Bluetooth Audio vs. Aux Connections

Before the rush of options like Bluetooth, auxiliary inputs, USB, and others, listening to music in your car used to be a pretty simple proposition. For the better part of a century, the only choice for car audio was between AM and FM radio. Then portable media small and robust enough for automotive use showed up in the form of the eight-track, and nothing was ever the same.

Compact cassettes soon took over the road, followed by CDs, and now digital media, in one form or another, has left everything else in the dust. But even if you’re totally on board with the idea of listening to music from your phone in your car, the question remains: is Bluetooth better than a physical aux connection, or is it the other way around?

Where Did Aux Inputs Come From?

Car stereos have had auxiliary inputs for a very long time, so it may be tempting to dismiss the technology as outdated. In fact, the 3.5mm auxiliary jack on the front of your car stereo relies on technology that has remained virtually unchanged since the 1960s.

Aux inputs in car radios are basically just analog connections that have been called phone plugs, stereo plugs, headphone jacks, and a variety of other names over the years. The same basic type of plug has been used to connect everything from phones, to electric guitars and microphones, to headphones, and everything in between.

The technical term for this type of aux connection is TRS, or TRRS, which stand for Tip, Ring, Sleeve and Tip, Ring, Ring, Sleeve, respectively. These names, in turn, refer to the physical metal contacts present in the specific aux input.

Most car audio systems include a TRS connection that is designed to facilitate the transmission of an analog audio signal from your phone, or any other audio output, to your car’s head unit in the exact same way that you might plug in a set of headphones.

There are some issues with this type of audio connection, and it is possible to run into some audio quality issues when you pipe an analog signal means for tiny headphones into a car stereo. Using a line out instead of a headphone or speaker out, or using a digital USB connection instead of an analog aux connection, are both ways to solve this issue.

However, simply plugging the headphone jack of a phone or MP3 player into the aux input of a car stereo is an option that works just fine for many people. Since the connection is analog, there is no compression involved in moving the audio signal from the phone to the car stereo. So while the DAC in your typical smartphone may not be optimized for this type of usage like a good car stereo DAC, there’s a chance that you won’t even notice the difference.

Where Did Bluetooth Come From?

While the basic technology involved in the aux input in your car stereo was originally designed to transmit analog audio signals of a different type in the 1960s, Bluetooth was invented much more recently as a way to create secure, wireless, local networks.

The basic idea behind the creation of Bluetooth was to come up with a faster, wireless alternative to the RS-232 serial port connection in the realm of personal computers. The serial port was largely replaced by USB by the late 1990s, but Bluetooth eventually found its way into the mainstream as well.

While Bluetooth is used in a host of different ways today, the way that most people interact with the technology on a daily basis is via their phones. Since Bluetooth allows for the creation of secure, local, wireless networks, the technology has seen widespread use in connecting wireless headsets to phones.

The wireless headset and hands-free calling is the main vector by which Bluetooth arrived in our cars. Since so many phones already had Bluetooth built right in, and so many people were already using wireless Bluetooth headsets, automakers started to offer built-in Bluetooth hands-free calling.

Since Bluetooth also includes a profile for streaming audio, it was only natural that car stereo manufacturers would start to offer that option as well. With the right Bluetooth car stereo, you can stream audio, video, and may even be able to control various radio apps straight from your phone.

Bluetooth vs. Aux: Searching for High Fidelity Audio in Your Car

The question of whether Bluetooth is better than aux in terms of listening to music in a car comes down to two major issues: audio quality and convenience. Coming at the issue from a convenience angle, it is tremendously easy to hook a phone up to a car stereo via an aux connection. In some cases, all you have to do is plug the cable in, and you’re good to go. At the outside, you may have to manually select the correct auxiliary input.

Bluetooth, on the other hand, can be a little more finicky to set up. In order to connect a phone or other type of MP3 player to your car stereo, you have to set one as “discoverable” and then use the other one to find the first one. If the devices won't pair, you may have to repeat the process until it works. Once your phone and car stereo have found each other, you will typically have to input a short passcode that will allow the two devices to successfully pair.

