Older head units are a real hurdle for our modern, smart phone-obsessed world. If your car radio is not equipped Bluetooth or USB / Auxiliary input, your options for listening to audio from your mobile device are quite limited.

In such cases, most people use car FM modulators. These gadgets essentially add an extra input to any car radio.

What is a car FM modulator?

A car FM modulator is simply an RF modulator that is specifically designed for use with car audio systems. These RF modulators allow you to connect external components to TVs, home radios and car head units.

consumer FM modulator
Photo from the Amazon

RF modulators work by adding audio or video signals to a carrier. These signals are then processed by the TV or head unit as if they were received through essential ( OTA ) broadcast. But unless you’re dealing with very old equipment, FM modulators are not needed in your home theater setup. Modern TVs and audio receivers are built from the ground up with HDMI RCA, optical and other connections.

On the other hand, car radios are mostly equipped with FM radios anyway, making them ideal candidates for FM modulators.

Broadcast Basics

Both television and radio broadcasts, including AM and FM radio work almost the same. At a radio or television station, audio or video programs are added to the carrier using frequency modulation (FM) or amplitude modulation (AM). The modified carrier signals are then broadcast over the air in all directions.

When a carrier is sensed by an antenna, the signal is demodulated by hardware inside the TV or radio. This process restores the original signal from the modulated carrier, allowing it to be displayed on a TV or played back on the radio.

Before the digital age, televisions were mostly lacking in A/V inputs . RF modulators have been developed to facilitate communication between analog sources and digital media players.

Tricking the tuner with an FM modulator

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

In essence, FM modulators take advantage of this to force a car radio to play a locally transmitted signal. You still tune the radio to a specific frequency, but instead of searching for a radio station, it finds a signal from the digital device that the FM modulator has inserted. (The same method lets you connect DVD players, Blu-ray players, and game consoles to TVs that don’t have A/V inputs.)

FM modulator installed in car dashboard
Dave Parker / Flickr / CC 2.0

The FM modulator must be connected between head unit and antenna for proper operation. The signal from the antenna passes through the modulator to 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 connected to the module in this way, it performs the same radio function on a smaller scale: the audio signal is added to the carrier, which is then transmitted to the head unit.

Car FM modulators and FM transmitters

While FM modulators and transmitters are similar, there is a key difference in how head units receive signals. Due to laws limiting the power of unlicensed radio transmitters, car FM transmitters must be very low power. They are strong enough to transmit the few feet that separate them from a car antenna, but such a weak signal can be easily drowned out by a strong OTA broadcast signal. This is why they work best at frequencies with little to no reception.

FM modulators can still suffer from interference and they usually can’t match the sound quality of an auxiliary port, but they are a good option for head units that don’t have auxiliary ports.

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