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FM Transmitters: How They Work & Why They're So Bad!

The purpose for an FM transmitter is to allow you to listen to any external audio source played through a car stereo, or any radio with an FM band. An FM (Frequency Modulated) transmitter, which is also called an RF (Radio Frequency) modulator or FM modulator, is an aftermarket device that you connect the audio from the earphone jack of a portable audio device, such as an iPod, a Zune, an MP3 enhanced cell phone, an MP3 player, a CD player or satellite radio system, into the FM transmitter. The sound from the portable audio device is then broadcast through the FM transmitter as an FM radio station, and is picked up on your car radio as an FM station and played through the FM band out through the speakers.

Looking at how the FM transmitter works in a stage-by-stage description, the first stage is the converter that takes in the audio output from the external audio source and converts it into analog audio within the FM transmitter. In the second stage, the now convertered analog audio signal is converted again into an FM signal by modulating the audio using FM modulation. This FM modulated signal is then laid onto an RF transmitter signal from the FM transmitter on a specific frequency as an FM radio station signal in the final stage. You can then tune your carís radio to the specific FM station frequency that the FM transmitter is set to transmit on and listen to the audio in your car on the FM band as an FM station.

It doesn't matter if the FM transmitter is advertised as "new and Improved" or "brand new technology" or whatever. If it takes in external audio and you have to tune your car's FM radio to an FM radio station in order to listen to it, then its an FM transmitter and the operational description in the above paragraph is how it works.

There are two distinct types of FM transmitters, each having their own distinct requirements for installation, but in the end, operate pretty much the same way and share the same shortcomings. The ďwiredĒ FM transmitter is physically connected in between the radio and the radioís FM antenna. When the FM transmitter is in use, it has an internal circuit that electronically disconnects the antenna by grounding the FM signals coming in through the carís antenna and electronically connects the RF FM radio signal coming from the FM transmitter directly to the car radioís antenna input. When the FM transmitter is turned off, this same internal circuitry electronically re-connects the carís FM antenna back to the radio.

Back in the late 50's and early 60's, these "wired" FM transmitters were all that were available. The disconnecting of car's antenna from the radio was accomplished through an electro-mechanical process using a physical toggle switch and a relay. With the advancement of transistors and semiconductors, the electro-mechanical process was later replaced in the late 60's by the electronic method still in use today in "wired" FM transmitters.

The "wireless" FM transmitters, which began to appear in large numbers in the mid-80's, do not physically connect to the radio and do not disconnect the car's antenna. The input into the wireless FM transmitter from the audio source is still the same cable type connection as the wired FM transmitter, but rather than physically connecting to the factory radio they connect wirelessly to the radio by transmitting a low-powered FM radio station signal through the air utilizing a RF signal generator built into the FM transmitter. This low-powered signal is picked up through the carís antenna, just as any other FM station signal would be. Most wireless FM transmitters on the market are very low power and typically have a short range of no more than 100 feet.

All FM transmitters, both wired and wireless, operate within the FM frequency band from 87.7 to 107.9 in the US. The lowest-cost FM transmitters are typically internally designed to one specific FM frequency, and while that one specific frequency can vary from manufacturer to manufacturer, it usually falls within the 87.7Ė91.9 MHz band, which is allocated to educational broadcasts in the United States. Others FM transmitters with a slightly higher price will provide two, three or maybe even four separate preprogrammed FM frequencies from which to choose. A few at the upper-end of the price range allow you to tune the FM transmitter to a specific frequency anywhere up and down the FM band, which gives you an improved chance of finding a frequency in your geographic area that does not have a radio station already assigned to it.

INSTALLATION: Wireless FM transmitters generally require very little to be successfully installed in any vehicle. Basically, you connect it to a power source, connect the audio device whose audio is to the transmitted into your carís radio and youíre done. Since they only physically attach to a power source and the audio device that will play through them, the location for the FM transmitter can be pretty much anywhere. Obviously, the closer they are in proximity to the radio, the better the chances are for lower noise and interference.

