What's the Difference Between USB and Aux?

Aux Inputs Vs. USB Connections

Car dashboard unit with two 12V power ports along with an AUX and a USB port
 Marin Tomas / Moment / Getty

Phones and portable music players typically have both USB and auxiliary outputs, in the form of headphone jacks, and both can be used to pipe music into your car or home stereo. They're both equally convenient, since you can plug and unplug both types of connections pretty much at will, but they're actually pretty different in terms of how they work.

What's the Difference Between USB and Auxiliary Cables?

The main difference between a USB connection and an auxiliary input (aux) is that one sends unprocessed digital data to the head unit, and the other one sends a processed, analog audio signal. It may be easier to think of it as the USB cable transferring data like you would to a computer, and the aux cable transferring an audio signal like you would to your earbuds.

Although there are benefits to both USB and aux connections, you will almost always get better sound quality out of a USB connection. While an auxiliary jack on your car radio will usually provide more utility, in that you can use it with a wider range of devices, the fact is that your head unit is almost certainly better at turning digital files into analog audio than your tiny smartphone or MP3 player.

In some cases, USB also allows you to control playback, and other functionality, from the head unit. Since auxiliary jacks are only able to transfer analog audio signals, you'll never get that type of functionality from an aux connection.

What is a DAC, and Why Does it Matter?

In the audio world, DAC stands for digital to analog converter. This is a technology that you probably use on a regular basis, but you never really have to think about it. Your smartphone, MP3 player, car stereo, and a multitude of other devices all contain a DAC.

In very basic terms, a DAC takes digital data and turns it into an analog signal that can then drive speakers or headphones. Whenever you listen to a CD on your car stereo or listen to an MP3 on your phone, a DAC has to take the digital information and process it into an audio signal.

While auxiliary inputs and USB are both good ways to connect a phone or MP3 player to your car stereo, there can be a huge difference in quality based on the DACs involved. This is because an aux connection utilizes the DAC in your phone or MP3 player, while a USB connection allows the DAC in your car stereo to process data located on your phone or MP3 player.

What is Aux?

Auxiliary input literally just means an additional audio input method. It isn't a specific type of connection like USB, and there are actually a ton of different types of cables and connection types that can be used as auxiliary input.

The main type of aux input that you find on car head units is a 3.5mm jack, which is the same type of tip-ring-sleeve (TRS) or tip-ring-ring-sleeve TRRS connector that you see on headphones. So when you see “aux input” listed as a head unit feature, that’s what they’re really talking about—a jack that you can hook directly into the headphone jack on your iPhone, or iPod, or any other audio source, with a male-to-male 3.5mm TRRS cable.

Home stereos also use this same type of connection, but you'll also find a larger style of TRS connector, RCA type connections, optical connections, and many others.

Benefits of Aux Inputs

The main benefit to aux inputs are that they can be used with basically any audio device. Regardless of whether you have an iPhone, an Android phone, or even a decades-old Walkman, you will be able to use it with the aux input in your head unit or home stereo.

This is why one aux cable will work with pretty much all of your portable devices, although some will require an adapter, and changing or upgrading your music player is painless. It's typically a simple matter of just unplugging your old phone or music player, plugging in a new one, and you're done.

Drawbacks of Aux Inputs

The main drawback of using an auxiliary input has to do with the difference between a car stereo and earbuds. Earbuds are small and unpowered, whereas even the simplest car stereo system has much larger speakers and an amplifier, whether it is a powerful stand-alone amp or built  right into the head unit.

The issue is that when you use an auxiliary cable with a portable music player like an iPhone, the phone hardware has to do all the heavy lifting. The iPhone processes the digital music files that you have stored on it, and it transmits the resulting audio signal via the headphone jack to the aux input in the head unit.

Since iPhones are designed with earbuds and headphones in mind, and they don’t include line level outputs, additional noise may be introduced to the audio signal when it passes through the amplifier in your car stereo. Of course, noise can also be introduced via the aux cable and jacks as well.

Benefits and Drawbacks of USB Inputs

When you connect your iPhone, or any other compatible device, to a head unit via a USB input, something totally different happens. The iPhone or other device typically sends unprocessed data to the head unit instead of a processed audio signal. The head unit is then responsible for decoding and processing the song data into an audio signal.

Since head units are designed with amps and larger speakers in mind, they typically include DACs that are much better suited to this task than any portable music player—iPhone or otherwise.

The main benefit of a USB input vs an auxiliary input is sound quality, but these connections often come with other benefits. For instance, some head units can take direct control of an iPhone via a USB connection. This is sometimes referred to as direct iPod control, and it’s a lot safer and more convenient than fiddling with your phone whenever you want to change songs or adjust the volume.

Of course, the level of integration varies from one head unit to another. Some head units, like Pioneer’s AppRadio, include iOS-like touchscreen controls, and others are a little more cryptic.

Although USB connections will typically provide better sound quality than auxiliary inputs, they aren’t as universal. Whereas you can use an aux input with virtually any portable audio device, the compatibility of a head unit’s USB input is typically limited. For instance, Pioneer’s first generation of AppRadio head units wasn't initially compatible with iPhone 5.

Understanding USB to Aux Cables

With the understanding that a USB connection on a head unit handles raw data, while an aux input deals with analog audio signals, it seems like there shouldn't be any such thing as a USB to aux cable. Where car audio is concerned, plugging a USB cable into a 3.5mm aux input would be like trying to play a vinyl record in a laser disc player. Maybe you could make it fit, but what would be the point?

There actually are USB to aux cables out there, but it's important to understand what they're for and what they can't actually do. If you have a USB thumb drive, for instance, and you want to plug it into your head unit, you'll need a head unit that actually has a built-in USB port. Plugging it into a USB to aux cable, and plugging the cable into the head unit, isn't going to accomplish anything at all.

USB to aux cables actually do have legitimate functions, like plugging a USB headset into the 3.5mm headphone jack on a computer. Some phones and MP3 players are capable of outputting audio via their USB port, but these are edge cases. If your phone or MP3 player has an audio output, you're typically going to be better off just using that even if it is capable of outputting audio via a USB port.