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Commentary: Why Some FM Mods Go Over Limits

Recent tests conducted by the NAB and NPR have raised questions regarding wireless FM modulators. Some of these products are apparently over FCC Part 15 limits.

Recent tests conducted by the NAB and NPR have raised questions regarding wireless FM modulators. Some of these products are apparently over FCC Part 15 limits.

The view of this situation is somewhat different, depending on whether one is a broadcaster or a consumer electronics maker, for obvious reasons. As manufacturers, we generally look to comply with existing regulations as much as broadcasters do, but recent events have shown that not all regulations are being observed, or at least not with the same attention to detail.

When we’re discussing wireless FM modulators, or “mods,” we’re generally talking about the low-power intentional radiating devices that are expected to transmit a weak signal a short distance to an FM receiver. By “short distance,” I mean perhaps a meter or two.

FCC rules are located in Title 47 of the Code of Federal Regulations. CFR 47, Part 15 covers these items. Part 15.239 and Part 15.209 cover the requirements under which these devices operate, with measurement specifications in several places including ANSI/IEEE standard C63.4.

Was I speeding, officer?

Many consumer applications for these modulators are for the mobile (car) environment – using a modulator to play your iPod, for example, over your OEM car stereo. A wireless modulator that strictly complies with Part 15 limits will sometimes have noisy audio in a real-world environment.

The device would need higher than Part 15 power to get clean performance under the majority of normal conditions. Markets such as New York City don’t have any open channels, so there is an ongoing motivation for transmitting higher power.

Put another way, the existing Part 15 limits don’t allow for reliable transmission enough of the time that consumers will be happy with the product. This leads to consumer calls and complaints – on the CE side. Of course, transmitting too much power leads to calls and complaints to the broadcasters, and eventually a letter from the FCC.

Apparently, the Part 15.239 limits have been watched somewhat less than a desert speed limit. That is, “everyone” went over the limits, and “everyone knew” that it was “okay.” This situation has been developing since about 2002.

Information available at that time indicated to us that some small companies had started offering product that significantly exceeded the limits. Apparently, this situation has grown to include larger companies.

Now that the FCC has lowered the boom, there are certainly more than a few engineers at more than a few companies – large and small – scrambling for fixes, rework, etc., before they get their letter.

What do you mean by ‘measured,’ exactly?

Of course, even if you strictly complied with the FCC limits in the required test procedures, you don’t have a solid picture of what is happening with modulators and cars out in the field. The FCC testing is typically done on a wooden rotating table. Some of the industry testing involves measuring field strength outside the car, which brings to light an interesting point.

What appears to happen is that the device – when mounted in a car – is attenuated 10-15 dB from the standardized test results, based on the NAB report data (RW Aug. 2). That means that the device that can bang a signal into another vehicle from a distance is significantly over-powered. It is overcoming significant attenuation and still capturing the second car’s FM receiver.

Like, totally wired, dude

Another item that has been studied is the “wired FM modulator tied to the car antenna” concept. In this case, the type of product considered is an FM modulator with only antenna cable connections, and no antenna for wireless transmission.

Data submitted by the NAB indicates that wired modulators are not part of the larger problem mentioned above. NAB testing shows that the field strength resulting from intentionally mis-wiring a wired FM modulator can push the entire configuration over Part 15 limits, but not by anywhere near as much as the wireless FM mods do.

After the NAB report came out, we checked with our field staff. No one was aware that this “intentional mis-wiring” might be going on in the field. If it is happening, it is not a happening on a significant number of installs.

Also, wired modulators only make up a few percent of the total number of modulators sold. The unit sales for wireless modulators probably exceed the wired ones by a factor of 100 or more. Probably, more wireless modulators were sold in the fourth quarter of 2005 than the total number of wired modulators sold in the past 10 years.

So, since the wired modulators are outnumbered by 100 to 1 or more, and must be mis-wired to be considered, and even then are some 30-40 dB under the strongest wireless modulator output power, one can safely conclude that the wired units are unlikely to be the reason your listeners are hearing Howard Stern.

Feeling their pain

I have some sympathy for engineers who have been routinely ignoring the Part 15 “speed limit” and are suddenly facing a “traffic cop” for doing what everyone has been doing for years. I’d be pretty surprised if I had my car impounded for driving 60 in a 55 mph zone.

However, I believe broadcast engineers have a right to feel differently for two reasons: first, this is directly causing them headaches; second, enforcement of broadcast rules appears to be quite strict, so sympathy from broadcast engineers is not something we in the CE world should expect.

Current FCC regulations permit just enough power to get a signal through, but with noise. Wireless hardware makers have probably not spent enough time optimizing performance with compliant power levels as opposed to simply increasing power.

On the other hand, wired products provide an excellent solution when wired correctly, and appear to be not a significant source or problems even when intentionally mis-wired by lab technicians. Wired, dealer-installed products are not ideal for every consumer.

However, wired FM modulators are the best technical solution for the bleed-through problem today.

RW welcomes other points of view.