that required only one extra wire to be added.
The Orban 8600 has on-board provisions for switching from analog to digital, but use of external relay control is preferred.
For on-air processing, we use Orban 8600 audio processors. Those familiar with the 8600 may wonder if the built-in word-clock loss/silence sensor is used. It wasn�t because the built-in solution ping-pongs between source � meaning, if the source is going and coming, it will switch every time. We wanted this function to be �one-shot� only.
If the audio goes away on the main STL, we want to make sure the alternate STL has audio on both channels and then have the system switch just one-time, followed by an email.
Except for limited situations, like a backhoe fade, audio sources usually don�t just vanish � they are intermittent due to faulty equipment in the vendor�s office or RF interference at the transmitter site.
Remote controls such from such companies Audemat (Worldcast Systems), Burk and Davicom are great at interfacing with SNMP and making decisions based on logic.
As for that one wire we added? Because the Orban does not accept SNMP �set� commands for audio input change, our remote control fires a relay to make the input source-select change.
NON-PPM MARKETS HAVE IT EASIER
Newer Nautel transmitters are optioned to play MP3s straight out of the transmitter that is on-air. For a smaller market with no PPM, no web stream and no options for backup STLs, this may be useful. It is great to be able to go to the operations manager and say, �We lost something but within seconds audio was back on the air.�
However, you should know that solution will be of value only up to the point they realize they can�t segue the songs, and live talent can�t be heard. Still, it buys you time.
Often, I see monitoring devices added to the air chain to solve this issue when all the tools needed were already in place.
The more alerting you get via technologies like SNMP, the more frequently you, the engineer, can make an informed decision.
Any solution you design will likely involve human intervention at some point, to ensure the problem is really fixed, but an automated response based on stimuli gives you time to think and can keep the station on-air without interruptions.