Modern vehicle manufacturing nearly always includes a complete system of some kind from the factory when the car is assembled. This is referred to as the ‘stock’ system, or original equipment manufacturer (OEM) system. These systems can be supplied by an exclusive vendor to automakers specifications, or other company that specializes in design or production of such equipment. Additionally, there is a large market of products sold directly to consumers to replace, upgrade, or expand the vehicle’s stock sound system capabilities.
In modern cars, the primary control device for an audio system is commonly referred to as a head unit, and is installed in the center of the dash panel between the driver and the passenger. In older vehicles that had audio components as an option, such devices were mounted externally to the top of or underneath the dash. In rarer occasions, head units have also been mounted vertically directly between the driver and passenger.
The head unit itself is usually a multi-purpose device that houses multiple components in its housing. The most common components are a radio receiver/tuner usually with AM and FM bands, and a small amplifier for driving an audio signal to speakers. Other possible components include various media devices, such as (in older vehicles) a tape player CD player, DVD player, HD Radio, Satellite Radio, USB flash memory, and even a portable hard disk drive typically used in notebook computing. Many head units also feature a DSP component, and equalization component (such as bass and treble controls), or a control interface for another feature on the car (such as a back-up/parking camera, navigation system, trip odometer, etc.).