Here is all of the information that I consider relevant (and some non-relevant) when buying a head unit. Your mileage may vary. First of all, what do the published specifications mean and are they important?

Let’s take a look at the common specifications associated with a typical head unit, in this case the Alpine CDA-9851. I pulled these specifications off of crutchfield.com, but re-arranged them because there were a bunch of features mixed in with the specs, and I’ll address features later.

On with the show

Power specs:

  • RMS Power Output 18 watts
  • Peak Output 50 watts
  • RMS Power Bandwidth 20-20kHz
  • Preamp Voltage 2 volts

Power:

So, what does this crap mean? Well, unfortunately, RMS and peak power numbers no longer represent what they are supposed to. RMS power and peak power, when used properly, both describe the same exact output, but in 2 different ways. What it boils down to is that peak power is double the RMS power. For a pretty straight-forward explanation of what RMS and peak really means, go HERE, or just take my word for it.

So, what does this mean to you? In effect, power ratings on head units are meaningless. They may vary by a watt or 2 here or there between brands, but you’re only going to get so much, and among reputable brands, that amount isn’t changing enough to be a major consideration. Most of us will not use the head unit’s internal power unless it’s just a temporary stepping stone until we buy an amp.

RMS power bandwidth is a bit ambiguous in its meaning. The way I first interpreted this spec was to assume it meant that the amp can make its 18 watts RMS when driving a full-bandwidth output, and not being measured at 1 frequency like most power ratings are done. However, I don’t think this is the case. I think this is just a poor way of saying that the head unit output has a frequency response of 20-20kHz. This type of word gimmickry is part of what makes specs so useless. This winds up telling us very little, other than the head unit has the frequency response that we all expect it to have.

Pre amp voltage is the amount of voltage that the head unit can send through the pre-amp outputs. Most head units are between 2 and 4 volts, and almost every amplifier has a wide enough input gain variability to accept anything within and somewhat beyond that range. The benefit you get from having a higher pre-out voltage is that it will allow you to turn down the input gain on the amp, thereby lowering the amp’s noise floor a tad. That’s great, except that a good amp will have a very low noise floor to begin with, so lowering it by 3dB by doubling your pre-out voltage is going to basically be a non-issue. Also, the higher voltage that’s running through your RCAs, the less noise it’s likely to pick up, but remember that the 2 or 4 volts is the peak output (i.e. it’s maximum output at maximum volume) and that during the vast majority of the time, you’re at a very small fraction of that. In most listening cases, you’re probably only pushing 50mV or less through the RCAs, so if you double that to 100mV, you’re still talking about very little voltage and a high noise susceptibility either way. Proper routing of your RCAs will have a far greater impact on noise than your pre-out voltage.

Tuner specs:

FM Sensitivity 9.3 dBf

A fairly useful number, except that all head units are relatively similar in their FM sensitivity numbers. Up a dB here or down a dB there isn’t likely to have a major impact on how the radio comes in. A much more important aspect to tuner performance is how the head units manage their mono-convergence, but good luck ever getting this spec. Basically, the way your radio handles poor reception is to gradually step the stereo signal more and more toward mono, which helps it eliminate noise by canceling anything that’s not the same in both channels. Most aftermarket head units try to maintain the stereo separation in order to increase fidelity with strong signals, but if you live in a rural area with a weak signal, the result is usually radio reception that seems worse than your factory radio had. If you live in an area with poor radio reception, and you’re a big radio listener, sometimes sticking with your factory radio is a better decision than upgrading to aftermarket. However, in an area with a strong radio signal, an aftermarket radio will probably have a better sound quality because it’ll better maintain the stereo separation.

That leads me to another spec that isn’t published here, and that’s fm stereo separation. This states how much of a maximum difference there can be between 2 stereo channels. I typically see numbers around 50dB when I see this spec. It’s a neat spec, but again, it’s not going to vary enough between one brand and the next for you to bother worrying about it.

CD specs:

  • CD Frequency Response 5-20kHz
  • CD Signal-to-Noise 105 dB

CD specs are all worthless to the normal consumer. The frequency response of the cd player really doesn’t mean anything if it’s wider than the response of the head unit that’s transporting that signal. Who cares if the cd player can go down to 5hz if the head unit’s low frequency capability is only 20hz?

CD signal to noise ratio is another crock. The absolute maximum signal to noise ratio that can be achieved with a 16 bit digital source is 96dB, so if the cd player is capable of more, hooray for it. It doesn’t matter.

Sometimes you’ll see other specs, such as jitter, wow and flutter, and a bunch of other crap that’s all too small to matter.

So… I just spent a half hour telling you that the published specs for a head unit don’t matter, so now you’re wondering how to decide between different head units.

What it boils down to is features, price, aesthetics, ergonomics, reliability and sound quality, although I’ll hit that last issue a little later. For now, let’s look at features, price, aesthetics, ergonomics and reliability.

Features

So, what features might you want your next head unit to have?

They all will play AM, FM, and cds. Most will play MP3s. Some play DVDs. Do you want to add a cd changer, Ipod control or satellite radio? Decide which of these music sources are important to you.

Now that you’ve established what you want to hear, decide how you want to hear it. If you’re just starting to build your system, you need to look into your crystal ball and realistically decide where you think you want to take the system. Buy a head unit that’s too basic, and you might find yourself spending more money on other equipment (or another head unit) to work around the problem. On the other hand, be realistic with yourself. There’s no sense in spending crazy money for every feature your heart desires if you’re not going to actually use them. That money could be better spent on more important equipment. You’ve got a lot to consider here. The number of pre-outs, Crossover flexibility, time alignment, equalization, or other sound-shaping features all come into play here, and this can become a bit overwhelming.

Pre outs

Let’s start with the number of pre-outs. How many do you really need? Some very cheap decks come with 1 set. Most come with at least 2 sets, although most come with 3 now. The minimum you need if you decide to use a separate amplifier for your stereo is 1 set. This is all I currently use in my IROC, although I intend to run a separate set soon. Almost every amplifier on the planet has built in crossover control that serves the same purpose that the one in your head unit serves, so you can run a whole system with 1 set of pre-outs. But, having multiple pre-outs gives you the flexibility to control crossover options at the head unit instead of at the amp. For most it’s a set and forget issue, so running 1 set will usually suffice and will save you money on RCA costs. I’ll go more into depth on this issue when I talk about crossover features, because how you use them will have a direct effect on how many sets of pre-outs you need. One additional thing to think about is whether you intend to amp 2 sets of speakers (like a front set and a rear set) and if you want independent fader control. If you do, you need a minimum of 2 sets of pre-outs.

Crossovers

Most decks with at least 2 sets of pre-outs will have some level of crossover control. First a bit of terminology. You’ll run into filters called a “low pass” and a “high pass”. They do just as their name implies. A low pass filter will allow the low frequencies to pass, but will block the higher frequencies. The black line is the sum of the two. A head unit that has subwoofer crossover control will typically provide a low pass option for 1 set of pre-outs, and a high-pass option for 1 or more sets of pre-outs. Most allow you to choose from at least a couple different frequencies, some allow you to choose from a significant amount.

Typically you’ll be choosing a frequency somewhere around 80-100hz, but in cases where you’re running 4×6 front speakers, you may need to bump the frequency up a little higher. Since subs can get a little funky sounding if you try to run them too high, you may want to be able to set the frequency for the high pass and the low pass separately.

For example, run the low pass for the subs at 100hz but run the high pass for the 4x6s at 150hz. Yes, you’ll have a gap in the sound, but quite often that can be more pleasant than forcing your sub to play up to 150 or trying to get the 4×6 to play down to 100. Having the flexibility to do this is nice. More advanced head units may have a much larger number of crossover frequencies to choose, and some may even allow adjusting the slope. If you have a crossover frequency of say, 80hz, that doesn’t mean that everything above 80hz is completely removed from the signal. The filter provides a gradual slope that starts around the selected frequency. Most are 12dB/octave, which means for every octave above the selected frequency, the signal is 12dB quieter. That means at with your 80hz filter, your output at 160hz is 12dB quieter, at 320hz its 24dB quieter, etc. Having the ability to choose a steeper slope (such as 18dB/octave or 24dB/octave) gives you a faster roll-off, which is generally better, but sometimes can give a disconnected sound between the drivers, so experimentation is usually required.

This isn’t something that the average Joe needs to concern himself with, but for some, it’s a worthwhile option. Some heads also have a yet more advanced filter network that allows you to run a full active system. Rather than just providing subwoofer crossover control, it will provide another band or 2 of crossover control so that you can control the filter for your subs, mids and tweeters all from the head unit.

This is a fantastic option if your knowledge and your system is advanced enough to take advantage of it, but will be unused by the vast majority, and will be miss-used by the vast majority who do use it.

Time Alignment

One somewhat advanced feature that’s starting to get more common is time alignment. What this does is allow you to program in a certain amount of delay for certain speakers. This can be a fantastic tool for really creating a good wide soundstage with accurate imaging. In a car, you’re in a non-ideal environment that places you much closer to one speaker than to the other.

