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King Harald Gormsson united the tribes of Denmark into a single kingdom during his reign (958 – 986 CE) – he had a discoloured tooth that led to his being nicknamed Bluetooth.
When Intel, Ericsson and Nokia were developing a way of offering short-distance wireless connectivity between electronic devices, the word Bluetooth was suggested as a placeholder until a “proper” name for the technology could be agreed; because this tech would unite devices like King Harald united tribes. No better name was forthcoming, and the adoption of a logo using the rune of Harald’s initials sealed the deal.
Bluetooth technology can connect many types of fixed or mobile electronic devices, of course, but in this article, I’ll be explaining the specifics of wireless headphones and their connection to a smartphone or some other source of digital music. More specifically still, I’ll be looking at what Bluetooth codecs are, and the impact they can have on the sort of sound quality you’re entitled to expect. So if you thought all wireless connections were the same, think again…
How does Bluetooth connectivity work?
Bluetooth uses ultra-high frequency radio waves to create short-range (around ten metres) wireless connections in order to transfer data between electronic devices. These radio waves are typically in the unlicensed 2.4GHz range that’s often used for wireless communication – your Wi-Fi router operates in the same range.
Bluetooth uses what’s called frequency hopping to maximise signal stability and minimise interference. This means it switches between radio frequencies as often as 1,600 times per second in order to maintain signal integrity. Frequency hopping also helps to keep these short-distance transmissions secure.
Naturally, all of this wireless activity requires power, which is why we have Bluetooth Low Energy (BLE). Launched in 2010 and initially branded Bluetooth Smart, it’s optimised for low power consumption and is ideal for those applications where a wireless connection needs to be maintained for a long period.
The Bluetooth SIG (special interest group) introduces new features from time to time. Multipoint connectivity is one (Technics’ EAH-AZ100 earbuds, for instance, can connect to three devices at once), and BLE mentioned above is another.
What is a Bluetooth codec? And why is it important?
As Bluetooth standards advance, in terms of extended wireless range and other areas, new Bluetooth versions are created – currently we’re at Bluetooth 5.4 – and as standards move forwards, new codecs become available.
A Bluetooth codec is the software that compresses, packages and encodes digital audio files to make them ready for wireless transmission in the form of packets, just like data sent over the internet and around your home network. Once they’re transmitted, the receiving device reassembles the files, then decodes and unpacks the audio ready for you to hear. It’s a process that needs to balance efficiency with accuracy, and the codec is ultimately what governs how this balance is struck.
The data rate of a Bluetooth connection is limited (up to 3Mbits/sec max), so the codec also needs to compress the data before it sends it across, and Bluetooth codecs use lossy compression schemes, which means some of the data is permanently removed in the process.
So the less ‘lossy’ a Bluetooth codec is able to be, the more likely you are to retain that information that might make all the difference when it comes to the eventual audio quality you get to hear.
Are there different codec types? Why?
If you have a Bluetooth-enabled device, whether it’s a smartphone or a wireless speaker or a pair of wireless headphones, its specification sheet will probably let you know about the Bluetooth codecs it’s able to support.
It’s almost certain your particular device will support the SBC and AAC codecs explained below. After that, it depends entirely on what the manufacturer has decided on. And it’s important to understand what your particular device is capable of, because the effect the Bluetooth codec can have (on battery life as well as audio quality) is huge.
Where sound quality is concerned, what’s most important is the amount of compression the digital audio file undergoes, and the resultant resolution of the file when it arrives in your headphones.
Bitrate – the rate at which data is transferred – is the most significant measure here and is one of the biggest differentiators between codecs, because the higher a codec’s bitrate, the more data it can transmit and the better able it is to minimise loss of information during the wireless transfer.
Codecs may be able to deal with very high-resolution audio content (way beyond the 16bit/44.1kHz CD standard), but the compression they introduce and the loss of information that comes along with it means bitrate (measured in kilobits per second, or kbps) is your best indication of what to expect when it comes to sound quality.
Bluetooth is constantly evolving, and its effectiveness is evolving too – which means that as long as your headphones, say, can support the same codecs as your smartphone, optimum wireless performance can be yours…
Key Bluetooth codecs
SBC
The basic codec, SBC has been around for a couple of decades. It’s open source, so it’s free to use, and its implementation is mandatory for all devices supporting the AD2P audio data transfer standard, which means every audio-based Bluetooth device will support it. It has a maximum bitrate of 320kbps, but it’s often implemented to work at 256kbps in a drive for greater energy efficiency.
