Using a USB-to-Aux Cable in Car Audio

Car audio systems tend to lag behind audio options and consumer electronics in general, so you're more likely to find a 3.5 mm auxiliary jack in your car radio than a USB port. If you've seen a USB-to-aux cable, you may have wondered whether you could use it to connect your phone or a USB thumb drive to your car radio. The answer is probably no, but the situation is more complicated than that.

Do USB-to-Aux Cables Exist?

USB-to-aux cables exist, and they work for the purposes that they were designed. However, they don't work as a conduit for digital music files to your car radio.

If you plug a USB thumb drive into a USB-to-aux cable and plug the cable into a head unit, nothing happens. The same is true, in most cases, if you plug a USB-to-aux cable into a phone and connect it to a head unit.

A few phones and MP3 players are designed to output audio signals via a USB connection, such as the original HTC Dream that used a single micro-USB connector for both power and audio output, but that isn't always the case.

Using USB vs. Auxiliary in Car Audio

USB is a digital connection that transfers digital information, and a standard 3.5 mm TRRS auxiliary jack is an analog connection that expects an analog audio signal. There is some overlap between the two, as USB headphones exist, but USB headphones 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 the processing of audio data to the head unit. In contrast, aux connections are only capable of taking in an already processed signal.

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, it typically provides better sound, and USB also provides better sound quality, but only if the head unit has a USB connection.

Can You Plug a USB Drive Into a USB-to-Aux Cable?

When you put music on a USB flash drive, phone, or any other storage media, it is stored as a digital file. The file is usually compressed in an MP3, AAC, OGG, or another format unless you buy high-resolution digital music.

To listen to those files, you need a program, app, or firmware capable of reading the data and converting 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 the head unit in your car, the process is essentially the same.

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

Nothing happens 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. The thumb drive can't output an audio signal, and the aux input on the head unit can't read the digital information stored on the drive.

Can You Plug an MP3 Player Into a Car Head Unit?

The same is true for phones and MP3 players that aren't designed to output sound via USB connections. The USB connection can transfer digital data back and forth and can likely be used to charge the device, but it may not 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 a head unit is if the phone doesn't include a headphone jack. Some phones omit the headphone jack in favor of the ability to output sound via the USB connection.

Uses for USB-to-Aux Cables

USB-to-aux cables have some uses, but they are far from universal across all devices. Some devices are designed to receive power over a 3.5 mm TRS connection, in which case you can typically power them with a USB-to-aux cable.

In another example, you can sometimes use a USB-to-aux cable to connect USB headphones to the 3.5 mm headphone jack on a computer. This is usually only possible if the headphones don't require power over a USB in addition to an audio signal.

This works for some headphones that are designed to accept an analog audio signal in this way. Still, it doesn't work for other headsets that expect a digital output from the computer or require power via the USB connection.

Phones and MP3 Players With No Headphone Jack

The one case where a USB-to-aux cable would be useful for listening to music in a car involves a phone or MP3 player that has a micro or mini USB and no headphone jack.

Phones and MP3 players like this can output 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 3.5 mm aux-out for sound and a pass-through USB connection for power.