The Head Unit: Understanding Car Audio Systems, Part 2

The Component That Makes the Sound in Your Vehicle

It's often called a radio, but "head unit" is a more appropriate term for this component since it is the brains and command center for a car's audio system. The head unit is what lets you choose the audio source, set the volume, determine the specific song you want to hear or pick the radio station you want to listen to.

All audio signals also start with the head unit. It generates sound from both "over the air" signals, such as radio, and recorded formats, such as a CD and other digital media. A car head unit typically has everything but the speakers crammed into that one box in your dash — far different from a component home theater system, in which devices such as signal processors and amplifiers can be housed in separate units connected by an array of cables.

Here's an in-depth look at the various elements that make up an automotive head unit.

The Tuner
In the early days of automotive electronics, the only entertainment option was to tune in an AM radio signal — hence the name "tuner." Next came FM radio. In metropolitan areas, radio stations now might also broadcast HD Radio.

An HD signal is a high-quality digital simulcast of standard analog AM or FM programming. Some stations also broadcast completely separate programming on sub-frequencies that are only available with an HD tuner. Of course, your car's head unit must have a dedicated HD Radio tuner to receive these broadcasts.

More automakers are adding HD Radio capability to their vehicles as standard equipment or as an option, even though consumers are still largely unaware of HD, despite the improved sound quality and extra features. You'll want to check HD Radio availability in your area before paying extra for the feature.

Satellite radio from Sirius and XM is more popular with consumers. These subscription-based services offer hundreds of channels of programming. Stations don't fade the way AM and FM does as you drive away from an area, and the sound of satellite radio is higher quality than that of analog radio. Aftermarket satellite radio and HD Radio tuners can also be added to head units that do not have them built in.

The antenna is a critical part of the radio tuner. For years, the antenna was a metal mast mounted on the exterior body of the car. Many automakers have recently adopted a "shark fin" style of antenna for better aerodynamics and a more streamlined look. If you don't see either type of antenna on your vehicle, it probably has an array of thin wires embedded in the windshield or back glass that serves as a radio antenna.

Disc-Based Music Sources
Car head units also play recorded material from various sources. Eight-track and cassette players had their heyday. Even record players briefly appeared in cars in the 1960s.

A compact disc player is the main non-radio audio source in cars today. Some head units can control remotely mounted CD changers. Head units that play both CDs and DVDs are also popular, since they let consumers play DVD videos on separate screens in the back — and in the front, provided the car is parked.

Many modern CD players can also play MP3 music files that have been burned onto a disc, which makes many more songs available. Most head units that play DVDs can also play MP3 files burned onto those discs. DVDs can hold significantly more files than a CD.

Some head units also have a built-in hard-disk drive that can store music files transferred from a CD or DVD. These disk drives range in size from 10GB to 30GB, with larger sizes allowing more music storage.

Portable Music Sources
Most modern head units offer some way to integrate portable music players like the iPod into a car's audio system and control them through the head unit. The most basic connector is the "aux-in jack." You plug an inexpensive accessory cable into the aux-in jack, which in turn connects to the headphone jack of an iPod or other music player. You operate the device using its own controls with an aux-in jack, although a car's head unit will adjust the volume. An aux-in jack also won't charge a music player's battery.

An even better way to connect a portable music player is via a USB port. In most cases, you can operate the device using the head unit's controls. The battery will also be charged when it's connected via USB. Most cars with USB ports will play music files directly from a USB thumb-type memory drive as well. A few vehicles also have a SD card slot for access to music files on that portable media source, which is most often associated with digital cameras.

Finally, a technology called Bluetooth audio can wirelessly send music files from a compatible device to the car's audio system, just as Bluetooth for hands-free phoning allows you to hear a caller's voice over the sound system's speakers. There's a caveat with Bluetooth: The controls are limited to basic functions such as play/pause and track-skip forward/back, and the head unit usually doesn't show information such as an artist or track name.

The Amplifier
Another article in this series covers amplifiers in depth. But it's worth a quick overview here since most automotive head units have a small built-in amplifier. The amplifier is actually made of two components, a preamplifier (preamp) and a power amplifier.

The preamp takes data from a radio tuner, CD player or other audio source and slightly boosts it before sending it to the power amplifier. The power amplifier further strengthens that signal so it's powerful enough to move the voice coils of the speakers in the system. (You'll find more about speakers in a separate installment.)

In addition to operating every audio source, the head unit can also tailor the sound with tone controls for bass and treble or with a built-in equalizer. Higher-end systems also use sophisticated digital signal processing (DSP) technology to further shape the sound and combat the ambient noise that's common in a car's cabin from engine, road and wind noise.

Of course, a head unit also controls the overall volume of the system. But today you may not even have to touch a knob or button on the dash to operate certain aspects of a car's audio system. Many car audio systems have volume controls on the steering wheel that can change radio stations or CD tracks and switch audio sources.

Some cars also have steering-wheel switches that allow the driver to operate an audio system via voice commands. And some even let the driver switch audio sources or change radio stations and CD tracks just by pushing a button and saying a command.

The Elements of Audio Sound
There are a lot of components that come together to create a car audio system. Understanding the role of each one will allow you to make a sound decision when you're buying a new car, or when you're shopping for new aftermarket audio components for your current car.

You can find an overview of the basic components that make up the audio system in a vehicle in Part 1 of this series. The rest of the series looks at each of those components and their functions, including speakers and amplifiers.

You can reference other installments in this series here: