The first component in the music chain is the head unit or "deck", often referred to simply as the radio though the head unit does much more than just tune stations. The source you choose for your system depends on your budget and your current collection of music. There are two main categories of source units available. These are CD receivers and external source units (that use an iPod, Zune or Pandora as the source). There are still cassette players being produced but since their market share is so small we will ignore that as an option. Some companies still produce head units that offer both cassette and CD playback in one unit. Typically you will need a double DIN dash opening for these oversized players.
Compact disc receivers are available in the price range from less than one hundred dollars to more than two thousand for extremely high-end units. Many companies offer CD receivers and quality ranges from good to excellent depending on craftsmanship and components used in the manufacture.
The first decision to make when choosing a source unit is whether or not to replace the factory head unit. The quality of original equipment manufacturer (OEM, those that come with your vehicle from the factory) head units has increased to the point where, if features are ignored, the unit will be acceptable for most listener's tastes. If the automobile is lacking an adequate deck or if the automobile is not a recent model, a replacement head unit is needed. Keep in mind that factory head units are usually easier to use with larger and fewer buttons and of course look like they belong in the dash (because they do). Also, a CD changer can be added to a factory deck (or deck lacking CD changer controls) through the FM antenna using what is known as a modulator.
Your factory head may also have inputs and controls for a CD changer which was not installed at the time of purchase. These head units can either have a factory CD changer added later or certain aftermarket changers added with special adapters available through a few select companies such as PIE (Perfect Interface Electronics) and others. Typically it will be cheaper to have an aftermarket changer installed with an adapter than to buy a factory unit. The advantage over an FM modulated changer will be higher sound quality and one less controller in the dash. Factory units may also have controls built into the steering wheel which can be difficult to replicate with an aftermarket head unit. Also, some cars have additional functions built into the head unit that are not audio related that can prohibit the removal of the factory deck. These are usually worked around in later generations of factory adapters. These adapters are also available for using an iPod with a factory head unit which is a very popular option.
Manufacturers do not want aftermarket head units in their cars because this cuts into their profits on premium sound systems they want to sell. These additional functions built into the radio are part of this plan. They have also changed the size of head units from the standard DIN (single height head unit) to 1 ½ DIN, double DIN, oval shaped, and split units where the controls are broken into different parts of the ash. Manufacturers such as Metra and Scosche offer dash kits that will facilitate the addition of standard DIN radios in these vehicles. Some aftermarket head unit manufacturers also offer non-standard size units to fit the more popular vehicles with 1.5 and double DIN.
If you have a classic automobile with a vintage radio shaft pattern and do not wish to cut into the dash to install a DIN unit you should contact a specialty radio manufacturer such as Custom Autosound that supplies these style radios.
The most important criteria to look for when shopping for a CD player or other head unit is the company’s reputation for building quality product. Most decent CD players will have a detachable faceplate. I wouldn't recommend purchasing one that did not as these are usually the lowest models offered by a company.
Audible Search: With this feature you will be able to hear the music that you are searching through which can be a very desirable feature.
Auxillary Input : In addition to iPod controls you may also find units that have an auxillary input. Generally these are 3.5mm jacks on the front of the unit that you can use to connect virtually any audio device with a headphone output.
Changer Controls: This is the ability of a receiver to control an external CD changer, MP3/iPod or satellite/HD Radio module through its front panel controls. With this option a digital source may be used in conjunction with an in dash receiver.
Dual Illumination: This is a receiver’s ability to change the color of its display between two or more colors, usually amber and green. This is a nice feature to have if you go through a lot of vehicles and are never sure of your next vehicles dash color. Some units have a much wider color selection in addition to these two basic colors.
Frequency Response: This is the portion of the audio spectrum a piece of equipment can produce. The average human can hear sounds in the range of 20Hz-20kHz. Most good CD players can produce this entire range.
Line Output Voltage: This is the output voltage that a deck can deliver, in volts, through its pre-amp line outputs. The higher than number the less chance that noise will be delivered to the next piece of equipment in the chain. Most lower to mid priced decks have an output of 500 mV (0.5 volts) while some higher end units have 4 volts or more.
iPod Control: This is the ability of a receiver to control an iPod or at the very least, accept the music output of an iPod into the unit. Look for units that can directly control the iPod. Some have the ability built in while others require special adapters or cables. The more control you have of the iPod from the head unit the better.
Loudness Control: This is a feature that boosts the lower and sometimes the very upper frequencies to compensate for the human ear's insensitivity at low volume.
MP3/WMA capability: Some CD head units have the ability to play MP3/WMA encoded discs. This can be a real benefit if you have a CD burner and a collection of MP3s on your computer as it eliminates the need to have additional equipment to play your MP3 collection in your automobile.
Power Output: This is the amount of power, in watts, that your receiver should be able to deliver to the speakers. The number printed on the face of the receiver is usually a large exaggeration. Most receivers actually have between three and fifteen watts RMS per channel. Keep in mind that the sound will become quite distorted at this level because of the size and current limitations of your receiver. The best sound is always found in dedicated external amplifiers.
Pre-amp Outputs (RCA jacks): These are the most common connectors used for external power amplifiers. If you plan to add amplifiers to your system I strongly suggest that you purchase a receiver with these outputs. More than one set of outputs are available on some of the better units and are handy for retaining front/rear/subwoofer fading capabilities.
Remote Control: Most people laugh at the idea of having a remote control for an in dash receiver but it is actually much safer to use when on the road. Models are available in large TV sized remotes, handheld remotes, credit card remotes and even steering wheel mounted remotes.
Station Presets: These are the areas of memory in which your receiver stores radio station frequencies. The more the better here. Some units even allow you to assign names to the various presets.
Seek/Scan: These functions are used to seek for the next station that is available and scan through your preset stations respectively.
Tuner Sensitivity: This is the ability of the tuner in your receiver to pull in weak stations. It is measured in dbf and the lower the number the better. Eight is about the best you will likely see and thirteen is about average.
USB Input : Usually a front panel USB jack that will accept direct connection of a USB thumb drive or a similar USB storage unit. This allows you to plug in a small thumb drive and access the compatible digital music you have stored there. This can be more convenient than burning a CD everytime you want to change the MP3s you take with you.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including car audio head unit installation. Click here.