What is an In-Car FM Modulator?

phone playing music with an fm modulator in a car
An FM modulator can provide a high quality, wired, connection from your phone to your car stereo.

Peopleimages / E+ / Getty 

There are more ways to listen to music and other audio content in your car than ever before, but most of them don't exactly play nice with older head units. Unless your car radio came with Bluetooth or an auxiliary input, your options are actually pretty limited. That's where in-car FM modulators can really help, since these devices essentially add an auxiliary input to any car radio, and they do it a lot better than your average FM transmitter.

What is an In-Car FM Modulator?

An in-car FM modulator is just a radio frequency modulator that is specifically designed for use in car audio systems. Radio frequency modulators are essentially just workaround devices that were originally designed to allow external components to be hooked up to televisions and home radios.

fm modulator
An FM modulator mixes the input from an MP3 player or your phone with the input from your car antenna, and sends the combined signal along to the radio.

Since both televisions and home radios were originally designed to accept only RF inputs from antennas, RF modulators work by adding an audio or video signal, or even both, to a carrier wave, which can then be processed by a TV set or a head unit just as if it had been received via an over the air broadcast.

FM modulators are largely unnecessary in home theater setups today, unless you are dealing with extremely old equipment. Modern televisions and audio receivers are built from the ground up with HDMI, RCA, optical, and other connections in mind.

Car radios are another matter, and basic car radios, even brand new ones, often lack any way to connect external equipment. Since the one thing just about every car radio does have is an FM radio, that opens the door for FM modulators.

The Basics of Broadcasting

Both television and radio broadcasts, including AM and FM radio, work in essentially the same way. At the radio or television station, audio and/or video programming is added to a carrier wave via either frequency modulation (FM) or amplitude modulation (AM). Analog television broadcasts used to use vestigial sideband modulation, which was a type of amplitude modulation, and digital broadcasts use a number of different types of modulation. The altered carrier signal is then broadcast over the air (OTA).

When a carrier wave is picked up by an antenna, the signal is demodulated by hardware inside the television set or radio, which is a process that reconstitutes the original audio and video data from the modulated carrier wave. The signal can then be displayed on the TV or played on the radio.

Until relatively recently, television sets lacked A/V inputs other than the antenna hookup, and a lot of car radios continue to lack any type of auxiliary input. In order to facilitate the connection of devices like VCRs to televisions, and tape decks or CD players to car radios, RF modulators were developed.

Tricking the Tuner with a Car FM Modulator

Car radios are designed to receive programming across a very specific range of the electromagnetic spectrum. When you tune a car radio to a channel, you set it to look for a broadcast on a specific frequency.

In effect, FM modulators take advantage of that to trick the car radio into playing back something other than an OTA broadcast. You still tune the radio to a specific frequency, but instead of finding a radio station, it finds the signal from your phone or MP3 player that the FM modulator has inserted.

This exact same method can also let you hook everything from VCRs, to DVD players, and even video game systems to television sets that lack A/V inputs.

fm modulator installed
FM modulators have to be installed in between the head unit and the antenna, so they are usually hidden behind the dash. Image courtesy of Dave Parker, via Flickr (Creative Commons 2.0)

In order to accomplish this feat, a car FM modulator has to be wired in between the head unit and the antenna.

The signal from the antenna passes through the modulator and into the head unit, but the modulator also has an auxiliary input that can be connected to a phone, CD player, iPod, generic MP3 player, or any other audio source.

When a device is plugged into the modulator in that manner, it essentially does the same thing that happens at a radio station: the audio signal is added to a carrier wave, which is then passed through to the head unit.

Car FM Modulators and FM Transmitters

While car FM modulators and transmitters are very similar, there is one key difference in the way that the head unit receives the signal. Due to laws that restrict the power of unlicensed radio transmitters, car FM transmitters have to be very low power. They’re strong enough to transmit the few feet that typically separate them from the car antenna, but it’s easy for such a weak signal to be drowned out in an area where there aren’t any dead spaces on the FM dial.

Since car FM modulators pipe the signal directly into the head unit, there’s less of a chance for interference. These devices can still suffer from interference, and they usually can’t match the audio quality of an auxiliary port, but they are a good option for head units that don’t have auxiliary ports.