USB-C is a highly versatile connection. It can be used to transfer files, connect peripherals like high-speed storage drives, displays, and more. One key feature of USB-C that is used widely on modern smartphones is audio transmission. But how does USB-C audio (digital) differ from the traditional (analog) headphone jack? Let’s take a look at what USB-C audio offers, as well as some of its drawbacks.
A Tale of Two Data Streams
To understand what USB-C audio is and how it’s different, we need to first understand how a computer interprets audio. Sound travels through a medium (think of a pathway) like air, water, or something solid like our eardrums. It moves through the medium by traveling along vibrations, which you may know better as “sound waves.” That’s how your computer’s speakers make noise — they vibrate at specific intervals, which vibrates the air around them, creating sound waves. These waves carry the sound to your eardrum and cause it to vibrate. Your brain then interprets these vibrations as Beethoven’s 5th Symphony, the Gettysburg Address, or Cardi B’s latest hit single.
Computers work in much the same way as your brain, though they interpret sound waves and vibrations as digital signals. When you select an audio file to play on your phone or computer, the digital information (the 1s and 0s that make up the file) is passed through an encoder to a Digital to Analog Converter (DAC). The DAC is a computer chip specifically designed to turn digital information into an analog audio signal. The analog signal is then sent to an amp, which strengthens the signal and sends it to the speakers or headphones. These have drivers that vibrate a membrane (like a piece of special paper) and create the physical sound waves.
The DAC and amp are the primary components responsible for audio quality. A poor-quality DAC/amp system can make for lousy sound, while a high-quality DAC and amp can pump out high-quality music, speech, or other auditory data. This is primarily due to how a DAC receives the digital audio signal, and it’s why a cheap DAC doesn’t sound good: it results in a phenomenon known as “clock jitter.” A discussion on clock jitter is beyond the scope of this article, but suffice it to say that higher-quality DACs have little to no jitter, resulting in higher audio fidelity and better sound quality. In any case, the digital signal needs to be converted to an analog signal, amplified, and finally sent to a driver to make physical sound.
Computers interpret audio as a digital signal (the blue line). This needs to be converted to an analog signal (the black line) before it can be sent to headphones or speakers. (Image by ScreaminFX.com)
Bluetooth, 3.5 mm, and USB-C
Let’s look at smartphones as a prime example. As many new smartphones no longer carry a 3.5 mm jack, USB-C or Bluetooth are the only ways for these devices to send audio data to external speakers or headphones.
Bluetooth speakers and headphones contain their own DACs and amps, so they simply take the digital signal sent from the phone, convert it, and push it to their drivers. This is the same method used by the 3.5 mm jack (which a handful of smartphones still have). An onboard DAC converts the digital file to an analog signal, sends it through an amp to increase the signal, and then fires it out the 3.5 mm jack to your speaker system, headphones, or whatever else is connected to it.
USB-C isn’t so simple. With USB-C, audio data can be sent out the port either as an analog or a digital signal. If the signal is sent as an analog signal, a connected pair of headphones can simply carry that signal to its drivers to produce sound waves. If the data is sent as a digital signal, it will need to be converted to an analog signal before drivers can interpret it.
Herein lies the key weakness with USB-C audio — there is no consistency with how the signal is sent across the USB-C signal, and some USB-C headphones or dongles will not work with some smartphones.
With the 3.5mm jack, audio is converted from a digital to an analog signal before it is sent through the port. USB-C may send a digital signal through the port, which will need to be converted to an analog signal by the connected audio device. (Image via Sound Guys)
Active vs. Passive USB-C
The biggest problem with USB-C audio is its lack of standardization, which is ironic considering USB-C’s purpose is to standardize USB connection types. The problem lies not with the USB-C specification, but with the companies that design smartphones, dongles, and headphones. Here’s why this can be such a confusing mess: (Note that the remainder of this article will only reference USB-C headphones. Keep in mind that the same principles apply to USB-C dongles as well.)
Some smartphones send audio data through an onboard DAC (usually embedded in the phone’s processor) before sending it out through the USB-C port. If this is the case, almost any pair of USB-C headphones will work. If the audio stream is converted to an analog signal before it goes through the USB-C port, then connected headphones do not need their own DAC. These are known as “passive” headphones.
However, some devices send audio data through the USB-C port as a digital signal. As such, connected headphones will need their own onboard DAC and amp to convert the audio stream from a digital format to an analog format. USB-C headphones with an onboard DAC/amp combination are known as “active” headphones.
The Cable Matters USB-C to 3.5mm Headphone Audio Adapter is an active USB-C audio device, meaning that it has an onboard DAC.
Further complicating matters is support for Audio Accessory Mode. Audio Adapter Accessory Mode is a feature of the USB-C spec that tells the phone whether or not a connected audio cable is passive and requires an analog signal to be sent. Many smartphones do not support Audio Accessory Mode. As such, passive headphones are not supported and cannot be used.
If you’ve ever connected a pair of third-party USB-C headphones to your smartphone that did not produce any sound, then it’s likely the headphones were passive and your phone sent audio data as a digital signal.
In general, active headphones can be used with smartphones that send the audio stream as either a digital or analog signal and should work universally. Some smartphones use the extra wiring of USB-C for additional features, though. A prime example is HTC’s U11, which supports active noise canceling through the USB-C port in addition to audio. As such, the U11’s included earbuds (and ones that support this feature) are the only ones that work with the phone.
Benefits of USB-C audio
Despite the complicated drawbacks of USB-C audio, there are some benefits. The primary “pro” of USB-C audio is its versatility. USB-C can transmit video data over DisplayPort, HDMI, and more, though this isn’t supported on many smartphones.
USB-C devices, including headphones and dongles, can also send information about themselves through the USB-C port. Lastly, the USB-C port can be used to simultaneously charge a device and send audio data (provided the correct adapter is used). These added features are welcome additions.
To sum up, getting audio over USB-C can be confusing. Not every USB-C audio dongle or pair of headphones can be used with every USB-C port. Magnifying this issue is the historic inconsistency of manufacturers in labeling their USB-C audio devices as active or passive.
It’s always a good idea to do some research before buying a USB-C audio dongle or a pair of USB-C headphones. First, check your smartphone or other device’s specifications to see if it supports Audio Adapter Accessory Mode. If it does, then you should be able to connect a passive set of headphones or a passive dongle without much hassle. If it does not, you will need a pair of active headphones or an active USB-C audio dongle.
In general, using an active USB-C audio device (be it a cable, dongle, or pair of headphones) will mitigate most problems. Keep in mind that some devices use other features of USB-C and will only work with supported audio equipment.