Types
Head unit: This is the control unit for the car radio system and usually contains a CD player and a car radio, although modern versions can include a DAB car radio, USB key readers, memory card readers, car DVD, iPod connectors and even SatNav systems.
This is the business end of all the audio equipment you have in your car and may even link to a CD or car DVD changer that can be stored under your seat or in your car boot. The car radio and/or car DVD player will display information about what you are playing and some of the more expensive ones can even show album artwork. Certain models of car radio that are compatible with car DVD players may even have a fold out LCD screen that can play car DVD movies. Generally car radio head units come in a single size, known as single DIN, which is the normal size of a car radio, although some cars take replacement head units that are double DIN, twice the size of the single DIN.
Amplifiers: Often neglected because it is the least visible part of your car audio or car DVD setup, the car radio or car DVD amplifier is one of the most important parts of a quality car radio setup. It is entirely possible to link the car radio head unit straight up to the speakers, although the built in amplifier in the head unit is usually not powerful enough to supply the speakers properly. This is certainly true if you want clear, high volume sound coming from the speakers of your car radio, because a car radio head unit alone will tend to make the sound muddy at higher volumes.
The quality of amplifiers increases as they increase in price. It is also important to be aware of how many channels an amplifier is equipped with. You should ensure that their ohm rating and their wattage is compatible with the speakers and car radio you intend to use.
Speakers: Car radio speakers are known as mid-range, woofers and tweeters. Woofers are the large speakers that are known to produce the strong bass sound that modified cars are famous for. They may require custom installation, as few cars have space for bass speakers without modification. Normal car radio speakers reproduce the widest band of frequencies between the bass (usually below 40hz) or high-treble frequencies (usually 2khz to 20khz); these are usually fitted in regular door cavities. There are also tweeters, which tend to handle high treble frequencies (usually above 2khz).
Dampening: Dampening is an important part of any serious car radio or car DVD audio setup and is usually wrapped around the inside of doors and the metal surfaces surrounding the boot. The purpose of the dampening material is to ensure that certain frequencies are not bounced around inside the car, which can distort the quality of the car radio's sound. Clever use of dampening materials ensures that the best possible car radio sound quality can be maintained regardless of where the speakers are placed within the car, or what shape the car is.
Features
Car radio head unit: One of the most recent innovations in car radio has been the move from analogue to a digital signal. This has led to the introduction of a number of car radio head units that are capable of picking up DAB car radio (Digital Audio Broadcasting) radio signals. This has led to a significant increase in quality over traditional FM car radio. Another feature of modern head units is the car DVD drive - a particularly useful feature to entertain children if combined with backseat LCD screens. In family cars, LCD screens are often installed in the back of the headrests, so that a car DVD can entertain children during long journeys.
Some car radio head units also have the ability to connect to an iPod, which can be particularly useful if you have a large music collection that you are constantly updating, that way it can be kept in one place rather than constantly being copied to different devices.
Built in SatNavs are common in the more expensive car radio head units, although they have become less so recently due to the fact that dedicated SatNav units have become both cheaper and smaller. With the rise of MP3 music, a lot of car radio head units have moved away from only having CD drives and now also include hard drives that can store a selection of music, and also USB memory stick ports that can be used to put music onto the hard drives.
Speakers: When looking for speakers, whether for use with standard car radio or car DVD, it is very important to ensure that the wattage and ohm rating matches that of the amplifier. This is because an overpowered amplifier can blow speakers that are not designed to work with it. The ohms of the amplifier should be equal to those of the speakers, and the best indicator of volume and quality will be the RMS (Root Mean Square) rating that can be used as an indicator of how loud speakers can become without distorting the audio.
Security
Car radio security is now a serious business, mostly because car radio head units are seen as one of the easiest targets for smash and grab thieves. Traditionally, car radio head units were very quick to uninstall from a car and held their second-hand value well. Modern car radio security features have reduced this, and very often security features can make your car audio equipment unattractive to thieves who are seeking an easy target.
The most common security measure is probably a detachable faceplate which the driver can remove and take away. The car radio head unit itself would be largely worthless without the faceplate; hence the unit becomes much less attractive to thieves. Some units require a password function to be used, so anyone who doesn't know the password wont be able to use it.
Another interesting security feature that some car radios have is a 'key CD' where the unit asks you to insert a CD to become the key CD when it is first turned on. If the car radio is then disconnected from the battery it will then ask to be fed the same CD once again, or it won't work. Additionally, a blackout faceplate is a security feature where the head unit will become featureless when it is not on, leaving potential thieves unable to see whether there is a head unit in the car or not.