Audio & Electronics

Understanding Car Audio Systems: The Head Unit

In the first part of our car audio series, we took a general tour around the vehicle and got a sense of where everything is. We looked at all the main component groups and discussed, in brief, the functions of each. In this installment, we'll take a closer look at the head unit and examine how it interfaces with the other components.

As we said in our introductory installment, the head unit is the heart and soul of your car audio system. Without it, you would not have sound in your vehicle. More commonly referred to as the radio, the head unit acts as the central processing unit of your system. Think of the CPU in your home computer. Would your personal computer work without that big box that everything else plugs into? I don't think so. The same thing holds for your car audio system. If you had a huge hole in the dash where your radio once was, you'd be listening to whole lot less Howard Stern in the morning.

Radio systems first began appearing in American sedans in the '40s and '50s. By today's standards, these early efforts were rudimentary and primitive — some were quite literally the size of a breadbox. There were several reasons for this.

For one, there were very few competing systems in the vehicle. The dashboard held no air conditioner, no navigation system, no cruise control, no electronic mirror switches, no traction control, no airbags, not even an emergency flasher switch. Second, the vehicles were much larger in those days, with voluminous areas behind the dash. And lastly, the transistor had yet to be invented. It took a little Japanese company named Sony to change all that. Believe it or not, until the invention of the transistor, the first car radios used tubes to process and amplify the signal path. And there are two things that tubes hate: heat and vibration. So much for that experiment...

As if that wasn't frustrating enough, the first car radios played only AM radio. Honest! Would we joke about something like that?! The FM band had not yet arrived as a viable commercial medium, and the cassette and CD medias were decades in the future. Even the eight-track player was locked in the dustbin of some demented human's mind. "What rough beast," the poet Yeats wrote, "slouches toward Bethlehem, waiting to be born?"

All of this is a roundabout way of saying that it took a long time for the head unit to reach its current prominent position in the dashboard of the modern motor vehicle. Much of the impetus came not from the automakers, who had to be dragged kicking and screaming into the modern auto sound era, but from the aftermarket players. Companies like Blaupunkt, Pioneer, Panasonic, Sony, Kenwood, Clarion and Alpine, to name a few, led the way in a revolution that continues unabated today.

But back to our point. The head unit acts as the nerve center of your car audio system. And that's not the whole story, because the head unit is actually a collection of different systems crammed into one box. Let's take a closer look at each component group.

The tuner section. This group of circuits is responsible for delivering AM and FM radio signals to the head unit. In recent years many automakers, such as Mercedes-Benz and General Motors, have also begun offering weather band, marine band, and other channels to the radio dial, so that drivers may keep abreast of weather patterns that might affect traffic flow, for instance, or decide whether to take the boat out for a quick spin through the harbor.

The tuner section includes the presets and all the receptors for the radio signals. The tuner section is typically attached to an exterior antenna, which serves to pull in a stronger signal and improve reception. Many automakers, particularly upscale sedan manufacturers, now use a radio antenna built into either the windshield or rear window for this purpose. In addition to creating cleaner-looking cosmetics, it reduces wind resistance, cuts down on vandalism, and protects the antenna against undue harm (corrosion, rust, physical damage). Other automakers use an automatic electronic antenna. In this scenario application, a lead runs from the tuner section to the antenna. When the radio is turned on, the antenna goes up; when it's turned off, it goes down. And then there's the good ol' coat hanger: a thick metal wire attached to the fender. It works just as well, but isn't nearly as fashionable.

The cassette. Many head units, either aftermarket or original equipment, have a built-in cassette player. Although the cassette is considered passé in audiophile circles these days, the format has been around for 30-plus years and is a very cost-effective way to transmit information and/or entertainment. Books on tapes and lectures are just two examples. As with the tuner section, the cassette is actually a separate group of components dropped into the head unit. It's conceivable, therefore, to have a defective cassette deck and not have to trash the whole head unit. A technician can open the "lid" on the radio and repair or replace the defective cassette player, and down the road you go. Cassette players are more prone to breakage and wear-and-tear than, say, the tuner section, because they have many more moving parts. Also, the movement of the tape across the tape heads causes wear on the heads.

The CD player. More and more cars also have a compact disc player built into the head unit. Many upscale cars don't even offer a cassette anymore, or offer it only as an option, since CD has now become the dominant high-fidelity music format. CD will eventually be replaced by DVD, MP3 or some other digital technology down the road. The principle here is the same as the cassette deck: the CD is actually a separate component built into the head unit. As with the cassette, the CD player, because it contains moving parts, is subject to wear and service issues; but, similarly, it can be repaired or replaced without trashing the whole radio. Many vehicles also incorporate a six- or 10-disc CD changer into the sound system, located somewhere else within the car, which feeds into the head unit through a separate circuit.

The amplifier stage. OK, then how does the signal get from the tuner or cassette or CD to the speakers? That's actually a really good question.

The answer is the amplifier stage. Actually, it's a little more complex than that, since the amplifier stage is composed of two subcomponent groups: the preamplifier section (preamp, for short) and the power amplification section. Although we'll delve into this more deeply in our next installment (Part Three: The Power Amplifier), let's take a quick look right now.

You have all these components -- the tuner, the cassette, the CD -- producing signals that we eventually hear as music. But how do those signals arrive at the speakers to produce sound?

The signals are first carried by wire or circuit board to the preamplifier stage. The preamp routes the chosen signal to the power amplifier, which then boosts it into the audible range and sends it, via wire, to the speakers. How does it do this?

Well, you've got all these buttons on the front of the radio, right? These are actually part of the preamplifier section. When you press the FM button, for instance, the circuitry inside the preamp directs the signal from the tuner, through the preamp, to the power amplifier. Likewise for the CD player and the cassette. The preamp stage also includes all the tone controls (bass, treble, etc.), sound processing (balance, fade and the like), and other signal processing circuits. For instance, some vehicles, such as Chrysler, offer a three-, five-, or seven-band equalizer built into the head unit. In this case, the equalizer is actually an expanded tone control. The signal is fed through the equalizer in a loop, exiting out of the preamplifier and returning back to it with the signal altered per your adjustments. The preamp then sends the altered signal onto the amplifier. All the other controls typically operate in a similar "loop" scenario.

If you have any steering wheel-mounted controls, these are also usually directed through the preamp section.

The back of the head unit holds a group of wires known as the wiring harness. These wires carry the signal to other components in the system. In more sophisticated setups, a remote power amplifier will typically be located in the rear of the car. In this scenario, a lead coming directly from the preamplifier stage exits the rear of the radio and carries an inaudible signal to the power amplifier, where it is boosted into the audible range and sent on to the speakers. This kind of radio is known as having a "pre-out." In more conventional setups (i.e., systems with lower power), a small power amp is attached to the rear of the head unit. For reasons of heat dispersion, the power amp will almost never be located inside the radio. In this situation, the preamp signal travels the short distance to the power amp, where the amp increases it and forwards it on to the speakers.

Remember, if an individual component goes bad in your head unit, you can usually get it repaired without have to replace the entire radio.

In our next installment, we'll take a closer look inside the power amplifier.

We've included the link to our opening installment below.