For proper imaging, you want to have the two distances be as equal as possible, which is part of why kick panels offer such vastly improved imaging and soundstage compared to dash speakers. In the dash, one speaker is about 3 feet away, and the other is 5 or 6 feet away. Move them to the kicks and one’s 4 feet away and the other is 5 (give or take). Sure, you can crank your balance control over to help compensate, but in the critical midrange frequencies, your ears determine direction mainly by time delay, and less by loudness.

By compensating with a balance control, you’re just lowering the output level of the closer speaker, and while somewhat effective, will cause the image to blur and not be as stable as you’d like. By using some time alignment to compensate for speaker differences, you can achieve a stronger, more solid soundstage for at least 1 listener. There’s no free lunch, and the more you optimize something for the driver seat, the worse it gets for the passenger, but hey… they didn’t pay for the stereo, you did. Time alignment can be a nice feature for those who choose to take the time to experiment with it, but won’t be used by most.

Make sure that if you decide to pay more for this feature that you’re going to be comfortable using it properly. If you want this feature, spend time messing with it in showrooms to make sure that it will offer the resolution and type of control that you want. If it only allows you to pick “driver” or “passenger”, it’s going to be much less useful than one that allows you to select speaker distance in 1″ increments. But, the latter will require more expertise and more experimentation.

Equalization

Equalization is a biggie for a lot of people. Simple radios will give you a bass and treble adjustment and not much more. This is ok if you just want to crank up the bass and treble, but is fairly useless as an actual tuning aid to help you smooth out a poor frequency response. Many head units offer multiple bands of equalization, which gives you a lot more control. Of course, most people still just crank the bass and treble, so in reality, they didn’t need all of that extra control. The extra bands can also benefit you if you’ve got specific tuning issues that you need to overcome… such as an upper-midbass problem common with kick panels. With enough resolution to be able to cut the 120-400hz range, you can help cure those problems. A lot of the better head units come with what’s called a parametric equalizer. This is a much more powerful type of equalization because it allows you to choose what center frequency to boost/cut, plus how wide of a range to apply the boost/cut to. This will let you smooth out issues like that midbass bump from your kicks, or any sort of standing wave issues that occur over a wide or a narrow band. However, if you just want to bump up the bass and treble, it’s going to be a lot more work with the parametric eq, so keep that in mind.

Other sound features

Other sound shaping features usually include some sort of loudness control, although these days every brand has its own name and gimmick associated with it. A loudness control is designed to provide additional bass (and sometimes treble) to accommodate for the ear’s loss of sensitivity to these frequencies at low volumes. Of course, it’s evolved into just being a bass-boost for bass heads to get an exaggerated bottom end. Some head units have very advanced features that include things like BBE processing which uses phasing and frequency response shaping to give a more enveloping, full, rich sound. A lot of these can seem very pleasing when you first use them, but after a while they just become one more deviation from the accurate sound that some of us are after. But… most people love this crap, so when you audition the head unit, play with it and see how you like it. You will probably find that this type of feature will differ considerably from brand to brand and may have a major influence on your perceived sound quality, or at least your opinion of the deck. Sometimes these types of features can help breathe a little life into a flat sounding MP3 or satellite radio broadcast.

I think I’ve hit most of the major features, but a lot of decks have an overwhelming abundance of bells and whistles above and beyond the stuff I’ve highlighted here. I’ve listed those that I feel are most fundamentally important (and also most popular), but you need to use your discretion when evaluating other features. Chances are, you don’t need them.

Aesthetics

When it comes to aesthetics, this is really a subjective qualification. About the only advice that I can offer is to try and think about how this thing is going to look in your car, and whether you’ll be happy with it long term. Some people like a flashy head unit that packs all the appearance of a self contained night club, complete with billboard display, 3 million variable color illumination, and a built in fog machine. Others like a much more tasteful, stock-type appearance that doesn’t draw attention to itself. One major problem for those of us who like a tasteful looking deck, is that they are becoming much rarer. When you’re buying a product whose primary buying age is something that ends in “teen”, it gets tough to find a laid back, simple looking product. But, that aside, most people buying these head units do like how they look, but you can make it a better experience by getting one that at least ties in with your car’s appearance. Something that’s got amber buttons or a display will look better in a firebird, and something with a white type of display may suit Camaro owners. No matter what you get it’s going to stick out like a sore thumb in an 80′s interior, but the more you minimize the effect, the better it’ll look long term. You also need to decide if you want a dancing dolphin screen saver, 3 million variable colors, pinwheels, graphic analyzers, black, silver, chrome, etc. Have a blast, go wild.

