Build quality:
Some head units are flimsy and weak. Some are built to last. Usually the best way to determine this is by the weight of the unit. Also, check to see if anything doesn't "fit", or if there are parts that are cheap or flimsy. The truth is this: The bigger name units tend to be better. Also, the more expensive units tend to be better. This isn't always a big deal for everyone, but I believe that build quality is very important.
Basic Head Unit Features:
Power output:
Most head units have built-in amplifiers. These drive the speakers. Some head units do not have internal amps, and therefore, require the use of an external amp. Head units without amps are usually high-end models. Anyway, power is measured in WATTS. We'll talk more about watts later, but for now, just see more watts as more volume. Most head units (unfortunately) are deceptive about the power output. Usually, the power output spec on a unit will say "50Wx4 MAX output". This means that when the laws of physics are on your side, it's a clear day everywhere in the world, the planets are aligned, the bass hits really really low and hard, and for about a tenth of a second, yeah, your head unit will put out 50Wx4. However, most of the time, your head unit is probably putting out a quarter of that. It's not that the head unit manufacturer is lying, it's just that they are exaggerating the truth, AND MOST PEOPLE DON'T KNOW BETTER. So, anyway, many people put a lot of importance on a head unit's power output. I don't because head unit amps suck, and no serious car audio person uses the head unit's amp anyway. Most head unit brands have about the same power output anyway. 9 times out of 10, an aftermarket head unit will put out more power than your factory head unit, and as such, will sound better.
RCA Preouts:
These are used to connect external amps to your head unit. These are the same red and white cables that you use for your home system. Basic head units usually have one set of preouts (front left, front right). Some head units have two sets (front left, front right, rear left, rear right), but most decent head units have 3 sets (front left, front right, rear left, rear right, subwoofer left, subwoofer right). One quick note about subwoofer outputs: Sometimes they're called "non-fading" outputs because they are not effected by the head unit's fade control (front to back balance). If you're not planning on adding an external amp, you don't need to worry about RCA pre-outs. If you want to add an external amp (or amps), I would always recommend getting a unit with three pre-outs.
RCA Pre-out voltage:
Usually this spec is next to pre-out spec. It is measured in VOLTS (which we'll go over later). Basically, the higher the voltage, the LOUDER the music will be at your amp. Don't look at this as "free power" because it is not. In addition, the higher the voltage, the less noise that will be introduced to your system. Again, this spec doesn't matter if you don't plan on using an external amp. If you do plan on using an external amp, try to find the HIGHEST voltage you can. 2 volts is basic, 4 is standard for a good unit, sometimes you'll see units with 5 volts. I know Eclipse units have 8V(!) pre-outs. In short, more voltage=less noise and cleaner sound.
Digital time correction:
This is used to simulate a perfect stereo environment. If you're at home listening to music, you want to situate yourself in the middle of the two speakers to get the best sound. The same is true in the car, EXCEPT that it's a hell of a lot more difficult. Usually, the passenger side speaker will be farther from your ear than the driver side. Since sound travels fairly slow (comparfed to light), you'll have one ear receiving a portion of sound before the other ear. The digital time correction delays the music slightly so your ears will be perceiving the same music at the same time. This is a very important feature if you are planning an SQ system.
Crossover:
This feature is incorporated into many components in a car audio system. If your head unit doesn't have it, don't worry. Just be sure your amps have it. Basically, this makes sure the right speakers are playing the right sounds. I will go into more detail about crossovers later.
Equalizer aka EQ:
This feature allows you to tweak the sound. The most basic EQ is a two-band, and controls BASS and TREBLE. Some head units have 3-bands, BASS, MIDS, TREBLE. More advanced head units have 5-band EQs. If you are serious about tweaking your sound to perfection, you won't use your head unit's EQ; You will get an external unit, and some of these units have 30 bands! I will go into much more detail about EQs later.
Theft Deterents/Security Features:
There are a few options when it comes to head unit security:
Detachable Faceplate:
The most common method is a detachable faceplate. Basically, you remove the faceplate of the unit (where the screen and controls are), and the unit won't function. This will deter thieves, but please don't leave the faceplate in your car. Thieves know most people do this, and will still break into your car, find the faceplate and steal your head unit.
Password:
Some other units have a password function. When the unit is disconnected from power, then reconnected, the unit will require some sort of password. The unit will not function until the password is provided.
Key CD:
A few units use a key CD. Basically, the first time the unit is used, the unit will ask for a key CD. You insert any CD, and the unit will now designate that CD as the key CD. If the power is disconnected, the unit will ask for the key CD before it will function again.
