aptX: Everything you need to know about the wireless Bluetooth enhancement

If you're the type of person who loves to scrutinize speaker and headphone feature lists, you've likely come across aptX hyped as an improvement in quality for the audio transmission between your source, like a phone or tablet, and your headphones or speakers. In the early days of Bluetooth, aptX got a lot of attention due to its potential for far higher quality compared to the stock Bluetooth transmission. 

Bluetooth technology has come a long way in the last decade or so -- it's currently in its version 5 iteration -- and as such, aptX isn't the panacea it once seemed. That doesn't mean it's not potentially useful, just that it doesn't necessarily mean there's much, if any, audible improvement. It can have other benefits, though, so it's definitely worth learning about.

But before we can explain aptX, we need to talk about Bluetooth itself.

Read more: The best headphones of 2020

Bluetooth audio: A quick primer

Digital audio is a collection of samples of what a sound wave looks like at a specific moment in time. Instead of a steady wave of sound, it's a series of snapshots.

With enough of these snapshots, or "samples," a playback device (an iPhone, say) can convert them back into a smooth sound wave. Take good old-fashioned CD. The audio on a CD has 44,100 samples every second, and each sample has a value of somewhere between 0 and 65,535 (also called "16-bit"). So in other words, there are 44,100 snapshots per second, and each snapshot has one of 65,536 potential values.

That 16-bit/44kHz rate of CD equates to about 10 megabytes per minute for stereo, or over a megabit per second. While that isn't a lot of data these days, it's still a lot more than what's easily transmittable between a portable device and Bluetooth headphones. It'd also chew through your mobile data if you were to stream it.

Hence the ubiquity of compressed audio, like AAC and MP3. These "codecs," a portmanteau of compression/decompression, remove what you theoretically can't hear using a method called "psychoacoustic modeling." This lets them reduce the size of the audio files or streams significantly. MP3s, for instance, are often around 1MB per minute. That's a 10x decrease, with a corresponding decrease in file size. 

The trade-off for convenience is quality. These codecs are called "lossy," as the sound removed is lost, and can't be recovered. Compressed audio, in theory, sounds worse than the uncompressed version of the same song. This isn't as noticeable now as it was in the early days of the format, thanks to better encoders, higher bit rates and more, but sometimes you can still tell the difference even on the cheapest headphones.

Which brings us back to Bluetooth. Bluetooth is a low-power wireless transmission designed to allow two devices to easily transfer data over short range. It's shorter range, lower power and transfers less data than Wi-Fi. As the Bluetooth standard has advanced so has the size of its limited wireless "pipe." Which is to say, far more data can be transmitted over Bluetooth than when it first came out. 

In the early days of Bluetooth, audio had to be compressed significantly to work at all. This was done using a codec called SBC or "low complexity subband codec." It wasn't designed with perfect audio fidelity in mind. It was designed to use as little processing power as possible, given the lower power devices for which it was intended. It has gotten better over the years, and on most devices runs at a bit rate of up to 345 kilobits per second. For reference, if you pay for a Spotify Premium account, you can get up to 320 Kbps, but more on that in a moment. This isn't to say that any two codecs at the same bit rate will sound the same, but it gives us something simple to compare.

There's no guarantee SBC will run at 345 Kbps, however. It's possible a device will use a lower bit rate, which means that's the max you'll get, regardless of the capabilities of what you pair it with. At its best, SBC isn't radically different in audio quality to MP3 or AAC. It's not necessarily always at its best, though.

It was into this lossy compression miasma that aptX was born.

Enter aptX

aptX is still compression; it's just a different kind of compression. Where MP3 uses psychoacoustic modeling to take out data, aptX uses "time domain ADPCM," which is a whole rabbit hole of fun research for any of the technically inclined readers out there. The oversimplified version is that ADPCM uses fewer bits per sample, so the files are smaller.

Qualcomm, the company that currently owns the aptX patents, makes big claims about its technology. "aptX audio is a bit rate efficient technology that ensures you receive high quality audio from your Bluetooth device, so you have a better listening experience." 

The trick is, both products -- the phone and the headphones, for example -- must have aptX to get any benefit. If only one has it, you don't get aptX. 

The initial version of aptX is still around, and fairly common. The biggest improvement over SBC is that it requires a bit rate of 384 Kbps. There won't be much, if any, audible difference between aptX at 384 and SBC at 345. If one or more of your devices doesn't run SBC at its best bit rate, but does offer aptX, then you might hear an improvement.

aptX HD goes a step further, offering 24-bit audio at a bit rate of up to 576 kbps, while the new aptX Adaptive theoretically offers the sound quality of aptX HD at a lower bit rate and has the option of a 96kHz sampling rate.

