Once connected, you can view and select songs through the head unit controls. This is one of the most streamlined ways to listen to an iOS device in your car—be it an iPhone, iPad, or iPod. Some head units can even read media from a USB flash drive, but most simply have a standard USB output. You will need to supply the cord that works with your device. Bluetooth is increasingly common in vehicle head units. However, because most of these interfaces also have USB outputs, most people opt for the latter. Bluetooth is generally preferable to Aux inputs because they allow for hands-free control. You will need a cable with two 3.5mm male ends. Make the connection and select the Aux audio source on the head unit. Because the line-in is an analog input, you will need to use your MP3 player to select and play songs. Audiophiles may also notice inferior sound due to the lossy compression of digital-to-analog audio. These adapters were originally used with portable CD players, but they work just as well with MP3 players. They look like cassette tapes, except they don’t actually contain any tape. Audio is transferred via a cable to the adapter and then passed through the tape heads. Cassette adapters don’t provide the best sound quality, but they’re a lot cheaper and easier than a brand new head unit. Most FM transmitters plug into an MP3 player just like a cassette adapter or auxiliary input. The best sound quality is typically achieved by selecting a frequency that does not have much or any idle reception. Some FM transmitters use Bluetooth technology. These devices can be paired with MP3 players or phone that are also Bluetooth-compatible. This allows for a wireless interface between your media player and stereo. If your radio does not have an auxiliary input, an FM modulator is likely the next best thing.
USB: Many cars have built-in USB inputs, allowing you to plug your phone directly into the head unit as you would a phone charger or external hard drive. Auxiliary (Aux): Some head units include a 3.5mm auxiliary input that you can use with any phone, MP3 player or audio device with a standard headphone jack. Bluetooth: Increasingly common in car head units, Bluetooth allows for hands-free wireless control of your MP3 player or smartphone. Apple CarPlay: Some head units have built-in compatibility with iOS devices. Carplay allows your iPhone, iPad, or iPod to take over the infotainment system, replacing the original vehicle interface with something more Apple-friendly.
You will get the best sound quality with a USB or Lightning connection. This is because, with USB, the digital media on your mobile device does not have to be compressed into an analog signal, as it does with an aux connection. An aux input is still better than an FM transmitter or a cassette tape adapter, although both of those options may still be better than having no access to digital audio at all.
