Bluetooth A2DP and aptX are two of the most popular audio codecs used in smartphones and tablets. Bluetooth A2DP is a codec that uses Bluetooth to send audio data between devices, while aptX is a codec that uses digital audio signals to improve the quality of audio. The main difference between Bluetooth A2DP and aptX is that aptX is designed for high-definition audio, while Bluetooth A2DP is designed for low-definition audio. Additionally, aptX supports multiple devices at the same time, while Bluetooth does not.
Bluetooth headphones are all the rage now, after spending the better part of a decade as a niche restricted to tech enthusiasts. Now you can find an incredible variety of Bluetooth headphones on electronic store shelves, and even more online. But as with almost all product categories, not every set of wireless headphones is created equal.
We’re going to talk about three Bluetooth technologies that relate to exactly how good your Bluetooth headset sounds, and what you’re looking for in a new pair. A2DP is the basic Bluetooth stereo streaming protocol, aptX is an advanced codec specifically designed for Bluetooth, and Apple’s W1 chip system is proprietary and only works with Apple hardware.
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A2DP: The Default
A2DP stands for Advanced Audio Distribution Profile, which means—well it doesn’t mean a whole lot in the context of something that’s already streaming audio. But as one of the oldest parts of the combined Bluetooth specification, A2DP is more or less the default for streaming audio over Bluetooth. Any Bluetooth audio product you purchase—headphones, speakers, mobile phones, laptops—will support A2DP at the very least, whether or not it can also work with aptX.
The A2DP standard operates in stereo and supports most of the standard audio compression codecs. The recommended sub-band coding (SBC) codec supports up to 345 kilobits per second at 48 kilohertz. That’s approximately one third the quality of standard CD audio—roughly the equivalent of a high-quality MP3 recording. Due to high “lossy” compression in the SBC codec, the reality of the audio quality is considerably lower, somewhere in the range of 256kbit/s.
The system also supports other popular methods of encoding and compressing audio, like MP3 itself. If the audio source is already compressed in a format like MP3, AAC, or ATRAC, then it doesn’t need to be re-encoded in SBC in order to be broadcast from the source device. With A2DP’s maximum audio bandwidth of 728kbit/s, it’s at least possible to start approaching what we’d call “high-quality audio” with the basic standard alone. (CD quality audio, uncompressed, is approximately 1400kbit/s.)
Unfortunately, very few hardware makers seem to be actually using this capability, and most A2DP-only devices are re-encoding audio to SBC and de-encoding on the receiver end. This makes the whole process more complicated, resulting in poorer audio quality.
aptX: The Upgrade
AptX is also a compression standard, like SBC or MP3. But it’s an altogether better one, and one that’s designed to work within the limited bandwidth and low power available to Bluetooth devices. CSR, the developer that created aptX, says that it uses a proprietary compression method that preserves more of the audio’s full range of frequency while simultaneously “squeezing” it down to fit in the limited data pipe that A2DP offers.
In layman’s terms: think of the A2DP profile as a McDonald’s double quarter-pounder hamburger, and aptX as the “special sauce” that makes that burger a Big Mac.
[Editor’s Note: Michael, your metaphors need work.]The company claims that this advanced compression results in “CD-like” sound quality, and while that might be a bit embellished, a full aptX system does sound dramatically better than most A2DP-only systems. The codec is also faster to encode and decode, resulting in less of a gap between the screen and the speakers when watching a video with Bluetooth audio enabled. AptX HD is an even higher-quality standard, with 24-bit/48kHz audio, and streaming at a slightly higher bitrate.
Unfortunately, aptX requires the codec to be supported by both the broadcasting device and the receiver. If your headphones or speakers don’t support aptX, they’ll default back to A2DP alone, resulting in the lower level of Bluetooth sound quality with which you already may be frustrated.
Apple’s AirPods and W1 Chip: The Other One
What about the iPhone? Does it support aptX, and do those fancy wireless AirPod headphones use it? Nope. While the AirPods do use Bluetooth (not AirPlay, which is more of a Chromecast-type Wi-Fi audio protocol), they use a proprietary W1 Bluetooth chip that’s fully supported by only Apple devices running iOS 10.2 or Sierra 10.12 (or later). This custom-made connection allows for more high-fidelity listening than standard A2DP (and a near-instant automatic connection), but it isn’t compatible with aptX, and connecting your iPhone to an aptX-capable headset or speaker will still use the lower-fidelity A2DP.
Note: You can use AirPods or Beats with non-Apple devices, or with Apple devices running older versions of iOS or Sierra. Those devices just won’t be able to take full advantage of the W1 chip. They will connect just fine over regular Bluetooth, and will default to using A2DP.
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How Do You Know You’re Getting aptX?
First, check your current device, which is probably your phone. Most newer Android phones sold over the last few years include this capability, especially those with Qualcomm Snapdragon processors. High-end phones from Samsung, LG, HTC, Sony, Huawei, and OnePlus all support aptX Bluetooth streaming. Apple’s iPhone is a the notable exception.
Next, make sure your receiving hardware—your speaker, car stereo, or headphones—also supports aptX. This is more rare, and you’ll want to specifically check the specification sheet to see if aptX is listed. This used to be restricted to only the most expensive models, but lately they’ve come down in price, and you can generally find aptX support on a wide range of designs. Everything from a $400 pair of Sennheiser noise-cancelling, around-the-ear cans to a $26 set of budget Aukey earbuds can handle the aptX codec. Look specifically for aptX HD support for even better audio.
Unfortunately, it can be hard to determine whether the actual audio you’re playing on your device also supports aptX streaming. Phone manufacturers in particular seem bad at informing the user of the codec or bitrate that’s actually being used when delivering audio. Once you’ve ensured that both your player device and your audio device are compatible, you’ll usually have to (ahem) play it by ear.