AAC
Advanced Audio Coding is the default codec used on iOS and MacOS devices (it’s on plenty of Android devices too). It can deal with files of up to 24bit/44.1kHz, is slightly more power-hungry than SBC, and its maximum 320kbps bitrate is a slight improvement on the default SBC implementation too. Which makes it good enough for Apple, a company that has never appeared to be interested in upgrading its wireless audio standard, despite there being a lot of hi-resolution audio content (up to 24-bit 192kHz) on Apple Music.
aptX
The first of Qualcomm’s Bluetooth codecs, aptX is a staple of many Bluetooth devices and a more effective way to transmit data than SBC, although with a maximum bitrate of 352kbps it’s not to be confused with any kind of lossless transmission. It’s a slightly more energy-efficient codec, too.
aptX HD
That HD is exciting, isn’t it? High definition? Now you’re talking. Released almost ten years ago now (Expert Reviews was very excited about aptX HD at the time), aptX HD can handle 24bit/48kHz content and operates at a maximum bitrate of 576kbps. Any number of “proper” headphones and wireless speakers still support it, including (but not limited to) our current favourite true wireless earbuds from Technics, the EAH-AZ100. Even turntables got in on the act. Cambridge Audio’s Alva TT could transmit wirelessly using this codec.
aptX Low Latency
A maximum data rate of 352kbps didn’t seem all that impressive at the time – but this codec’s super-fast sub-40ms latency made it an absolute gift for wireless gamers. You hardly ever see it mentioned any more, though, because aptX Low Latency has now become a part of…
aptX Adaptive
With the benefits of both its Low Latency and HD siblings, 2018’s aptX Adaptive includes the intensity of competing radio signals in its area to minimise drop-out and maximise audio quality by adjusting its bitrate real time. Adaptive can run at anything between 279 and 420kbps, and is the first aptX codec that can deal with 24bit/96kHz. Seven years since it first broke cover, aptX adaptive is still a staple of premium headphones, and is available in everything from the Bose QuietComfort Ultra Earbuds to Bowers & Wilkins Px8 and plenty more besides.
aptX Lossless
And here is the current state of the aptX art. Support for 24bit/96kHz and dynamic bitrate scaling of between 256kbps and 860kbps is impressive enough, but aptX Lossless, as the name strongly implies, can transmit CD-quality 16bit/44.1kHz audio losslessly. The knowledge that aptX Lossless tops out at 1,200kbps and that CD is 1,411kbps might imply that this is a lossy codec, but Qualcomm is having none of it; it insists no data is lost when aptX Lossless is doing the encoding and decoding.
Bowers & Wilkins’ brand-new Px7 S3 support aptX Lossless, as do the Sonos Ace headphones – and there are plenty of supporting source devices coming on stream too, including the Sony Xperia 1 VI. And speaking of Sony…
LDAC
Sony isn’t the only brand that likes to keep everything in-house if at all possible, and LDAC is the company’s attempt to create its own high-performing Bluetooth codec. LDAC can deal with files of up to 32bit/96kHz resolution and transmits them at a not-quite-class-leading 990kbps. And it’s a sign of how successful Sony has been with LDAC that there are plenty of companies producing plenty of different wireless products that have the technology on board – everyone from JBL, with its Tour ONE M3 headphones, to Astell & Kern’s Acro BE100 wireless speaker, is involved. LDAC is supported by all Android devices from version 8 upwards, so there are no huge issues about compatibility on the source side.
LHDC
Savitech wanted LHDC to work as an alternative to LDAC when it launched the codec in 2017, but although it’s good with 24bit/192kHz stuff and can transmit it at a very respectable 1,000kbps, uptake hasn’t been quite as extensive as the company must have hoped. Huawei was an early supporter (quite a few of the company’s smartphones included LHDC support), but then it hit upon troubles of its own…
LC3
2020’s ‘Low Complexity Communications Codec’ puts the emphasis on efficiency rather than audio quality. Its 345kbps bitrate looks like a throwback, but its super-low power requirement means LC3 headphones can enjoy far better battery life than was previously possible. Sony’s WF-1000XM5 and Earfun’s Air Pro 4 are just a couple of the models able to work for longer thanks to the Bluetooth SIG’s Low Energy Audio standard.
Do all codecs work with all source devices?
No, they most certainly do not. You’ll be hard-pushed to find a source device of any type that doesn’t support both SBC and AAC, and the same goes for wireless headphones and wireless speakers.
After that, though, you need to examine the spec-sheets of both your source of music and the wireless device that’s receiving it – because if one supports LHDC (for instance) but the other doesn’t, the wireless Bluetooth connection between them will default to SBC or AAC. And as you now know, that’s hardly the way to go about achieving optimal sound quality.