Black out face:
When the unit is off, the head unit has a completely black face with no visible buttons or controls. Come on. Criminals aren't so stupid that they can't see that the head unit still has its face attached.
Hidden faceplate:
This is common for DVD players with screens, and one unit that I used to own, the Alpine CDA-7998. Basically, the face folds into the head unit, and looks exactly like a unit with the faceplate removed. Neat, but the unit is still vulnerable.
In short, all of these tactics are imperfect. Pick the one that makes the most sense to you, however most units only have a detachable faceplate feature.
CD Changer controls/Optional equipment interface:
This is a feature that allows you to connect other peripherals to your head unit. These peripherals are often CD changers, digital music players, satellite radio units, and video equipment. With this feature on your head unit, you'll be able to add additional capabilities to your stereo system in a convenient way. However, most of these interfaces are only compatible with the same brand. In other words, you'll need an Alpine head unit with the Ai-net interface connected to an Alpine CD changer with the Ai-net interface.
Remote Controls:
Some head units have remote control capability. This is a handy feature because you can control the headunit without taking your eyes off of the road.
OK, so these are some of the features of head units. Now, let's look at the specs of specific types of head units.
Tuner Features:
Most head units have AM/FM capability built in. Check out this information on getting the best AM/FM performance possible.
Presets: This feature allows you to save your favorite radio stations, and instantly recall them at the push of a button.
HD Radio: A new radio format that is close in sound quality to CDs. Basically, a radio station broadcasts their usual programming, but with an additional signal that can be received by units with "HD radio" capability. The availability of units with this feature is still small, and most radio stations haven't yet embraced the technology. I do expect this technology to take off, however because the sound quality is better, and not everyone wants to pay for satellite radio.
RDS: AKA "Radio Data System". A new feature for FM radios. This technology makes it possible for radio stations to transmit text to your receiver. This text can be used to show the station title or the artist name, and other information.
FM Sensitivity: The ability of your unit to tune into a weak signal
FM Selectivity: The ability of your unit to reject interference from other stations.
FM Stereo Seperation: The ability of a tuner to recreate the proper left/right channel sepearation.
CD Players:
CD-R/CD-RW Playback: This feature guarantees that your unit will play a CD-R or CD-RW that was made with a CD recording device.
CD-Text: This feature displays CD text information that is encoded on some CDs. This information includes artist names, song titles and album titles.
MP3/WMA/iTunes Playback: This spec assures you that certain digital file formats can be played back on certain units.
ID3 Tag Display: Displays text (such as song title and album name) encoded into MP3 files.
DAC: Digital to Analog converter. This device takes the digital signal from your CD, and turns it into an analog signal for your amp. The better your DAC, the more "real" your sound will be.
CD Frequency Response: The range of frequencies that the CD player can reproduce. It should at least cover the 20hz-20,000hz range.
Electronic Shock Protection: This feature prevents your CDs from skipping. If your head unit doesn't offer this, don't buy it. However, most players have it, and technology has gotten pretty good lately, so it's unlikely that you'll have any problems with skipping.
Signal to noise ratio (s/n): A measure of how well a CD player silences noise. The higher the rating, the less noise.
Zero Bit Detect mute: When the CD player detects a series of 0's on the CD (such as between songs), it will automatically mute the output. This feature isn't really important, but is often advertised.
Oversampling: The CD Audio format has a resolution of 16 bits, with a sampling rate of 44.1khz. What the hell does that mean? I really don't know, but think of a digital picture. Zoom in really close to this picture, and you see pixels. Think of each pixel as a sample. When you increase the pixels, you increase the sharpness and quality of the picture. When a CD player has oversampling, it takes the 44.1khz, and turns it into a bigger number. More samples=More realistic sound. Confused? I know. I am too, but a unit with oversampling will sound better than one without. Of course, the good news is that pretty much every unit these days has this feature.
Tape decks:
Auto Reverse: A feature that will automatically play the reverse side of a tape at the end of a side.
Dolby noise reduction: There are many Dolby technologies incorporated into tape decks. Most of these features exist to lower tape hiss, and raise the signal to noise ratio.
Frequency response: Again, the frequencies the tape deck respond to. Normal human hearning is from 20 to 20,000 hz.
Wow and flutter: This spec tells you how stable the tape deck playback speed is.
Now that you know some of the features of head units, you are ready to buy the right unit for YOU.
Alan Bayer is an entrepreneur and Car Audio enthusiast. He currently resides in some lame suburb in northern California.
He owns an eBay business http://stores.ebay.com/honest-aebs-autosound/, and the website http://www.honestaebs-caraudio.com/.
I also have a newsletter at my website. Check it out!
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