What Is an In-Car FM Modulator?

FM modulators deliver digital audio to old-fashioned car radios

Phone playing music with an FM modulator in a car

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Older head units are a real drag for our modern, smart phone-obsessed world. Unless your car radio came with Bluetooth or a USB/auxiliary input, your options for listening to audio from your mobile device are rather limited.

In such cases, most people turn to in-car FM modulators. These gadgets essentially add an auxiliary input to any car radio.

What Is an In-Car FM Modulator?

An in-car FM modulator is just a radio frequency modulator that is specifically designed for use with car audio systems. These radio frequency modulators allow external components to be hooked up to televisions, home radios, and vehicle head units.

Consumer FM modulator
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RF modulators work by adding audio or video signals to a carrier wave. Those signals are then processed by a TV or head unit as if they had been received via an OTA (over-the-air) broadcast. But unless you are dealing with very old equipment, FM modulators are unnecessary in home theater setups. Modern televisions and audio receivers are built from the ground up with HDMI, RCA, optical, and other connections in mind.

Car radios, on the other hand, are mostly equipped with FM radios anyway, making them ideal candidates 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 or video programming is added to a carrier wave via either frequency modulation (FM) or amplitude modulation (AM). The altered carrier signals are then broadcast over the air in every direction.

When a carrier wave is picked up by an antenna, the signal is demodulated by hardware inside the television or radio. This process reconstitutes the original signal from the modulated carrier wave, allowing it to display on a TV or play on a radio.

Until the digital age, television sets mostly lacked A/V inputs. RF modulators were developed to facilitate connections between analog sources and digital media players.

Tricking the Tuner With an FM Modulator

Radios are designed to receive signals from a specific range within the electromagnetic spectrum. When you tune a radio to a specific channel, you are scanning the spectrum for a broadcast on a specific frequency.

In effect, FM modulators take advantage of that to trick the car radio into playing a locally broadcast signal. You still tune the radio to a specific frequency, but instead of finding a radio station, it finds the signal from the digital device that the FM modulator has inserted. (This same method allows you to connect DVD players, Blu-ray Players, and video game consoles to televisions that lack A/V inputs.)

FM modulator installed in a car dash
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An FM modulator must be connected between the head unit and antenna to work properly. The signal from the antenna passes through the modulator into the head unit, but the modulator also has an auxiliary input that can be connected to a mobile device or digital media source.

When a device is plugged into the modulator in such a manner, it performs the same function of a radio station on a smaller scale: 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 FM modulators and transmitters are similar, there is a key difference in the way that head units receive signals. 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 separate them from the car antenna, but such a weak signal can easily be drowned out by a strong OTA broadcast signal. That's why they work best over frequencies with little to no reception.

FM modulators 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.