Ergonomics

Ergonomics is a much overlooked aspect to head unit buying. 10 years ago, this really wasn’t an issue. Every head unit had the same functions, with the same type of controls and the same layout. About the only option you got back then was whether you wanted cd changer control or not. As a result, they were simple to operate, and simple to control while you were driving. That’s not the case anymore. The owner’s manual for my Kenwood XXV-01D is 68 pages thick, and is still so vague that you’re basically on your own. With head units cramming in so many features, and not adding to the button count, it became necessary for them to build extravagant menu-driven systems to allow you to control everything. None of the new head units are going to make it easy to change the more complicated functions without some serious concentration (or a trip inside for the owner’s manual), but the most commonly used controls like volume, track forward/backward, eject, station presets and the like should be easy to find and easy to turn/push/select. If you’ve got to dig through menus to change your radio station, it’s going to make it very difficult to do it while you’re driving.

Reliability

So, what about reliability? Head units have managed to literally cram 10 pounds of crap into a 5 pound box, and have gotten less expensive in the process. These days you can get more features than you ever dreamed of, but unfortunately, sometimes that means that you’ve just got more stuff that can (and does) break. In an effort to give you more for less, a lot of cost cutting has been done. Head units are more commonly made in china with lower grade components and very poor production tolerances. 10 years ago you could buy a head unit and expect it to last far longer than your desire to own it, but these days, a lot of stuff breaks a lot sooner. Keep this in mind when you are trying to decide where to buy it from. The best price you get on a head unit will usually be from a retailer who is not an authorized seller of that product, which means if you have any warranty problems, you might be out of luck. If you’re lucky, that retailer will take care of you, but how much are you willing to gamble? If you shop around, you can still get a good deal from an authorized retailer, and while it’ll cost you a little more than the cheapest EBay deal you can find, you’ll be much better off if you do wind up needing warranty work. It’s a gamble that you’ll have to decide if you’re willing to take. So, what brands are most reliable? This is hard to answer, because typically the stuff you’re buying is new enough that its reliability record may not be well established yet. Previous years models can give you a lot of insight, but with products changing so rapidly, it may not be a sure-fire indication.

Sound Quality

The sound quality debate has been ongoing since before I got into car audio, and will still continue long after I don’t care about this stuff anymore. Some people say that there is no difference (or a small enough difference that it’s not audible) and others will swear by the sonic qualities of certain decks. I’m somewhat neutral in that I do believe that there are differences from deck to deck, and that in come cases they aren’t even all that minor, but when comparing the vast majority of competently designed, well built, properly functioning head units, any sonic differences will tend to be minor when all sound-altering functions (loudness, DSP, equalizers, blah blah blah) are bypassed or set flat. Listen real hard and you might find a small difference between brand A and brand B, but usually not significant enough to have any effect once it’s in your car. If you have to strain to hear the most minor difference under the most ideal circumstances, once it’s in your car, either will be perfectly satisfying, especially after you realize that the car’s environment is inherently noisy to begin with. If you have a truely outstanding system, and a very good ear, then it may be worth your time to evaluate sonic differences, but until that point, give them a listen, but don’t pop a blood vessel trying to hear something that’s not there.

Brand suggestions

Here are some general suggestions for brands that I consider of repuatable quality in no particular order:

  • Eclipse
  • Alpine
  • Kenwood
  • Pioneer
  • Clarion

Brands of varying quality. This category includes brands that might have some diamonds in the rough, but might have some junk too. Don’t automatically dismiss a model from one of these brands, but just make sure you do enough homework to be sure that you’re actually getting a quality product:

  • Sony
  • JVC
  • Panasonic
  • Blaupunkt

Junk:

  • Audiobahn
  • Dual
  • Jensen
  • Pyle
  • Boss
  • Lanzar

Performance Technique

Most other brands that aren’t listed in the first 2 categories.

Don’t forget!

I’ll handle installation in another thread, but while you’re buying your head unit, don’t forget to buy your dash kit and your wiring harness. Don’t even think about trying to save $5 and skipping the wiring harness. Don’t be a hack and cut up your wiring, and also look to the future when you may want to return your car to stock, or at the very least, remove or upgrade the radio. All of this will be much easier if you can just unplug your head unit and walk away instead of trying to screw around with wires deep in the dash instead of doing your wiring on a work bench where you’ve got plenty of room and good lighting.

Useless conclusion

So… I hope that this has given you some insight into what things you should be thinking about before you buy a head unit.

Hope this helps