Suppressing radio frequency interference from the car's ignition system was initially thought to be an impossible task. This is because the signal from the radio broadcast station was weak in comparison to the interference from the induction coils, high voltage wires, and spark plugs. Even though the broadcast station transmitted thousands of watts of energy, the energy from the transmitter's antenna diminishes with distance. Several miles away, the signal's strength is reduced to a small fraction of the energy at the antenna. With the terribly noisy ignition system located so close to the car radio's antenna, few engineers thought it possible to receive broadcast signals with the engine running.

So the first step in mobile receiving was to remove as much noise from the ignition system as possible. As always, there were limits. For instance, by adding resistance to the spark plug wires, some of the radio frequency energy could be reduced; however, engine efficiency and horsepower could suffer. The trick was to eliminate just enough noise at the source and improve on the tuner as much as possible. With the tuner improvements, along came specifications that have carried forward to this day. There are many specifications that can be used to qualify a receiver, but two of the most important specs are sensitivity and selectivity.

Our first mobile radio receivers were in the AM band, and the first receivers were rated as to their sensitivity with respect to picking up far away broadcasts. The problem here was that not everyone measured sensitivity the same way. Some engineers applied a standardized signal to the receiver's front end. Then they measured how much input signal it took to achieve a designated output power. Complicating the specification, not all manufacturers designated the same output power for their sensitivity measurements.

Further complicating the measurement, some engineers defined sensitivity by considering the difference in the receiver's internal noise with and without a signal. Sensitivity figures for various brands of tuners soon became more marketing than engineering. And for car audio, the problem with tuners that are super sensitive is that noises from the car's electrical systems can become a problem.

But is the noise from the car's high-voltage systems necessarily on the same frequency as the AM or FM radio stations? The answer is not exactly. Early receivers were "wide open" and could pick up stations at various frequencies across the radio spectrum. This became a problem for ships at sea because the listening stations had to hear transmissions from hundreds of ships at the same time. The Titanic's distress call was subject to this problem - radio traffic had to subside before the rescue ships could communicate with the Titanic. When we were finally able to tune a transmitter, it was of little consequence until receivers could select between various stations.

The ability of a tuner to reject undesirable signals is known as selectivity. For instance, if a radio station is broadcasting on a particular carrier frequency with a designated bandwidth, an ideal receiver should only pick up the signal necessary to get all of that bandwidth.

Selectivity is the term we use to gauge the width of the reception window. A good selectivity spec would be just wide enough to accommodate the entire bandwidth of the signal. A tuner with poor selectivity would permit out-of-band noise to enter right along with the station.

When comparing selectivity specs between different brands of tuners, the shape of the filter is seldom mentioned. But receivers with identical selectivity specs can be totally different in reception because the shape of their filters is totally different. Today we can discuss "brick wall" digital filters; companies such as Blaupunkt utilize digital processing for some of their radio tuners. So the shape of the filter should be considered when discussing the selectivity specs of a receiver.

As we discussed above, purchasing a tuner by the specs alone is not a good idea. And last week someone asked, "Dave, where have all the FM Boosters gone? We used to see them hanging at auto parts stores, Radio Shacks, and car audio shops everywhere. With a good booster, any tuner will work great. Right?"

Engineers have always squirmed at the idea of adding a radio frequency amplifier between the antenna and the tuner. FM signal boosters typically cause severe "Front-End Overload" in the majority of car audio tuners. If an inexpensive booster could really increase the signal level without increasing the adjacent channel noise, sales might be strong today. So in your search for a tuner, forget about FM Boosters, they won't help. After all, if a booster could really help, wouldn't the tuner manufacturers simply add one to their decks?

Another poor idea is to audition tuners in a retail shop demonstration environment. Ham radio Rule #1 on tuners states that, "The most important part of any tuner is the antenna." Most car audio display boards have "daisy-chained" antennas, electronic antennas, or no antennas at all. Additionally, there are usually neon lights, noisy car audio amplifier power supplies, and fluorescent lights in the near vicinity of the car audio tuners. Don't evaluate car audio tuners in a car audio display environment. And don't believe any tuner demos in car audio shops unless you can personally verify that Rule #1 is identical for all tuners in the test.

The best place to test a tuner is in your own car. First of all, call around and find a shop that will let you test a tuner in your own car. (They may be surprised because most car audio decks are purchased for their CD specs or flashy displays.) Get a notepad and pencil and park your car into a clear parking space at the shop and test your OEM factory radio. Before exiting your car, use the pad to list the number of SEEK stations (either AM or FM or both) on your existing deck. If you're a bit of a tweak, go ahead and list the signal strength (1 to 5) of each station based on your perception. I'd leave the engine running for this test.

Now run into the shop and ask for, "The Best Tuner" available. The installer can run out to your car, yank your existing deck, and loosely connect "The Best Tuner" to your antenna and system wiring. Start the engine, and redo the test. Can you pick up that 25-watt junior college station 75 miles away? Did you give it a 3 or a 5 on the signal strength? You be the judge.

If tuner performance is important, then evaluating various tuners in your own vehicle, parked in the same place, at almost the same time, is the best way to make a scientific comparison. Good tuning.