If you've shopped for any wireless Bluetooth audio products lately, be it a pair of headphones or wireless speakers, or even a mobile phone, you might have seen mention of aptX.
AptX promises "CD-like quality" sound over Bluetooth. This comparison is always suspect, and it's as overused as it is vague.
But audio over Bluetooth is definitely compressed -- some of the data is discarded. It simply doesn't sound as good as audio transmitted over other wireless methods or a wired connection.
So can aptX help improve the sound of Bluetooth? And if so, how much?
Bluetooth audio: A quick primer
Before we talk about aptX, we need to start with digital audio and Bluetooth in general. I'll do my best to be brief.
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. While CD is by no means the best audio storage medium, it is the most familiar. 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.
This is an oversimplification, but it's a start so we can talk about the next steps.
That 16-bit/44kHz rate of CD equates to about 10MB per minute for stereo. While that isn't a lot of data these days, it's still a lot more than you'd want to stream over the Web, or store on a portable device. Hence the ubiquity of compressed audio, like MP3. By taking out what you theoretically can't hear using a method called "psychoacoustic modeling," MP3's file sizes are much smaller, about 1 MB per minute.
Compressed audio, though, often doesn't sound as good as uncompressed audio. Sometimes you can tell the difference even through the cheapest headphones.
Bluetooth is a low-power wireless transmission method designed to allow two devices to easily transfer data over short range. Like all wireless methods, it has limited bandwidth. As the Bluetooth standard has progressed since it was first introduced, the size of the wireless "pipe" to transmit data has grown. Even so, it doesn't have the bandwidth of something like Wi-Fi.
To transmit audio, Bluetooth uses "SBC," which stands for Low Complexity Subband Coding. It's compressed audio, much like MP3 is compressed audio. It wasn't designed with perfect audio fidelity in mind. It was designed to use as little processing power as possible (the "low power" thing from Bluetooth again). If you're using Bluetooth on a headset to transmit your voice for a phone call, SBC is fine. With music, however, a lot gets lost in the reencoding.
Worse, there are multiple levels of SBC, and as you can guess, the lowest common denominator wins. So if your new fancy phone can do SBC at a high rate, but your headphones can't, you get whatever the maximum rate the headphones can handle (or vice versa).
And remember, the compression used by Bluetooth is in addition to whatever compression is in the music. So if you've got MP3s on your phone, those MP3s get decoded, then recompressed with a different lossy codec (SBC) to get sent to your wireless headphones. This is not the way to high fidelity.
AptX is still compression; it's just a different kind of compression. Where MP3 uses the aforementioned 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.
CSR, the company that currently owns the aptX patents, makes big claims about its technology. "With the aptX audio codec source material is transparently delivered over the Bluetooth link, whether it is stored uncompressed or in an alternative compression (MP3, AAC, FLAC) format."
The trick is, both products--the phone and the headphones, for example--must have aptX to get any benefit. If only one has it, the other just runs SBC so the connection will work.
Here's a list of products that have aptX. And before you go clicking through, no, Apple isn't on the list. But plenty of Samsung, HTC, and Motorola phones have it, as well as numerous speakers and headphones.
Is it worth it?
Well, maybe. In my testing most Bluetooth headphones and speakers sound terrible, but that doesn't have to have anything to do with SBC. The transmission medium isn't the issue, they're just bad-sounding headphones/speakers. There are also some great-sounding ones, too, so it's hard to judge SBC or aptX on their own.
The trick is testing the same pair of aptX-enabled headphones or speakers with two different phones, one with aptX, the other without, running the same audio files. To my knowledge, no one has done this. I'll certainly aim to do just that in a future article, but feel free to post below if you've done this (but PLEASE describe your methodology).
Anecdotal reports on forums claim aptX is a huge improvement (but none mention doing an a/b test as I've described). There are some websites out there, like this interesting one, that use crowdsourcing to determine audio codec quality that show interesting tests with SBC, and they plan on adding aptX.
Turns out, we may not need aptX at all
Brent Butterworth, over at About.com, dug up an interesting tidbit in the spec for Bluetooth. Turns out it's written in the spec that if the source (your phone, say), and the sync (headphones, speakers, etc), can both decode a different codec, that can be used instead. This includes MP3 and AAC. In other words, if your phone and headphones both have the ability to decode MP3, you can stream MP3 without re-encoding the signal (via SBC or aptX) at all.
This would be awesome, except...no one is doing it, apparently. It would be a great feature to have, as the less compression/recompression, the better.
Could aptX offer better sound quality? Yes, better compression is usually a good thing. But the problem is, you don't know if you're getting aptX even if you have it with both pieces of gear. On most aptX-enabled gear, there's no indication that aptX is present, or active. That's something aptX really needs to work on. A little indicator light would go a long way.
Generally speaking, aptX seems to offer better sound quality over SBC, though how much depends largely on the gear and how it's implemented. It is possible, though, to get great-sounding gear without it, just by picking great-sounding gear in the first place.
Got a question for Geoff? First, check out all the other articles he's written on topics like why all HDMI cables are the same, LED LCD vs. plasma, active versus passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.