Here's a list of products that have aptX. Of note, there are no Apple products on the list. But plenty of Samsung, LG, Google and other phones have it, as well as numerous speakers and headphones. The list doesn't seem to be updated regularly, so if you don't see your current device or one you're considering, it's worth checking the specs page of the respective manufacturer.

Is aptX worth it now?

In a word: maybe. SBC isn't the quality-smashing issue it was in the early days of Bluetooth. Done correctly, it's on par with other lossy codecs. aptX will, potentially, give you a better guarantee that the transmission isn't the weakest link in your audio chain, but there's a lot more to it than finding the aptX logo and assuming the device will give you great sound. Some devices, for instance, will play the codec from the source directly. So if you've got AAC files on your phone, and your headphones support it -- and yes, most Apple hardware does -- you'll listen to that without a conversion to SBC, so aptX isn't necessary. 

Also, and perhaps more importantly, consider what you're listening to. On paper, aptX HD seems great, with a high sample and bit rate. Unless you have a high-quality source, however, this additional capability isn't important. A better transmission won't make your audio sound better unless it had been constricted by a lesser transmission type. To put it another way, if you're listening to a low-quality audio stream, on inexpensive headphones, changing to aptX HD isn't going to magically give you better sound. As mentioned above, even SBC can have a higher bit rate than Spotify Premium, most other music streaming services or most commercially available MP3s. The free version of Spotify is half that bit rate, and on mobile it's even less. So there's no point in aptX HD if your content isn't going to take advantage of it. The stock SBC that comes with all Bluetooth devices might be fine. Again, comparing bit rates isn't the whole story, but the difference isn't likely to be dramatic.

Now, if you're downloading high-resolution, uncompressed audio, the story is a bit different. This isn't something most people do, however, and can only be done on specialized services and sites. The step-up HiFi option on Tidal, for instance, offers "lossless, CD and Master Quality Authenticated quality (1,411 kbps versus 320 kbps for standard streaming)." There's also the step-up, additional-cost Amazon Music HD and others. Unless you've knowingly sought out high-res audio, it's highly unlikely your music is high-res enough to require anything other than a standard Bluetooth connection.

There are also other players in the game. Sony, for instance, has its own take on high-quality Bluetooth audio, called LDAC. It's not available in as many devices, though. 

It's important to keep in mind that the transmission codec is only one part of how the music sounds, and honestly, not a big part. The quality of the speakers and where they are in your room, the quality of the headphones and how well they fit, the quality of the source material -- they all have a much bigger effect on the overall sound quality than how it's transmitted from your phone to your speakers or headphones. That can be a weak link, but it is just a link.

That is perhaps a controversial statement, since aptX is discussed and praised extensively in audiophile circles. So instead of taking my word for it, test it for yourself. Long-time audio reviewer Brent Butterworth created a Bluetooth Blind Test so you can download files with different codecs to find out if you can hear a difference. Make sure you use wired speakers or headphones; don't add an additional level of Bluetooth to the test. 

The future of aptX and Bluetooth

There are some new versions of aptX starting to show up in products that have the potential to be far more interesting. I discussed Adaptive above, which expands in the audio quality direction. What I'm interested in is aptX Low Latency. 

You may have noticed when using Bluetooth headphones a lip-sync error, where the voices don't match up with mouth movement. This is caused by the additional processing required to get the audio to your ears via Bluetooth. Low Latency reduces the time from SBC's 200-300 milliseconds to around 40 ms. This should effectively eliminate lip-sync issues. The aptX Adaptive codec will have a similarly low latency as well, of around 80 ms.

What's also on the horizon is an update to Bluetooth's audio, in the form of the Low Complexity Communication Codec, or LC3. This codec should, in theory, allow for lower bit rates that maintain audio quality. It was announced at CES 2020, so don't expect it in products just yet.

The final word (for now)

Can aptX offer better sound quality? Yes, better compression is usually a good thing -- but it's not a guarantee of good sound quality. It's one link in a chain, and on most devices, the current link (SBC) works perfectly well. If you listen to all your music on YouTube and you're happy with your free Spotify account, the higher quality potential with aptX isn't likely to be audible. If you're trying to eke out the most performance possible on some high-end gear you already own, and you seek out new audiophile tracks and new download solutions, aptX and especially aptX HD and Adaptive might give you a slight bump over SBC. But if you're that much of a stickler for audio quality, I'm guessing that you're listening to everything wired anyway. 

For the most part, though, it's far more important to just get good sounding gear.

As well as covering TV and other display tech, Geoff does photo tours of cool museums and locations around the world, including nuclear submarines, massive aircraft carriers, medieval castles, airplane graveyards and more. 

You can follow his exploits on Instagram and Twitter, and on his travel blog, BaldNomad. He also wrote a bestselling sci-fi novel about city-sized submarines, along with a sequel.