Trends in Technology: Robust IP STL

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Years ago, as the chief engineer for KJAZ in San Francisco, I put together an STL system that I thought was pretty infallible—a dual-monaural 950 MHz radio link. This was a common method in the 1980s and early 1990s. The new equipment configuration allowed me to retire a very old composite STL and re-locate the station’s stereo generator to the transmitter site where I felt it belonged.

The system quickly instilled confidence because it was a lot quieter than the old composite radio. It sounded better, and, unlike the composite system, it had inherent equipment redundancy with dual radio pairs. Everything worked smoothly until one day when my pager went off with the KJAZ studio number. The disc jockey reported to me an incredibly annoying “thump” (for lack of a better description) that was going out over the air at seemingly random times. To make matters worse, it was occurring in both channels. We started looking for the interference source immediately, but it was difficult to find due to its random nature. The interference consisted of strong, wideband bursts of RF that lasted less than a second. I quickly ordered a set of equalized stereo phone lines from Telco—but we had to deal with the ‘thump’ until the lines were installed. It was bad.

The lesson here is that the system I had built lacked in diversity. Sure, it had frequency and equipment diversity—but that was not enough. As a result of this difficult lesson, I’ve maintained both radio and wireline links to the transmitter site for every station I’ve maintained over the last 25 years.

What is the most up-to-date means by which a station can build adequate diversity into an STL system in 2014? That’s what this article is about. It isn’t just about STLs necessarily; you could apply the same methodology we’re going to go over to any point-to-point connection, such as studio facility to studio facility, studio to satellite uplink, or studio to booster (SFN) site. The methodology used really depends upon how serious you are about maintaining the connection.

In order to develop a technologically diverse system, we’ll still make use of both radio and wireline (Telco) connections. The reason for multiple path technologies is because some catastrophe that “takes out” one likely will not affect the other simultaneously. For example, an intruding interference source that appears one day can effectively kill a radio system but won’t affect a wireline link. Conversely, something that impairs the closest telco central office (and thus the wirelines) likely won’t harm the radio link. In the past, there may have been just a single station at the transmitter site. Now it’s likely there are 2, 3, or even more, and the requirements for the link are much greater. Fortunately, we can now get both radio and wireline links that carry significantly more information than in the “old days.”

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