Wired FM transmitters are more difficult to install than wireless mainly because wired FM transmitters directly connect to the car radioís antenna and typically require you to remove the radio in order to get to the antenna socket at the rear of the radio. They are generally more costly to install because, depending on the make and the year of the vehicle, radio removal could require radio removal tools, and installation could require two sets antenna connector adapters, one input and one output, when being installed, neither of which are included with the transmitter. This sometimes will also complicate or restrict where the FM transmitter can be located in the vehicle based on the length of the input and output antenna cables that come with the FM transmitter.

Either type of FM transmitter can be self-powered or battery driven but both are usually powered through the use of the cigarette lighter socket, typically included in with the product. While this allows for easy power connection, in most cases, it is also the source of static and/or ignition noise appearing in the audio, as the cigarette lighter is notorious for being a ďdirtyĒ power source in most domestic and foreign cars. This is because of the cigarette lighterís need for itís circuit connection to be a high current pull circuit, and the cigarette lighter is most often connected directly to the carís ignition system. And because most FM transmitters are poorly designed for noise elimination, the static, bleed-through and ignition noise comes right through the FM transmitter and right into your audio stream.

The relatively low power output of FM transmitters, both wired and wireless, all of which is FCC regulated, make them unsuitable for use in large urban areas because of sheer number of FM radio stations up and down the FM dial and the FM transmitter's need to be set to a radio frequency where another FM radio station is not. In todayís world, there are FM radio stations signals playing up and down the FM band, a situation that did not exist when the 70 year old technology was created in the late 50ís, a time when large segments of the FM band was quiet and void of any FM stations at all. However, nowadays strong FM radio station signals can actually bleed into the FM transmitter and be imprinted onto the FM transmitterís signal being injected into your radio, even on the wired models. Removing a car's radio antenna connection completely from either wired or wireless units has been found to slightly reduce the FM transmitter audio noise. However, removing the carís antenna will also render the AM band totally unusable and will restrict FM reception to only the very strongest FM station signals in your geographic area.

Some newer vehicles that have car antennas that serve multiple purposes besides just AM/FM radio reception can also create issues. These multipurpose antennas handle satellite radio reception, hands free cell phone like OnStar, GPS navigation systems, etc., where AM and FM radio reception are connected to the radio in diverse, non-standard ways, which in turn have caused some issues with some poorly designed FM transmitters. Some wired FM transmitters which are of old school standard design have, in some cases, ended up inadvertently disconnecting the AM antenna of some cars completely, rendering the AM radio inoperable even though technically, the AM antenna remains connected.

There are additional problems that can develop when connecting certain models of external audio devices that have no way to adjust the earphone jack volume or audio devices that connect via line out connectors that also have no means of controlling the volume. When volume level is too high, it can over drive the audio circuits in the FM transmitter causing over modulation, audio distortion and possible radio interference When volume level is too low, the audio circuits in the FM modulator could operate below their input level requirement and produce poor fidelity audio, which would then be compounded by being required to have the car radioís volume operating close to maximum. In theory, you could use an automatic level control or audio limiter circuit in between the audio source and FM modulator to overcome this problem, but you are just adding complexity to something that should have been simple, and there are very few automatic level control or audio limiter circuit devices with such a capability that are readily available.

The audio quality difference between an FM transmitter and a direct-connect Auxiliary Input Converter is dramatic. We have touched a little on the technology issues with FM transmitters and briefly shown how that technology works by having to convert any digital quality audio input three separate times before it transmits the signal as an FM broadcast. This multiple-conversion process takes a toll on the fidelity and over-all quality of the audio file. The easiest way for us to graphically show the difference is by using a 1 to 10 scale analogy, with 1 being the worst audio fidelity reproduction of the original audio file and 10 being the best audio fidelity reproduction of the original audio file. Using this scale, we would grade a wireless FM transmitter as a 1, very, very poor quality; and we would grade a wired FM transmitter as a 1.5, slightly better, but seriously flawed, and lastly, we would grade a direct connection auxiliary input converter as a 12, off the chart. Whatever the audio sounds like when it leaves the digital MP3 player, will be the way it sounds when it comes out of your car radio speakers.

In conclusion, if you currently are using an FM transmitter now, then you will hear first hand the difference in audio quality by getting and installing a direct-connect Auxiliary Input Converter. Because as we said, the audio quality difference between an FM transmitter and a direct-connect Auxiliary Input Converter is dramatic!

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