The main benefit of Bluetooth in terms of convenience is that, barring unforeseen circumstances, you shouldn’t have to repeat the pairing process. When your phone comes into range of your car stereo, and both are powered up, the two should pair automatically. This is definitely more convenient than needing to physically plug in an aux connection every time you get in the car.

Are There Drawbacks?

Illustration showing a difficult Bluetooth connection and a clear AUX connection in a car
Lifewire / Miguel Co

The main drawback of using Bluetooth to listen to music in your car is audio quality. While it may be more convenient in the long term, the audio quality will typically be worse with Bluetooth than with an aux connection.

The reason that Bluetooth audio typically isn’t that great is due to the way that devices use the technology to transmit audio. Rather to transmitting an uncompressed analog signal, like a physical aux connection, sending audio via a wireless Bluetooth connection involves compressing the audio at one end and then decompressing it at the other.

Since Bluetooth audio transmission involves a form of lossy compression, some level of audio fidelity is necessarily lost whenever you use this type of connection. It is possible to transmit data via Bluetooth, in the form of complete files, without losing anything, but that doesn’t really come into play in this type of usage scenario.

If you are unsure about what all this means, and you have a Bluetooth headset or headphones at home, try hooking them up to a computer. If your device has the option to connect with the audio Bluetooth profile or the phone Bluetooth profile, try each, and check out the night and day difference between the two.

When you choose to use your Bluetooth headphones or headset on a computer via the “headset profile,” the audio transmitted to and from the device is encoded in 64 kbit/s or PCM, and the profile also allows for minimal controls like answering calls and adjusting the volume.

When you choose to use your Bluetooth headphones or headset on a computer via the “advanced audio distribution profile,” the audio can be transmitted by the low-complexity SBC codec, although the profile also supports MP3, AAC, and others.

The difference in sound quality between these two profiles is so obvious that just about anyone can immediately pick out which one is inferior. While the difference between Bluetooth and aux isn't as great, the reality is that some level of audio fidelity is lost with Bluetooth even with the A2DP profile.

The Hidden Advantage of Bluetooth Over Auxiliary

Even if Bluetooth does provide an inferior level of audio quality that you, personally, are able to detect, there’s one very important reason that you might still want to choose a wireless connection over a physical connection.

When you pair a phone to a Bluetooth car stereo or a compatible OEM infotainment system, the main purpose might be to listen to music. However, creating this type of connection also gives you access to hands-free calling without the need to establish a separate connection or fiddle around with a wireless headset.

In many cases, plugging your phone into your car stereo via a physical auxiliary connection will totally rule out hands-free calling. This is due to the fact ​that many phones will automatically want to use the wired connection to handle any incoming or outgoing calls when the wired connection is present. Of course, this typically will result in a situation where you can hear the person on the other end of the call through your car speakers, but they can’t hear you.

Using Bluetooth to stream music is the best way to avoid this type of problem since your phone and car stereo will typically be able to swap from the music-streaming profile to the communications profile during a phone call.

Does Aux Really Sound Better Than Bluetooth?

In practice, you may not notice a tremendous difference in audio quality between Bluetooth and aux. This is mainly due to the inherent weaknesses in car audio systems. If you have a factory car audio system or a low-end aftermarket system, you’re probably less likely to notice a difference than if you have a high-end aftermarket system. You’re probably also less likely to notice a difference between the two if you drive a vehicle that gets a lot of interference from road noise and other external sources.

The fact is that an auxiliary connection will always provide higher quality audio than Bluetooth, and a digital connection like USB can provide even better quality in certain circumstances. However, the difference between Bluetooth and aux is absolutely a matter of personal preference, especially if losing a little in terms of audio fidelity is worth the convenience of not having to plug in a physical aux cable every time you get in the car.