USB vs. Aux: What's the difference?

Which audio input is superior and when should they be used?

Most phones and media players have both USB and auxiliary outputs, which are sometimes referred to as "aux" or headphone jacks. Both can be used to deliver music from a phone to a car or home stereo, but they are quite different in terms of how they work.

USB vs Aux

Overall Findings

USB
  • Not as common or universal as aux inputs.

  • Safer and more convenient for driving: Allows for hands-free control of smartphone or tablet.

  • Superior sound quality, though not everyone will notice the difference.

  • Digital-to-digital: No lossy conversion of audio.

Aux
  • Universal: Available on most smartphones, tablets, CD players, head units, portable speakers, mixers, and even some musical instruments

  • Converts audio from digital to analog, which may result in noise or loss of information, though most people won't notice a difference.

  • Tends to wear out sooner than USB.

The main difference between a USB connection and an auxiliary input is that USB cords send digital information while aux cables send analog signals. Said another way, USB cords transfer data as you would to a computer, while aux cables transmit audio as you would to an amplifier or pair of headphones.

Which cord you should use depends on the speaker system and setup. USB cords are generally more convenient and deliver better sound, but they're only available on digital systems. Aux cords are useful in situations where there is no USB or digital interface, like an old car, record player, or home theater receiver.

In some cases, USB will allow you to control playback from the device you're connected to, like a car head unit. Since auxiliary jacks are only able to transfer analog audio signals, you often don't have the same degree of two-way functionality.

Aux Pros and Cons

Advantages
  • Universal: Available on most smartphones, tablets, CD players, head units, portable speakers, mixers, and even some musical instruments

Disadvantages
  • Wears out sooner than USB, leading to noise and hiss.

  • Inferior sound quality, but most people won't notice a difference.

The chief benefit of an aux input is that it's widely used, available on most smartphones, tablets, CD players, head units, portable speakers, record players, and even some musical instruments. (The biggest exception being every iPhone made since 2016.) Playback is also simple and easy, with none of the compatibility problems that sometimes plague digital connections.

The main drawback is aux cords tend to wear out faster than USB cords, due to the minimal surface area of the metal jacks. Aux cords also tend to introduce more audible noise due to shorts in the electrical flow. The cords themselves are often shorter, flimsier, and more expensive than USB. With Apple signaling its intent to phase out the 3.5mm standard on all its devices, the standard is not as future-proof as it once was.

USB Pros and Cons

Advantages
  • Superior audio quality, though most people won't notice a difference.

  • Digital-to-digital conversion: No loss of information.

Disadvantages
  • Not as common or universal as aux inputs, particularly on older devices.

When you connect your phone or mobile device to a head unit or other USB audio interface, the mobile device transmits the data unprocessed. The head unit or speaker system uses its own DAC (Digital Audio Converter) to convert the data into an audio signal, resulting in clearer sound with no loss of information. This contrasts with aux cords, which can only transmit digital audio if it's already been processed by the audio source, resulting in more lossy sound.

Some head units and audio interfaces can take direct control of a smartphone through the USB connection. This is sometimes referred to as direct control, and it’s a lot safer and more convenient while driving. The level of integration varies from one unit to another.

Despite their growing use, USB connections aren't as universal as aux or headphone inputs. Apart from modern iPhones, you can use an aux connection on almost every audio playing device. The same cannot be said of USB.

Formats and Definitions: Old vs. New

USB
  • Industry standard for connecting computers to peripheral devices like printers, audio interfaces, instruments, keyboards, external batteries, and hard drives.

Aux
  • Any type of auxiliary or secondary audio connection. Most commonly associated with 3.5mm "headphone jack."

An auxiliary input isn't a specific type of connection like USB. It just refers to an additional or secondary connection. There are many different kinds of Aux cables and connections. The most common is a 3.5mm jack, which is the same type of tip-ring-sleeve (TRS) or tip-ring-ring-sleeve (TRRS) connector used to connect headphones. (That's why they're sometimes called "headphone jacks.")

Anytime you see “aux input” on a head unit, home theater receiver, or audio interface, it is referring to this type of input—a male-to-male 3.5mm TRRS cable. Home stereos usually have the same connection, as well as RCA, optical, 1/4-Inch TS, and other connections.

USB (Universal Serial Bus) is an industry-standard for digitally connecting computers and peripheral devices. Having gone through several generations since its invention in the 1990s, USB remains the go-to wired format for connecting and controlling digital audio devices, as well as peripheral tools like storage drives, printers, keyboards, and modems.

What Is DAC?

DAC stands for "Digital-to-Analog Converter." A DAC takes digital data and turns it into an analog signal that can then drive speakers or headphones. Whenever you listen to digital audio on a car or home stereo, a DAC has to take the digital information from your phone and process it into an audio signal.

While auxiliary inputs and USB are both ways to connect a phone to a 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, while a USB connection allows the DAC on your car stereo or audio interface to process the data.