The author runs Burt I. Weiner Associates in Glendale, Calif.
I just finished reading Larry Langford’s guest commentary on tossing out the yearly NRSC proof for AM stations. Here is my reply.
Larry, I will agree with you on one point, and one point only: Most AM audio processors of recent vintage have the 10 kHz low-pass filter and meet the requirements (at the time of manufacture) of FCC Rule 73.44. But that doesn’t mean that these processors will continue to properly do their jobs forever, and some of them are already pretty long in the tooth. There is a lot of “stuff” downstream from the processor that can and does fail. The NRSC proof is not just about the mask limits, but if done properly takes into account harmonic amplitude, spurious signals and intermodulation products.
Yes, newer, high-efficiency transmitters are much different than older tube-type transmitters. They have switching power supplies, new methods of modulation such as PDM, PWM and a myriad of other modulations schemes. We no longer have to change expensive tubes in transmitters, instead, we now have to periodically change electrolytic capacitors and solid-state devices. Any of the above can and will eventually fail, often causing excessive occupied bandwidth, spurious signals and high harmonic content. Quite often these failures go unnoticed by the station’s staff.
I’ve been doing “NRSC Proofs” since they were first required back in the early 1990s. Since then I have seen many instances of transmission equipment failures resulting in RF spectral problems that station staff, including their engineers, were unaware of.
To cite a few actual cases …
In the high desert there was a problem with high noise levels across most of the AM band. This had been going on for a few years and was attributed to power lines. A station in that area called me to do their annual NRSC proof. What I found was that their high-efficiency, solid-state transmitter was spewing garbage across most of the AM band. The edges of the 10 kHz audio filter could not be seen on the spectrum analyzer. The problem turned out to be a blown capacitor in their transmitter’s PWM filter circuitry. Once this was repaired the station became a great example of what a good NRSC proof mask should look like. Coincidentally, the noise across the AM band disappeared.
A station that was in the process of commissioning a new 5 kW AM transmitter from a well-known transmitter manufacturer asked me to do a RF proof of the transmitter. One of the first things we noticed were symmetrical spurs each side of the carrier and only down about -14 dBc. This turned out to be caused by a lead in the PDM filter circuitry that had apparently come loose in shipping.
There was another station who for years had gone through their annual NRSC Proofs with no problems. During a regularly scheduled NRSC proof for this station we discovered they had a mix product that was being generated in their transmitter that was only -27 dBc and on top of another station about 15 miles away. This was the result of a station that had lost its land lease and in order to diplex with an existing station, had relocated to a site about six miles away from my client’s station.
The above stories are only a very few examples of what I have seen over the years. They were not the fault of the processors, but things downstream from the processor. Although, there have been situations where processors have failed, causing problems. These are not rare or unique situations.
I have discussed the value of the current NRSC proofs with many knowledgeable engineers. All agree that the yearly NRSC proofs serve a very useful function in reducing man made interference to the AM band.
Let me also add that in my opinion a yearly proof should also be required for FM broadcast stations. Is FM somehow magically immune from having transmission problems? I have documented proof that FM’s also have stuff fail or inadvertently misadjusted transmission equipment resulting in serious interference to stations several channels away.
So, in conclusion, I must say that I strongly disagree with your reasoning to dump the required annual NRSC Proofs for AM. It would be a foolish move. Regular checking of your RF transmission equipment and spectral emission as described in the Rules under 73.1590 and 73.44 is good engineering and the right thing to do. It’s a small part of the cost of doing business.
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