Using a USB to Aux Cable in Car Audio

A modern vehicle head unit.
George Hodan / / Public Domain

USB to aux cables do exist, and they do work for the purposes that they were designed, but they don’t work in the way that you are describing. If you plug a USB thumb drive into a USB to aux cable and plug the cable into your head unit, nothing will happen.

The same is true, in most cases, if you plug a USB to aux cable into your phone and connect it to your head unit. Some phones and MP3 players are designed to output audio signals via USB connection, like the original HTC Dream that used a single micro-USB connector for both power and audio output, but most phones and MP3 players come with a standard 3.5mm or 2.5mm TRRS headphone jack for a reason.

The Difference Between USB and Auxiliary in Car Audio

In simplest terms, USB is a digital connection that transfers digital information, and a standard 3.5mm TRRS auxiliary jack is an analog connection that expects an analog audio signal. There is some overlap between the two, as USB headphones do exist, but USB headphones still require an analog input via the USB connection.

The main difference between USB and aux in car audio is that USB connections are designed to offload processing of audio data to the head unit, while aux connections are only capable of taking in an already processed signal. If you couldn’t plug a set of headphones into a device, then you can’t connect that device to your head unit’s auxiliary input either.

There is a difference between headphone and line outputs, which is one of the reasons that people like to use USB to offload processing and amplification to the head unit. In most cases, when you plug a phone or MP3 player into the aux input in a head unit, you end up piping an already amplified signal intended for headphones rather than a line-level signal, which isn’t ideal in terms of sound quality.

If a phone or MP3 player offers a line output option, that will typically provide better sound, and USB will also provide better sound quality, but only if the head unit also has a USB connection.

Why You Can’t Plug a USB Flash Drive into a USB to Aux Cable

When you put music on a USB flash drive, or a phone, or any other storage media, it is stored as a digital file. The file is usually compressed as an MP3, AAC, OGG, or another format unless you buy high-resolution digital music. In order to listen to those files, something needs to read the data and convert it into an analog signal that can be used to drive headphones or speakers. Whether it’s software on a computer, phone, MP3 player, or even the head unit in your car, the process is essentially the same.

In the case of a USB flash drive, what you have is passive storage media that holds song data but that can’t actually do anything with that data. When you plug the drive in the USB connection of compatible a head unit or infotainment system, the head unit accesses it just like your computer would. The head unit reads data from the drive and is able to play the songs because it has the right firmware or software to do so.

When you plug a USB flash drive into a USB to aux cable and plug the cable into an aux port on a head unit, nothing happens. The thumb drive is incapable of outputting an audio signal, and the aux input on the head unit is incapable of reading the digital information stored on the drive.

The same is true of phones and MP3 players that aren’t specifically designed to output sound via their USB connection. The USB connection is capable of transferring digital data back and forth, and can likely also be used to charge the device, but it usually won’t be designed to output an audio signal.

The only case where you would want, or need, to output audio from a phone’s USB connection to an aux input in your head unit is if the phone didn’t also include a headphone jack. Some phones, like the original G1/HTC Dream, omitted a headphone jack in favor of the ability to output sound via the USB connection.

What Are USB to Aux Cables For?

USB to aux cables do have some uses, but they are far from universal across all devices. For instance, one use for a USB to aux cable is to connect USB headphones to the 3.5mm headphone jack on a computer. This will work for some headphones that are designed to accept an analog audio signal in this way, but it won’t work for other headsets that expect a digital output from the computer or require power via the USB connection.

The one edge case where a USB to aux cable would be useful for listening to music in a car involves a phone or MP3 player like the old HTC Dream that has a micro or mini USB and no headphone jack. Phones and MP3 players like this are capable of outputting sound via the USB connection, so you should be able to plug in a USB to aux cable and have it work. However, charging the phone at the same time in this type of situation is only possible with a Y cable that plugs into the phone’s USB connection and provides both a 3.5mm aux out for sound and a pass-through USB connection for power.