Bluetooth is everywhere: in phones, headphones, speakers, TVs, games consoles and smartwatches... basically, if it's powered then chances are it uses Bluetooth. This ubiquitous technology lets devices communicate with each other wirelessly, so you can send fitness data, images and – most importantly – music through the air.
But which Bluetooth codec is at play has a significant bearing on the sound quality you'll get between your source device and headphones or speaker. Different Bluetooth codecs have different bitrates and different ways of compressing and processing data during transmission. So if you want a chance of hearing music over Bluetooth devices at the best-possible quality, you will need to use a higher-quality Bluetooth codec than the 'standard' AAC and SBC ones (we'll go through those shortly too).
In this guide, we'll explain everything you need to know about each widely available Bluetooth codec so that you can understand the differences between them and know how to benefit from the better ones.
What is Bluetooth?
Bluetooth is a secure, short-range wireless technology that lets devices exchange information with one another without being physically connected by a cable. How short-range are we talking? It depends on the devices and type of content being shared, but typically ranges from 10m to 100m.
Devices can share all kinds of things over Bluetooth, including fitness data, music files, images, word-processing files and spreadsheets, to name just a handful. Big-data files are often better suited to being transferred over wi-fi because of the greater capability of 'wireless fidelity', though Bluetooth can carry larger files via compression (more on that later).
Essentially, a Bluetooth connection exists between a 'main unit' (a music source, say) and a 'peripheral' (a speaker or pair of headphones, for example). These two communicate with each other over Bluetooth using ultra-high frequency (UHF) radio waves – electromagnetic waves with frequencies of around 2.4GHz (2.4 billion waves per second).
If you have ever wondered what Bluetooth is named after, the answer isn't something you would correctly guess and scribble down in a pub quiz. It's actually named after the 10th-century Danish king Harald Bluetooth, who carried the nickname on account of his off-coloured grey/blue tooth. As the story goes, during an early meeting between Intel, Ericsson and Nokia about Bluetooth's inception, Jim Kardach from Intel suggested Bluetooth, saying “King Harald Bluetooth…was famous for uniting Scandinavia just as we intended to unite the PC and cellular industries with a short-range wireless link.”
It was originally just a placeholder until the creators could think of a better name, but that never happened. So 'Bluetooth' it remained...
What is a Bluetooth codec?
So that's Bluetooth. A Bluetooth codec is a software format that compresses and then encodes music so that it can be efficiently transmitted wirelessly between devices before being decoded by hardware that supports that same codec.
Compression reduces the file size lossily (meaning it loses information as it does so), as the less information that is transmitted, the smaller the file size can be. Compression can also be used to reduce audio-coding delays and minimise latency issues.
When a music file is compressed and passed wirelessly between Bluetooth devices via a Bluetooth codec, some of the song's detail is almost always lost forever in the process – to varying degrees depending on the codec's capability, which we'll get to momentarily. Indeed, not all codecs are created equal.
It's important to remember that even a small amount of compression is detrimental to sound quality, so pretty much every codec is lossy.
Bluetooth codecs are different from Bluetooth standards, such as Bluetooth 5.3. Bluetooth standards introduce new features (like Multipoint) and capabilities (like greater range). New Bluetooth standards may allow new codecs to emerge.
What is bitrate?
Now we're getting a bit technical. Bitrate is the rate at which data is transferred by a codec, measured in kilobits per second (kbps) – or megabits per second (mbps) in the case of the most capable codecs. It's the simplest way to compare the capability of each Bluetooth codec for transmitting music. The higher a codec's bitrate, the more 'bandwidth' it has, meaning the more efficiently it can carry higher-quality audio without losing information. Think of a codec like a tube, and the music file as something that needs to pass through it – the bigger the tube (bandwidth), the more music information can fit and more easily flow through it.
Now, bitrate is calculated using a digital music file's sample rate and bit depth, plus the number of channels (two for stereo).
Sample rate refers to the number of times samples of the audio signal are taken per second during the analogue-to-digital conversion process. CD-quality files have a sampling rate of 44.1kHz, meaning 44,100 samples were taken every second during that process. Hi-res audio files commonly have a sampling rate of 96kHz, meaning 96,000 samples were taken every second. Generally, the higher the sampling rate, the more accurate the digital recording (music file).
What bits largely buy you is dynamic range – the difference between the quietest and loudest sounds on the recording. For reference, CD quality is 16-bit and hi-res quality is typically 24-bit. But the higher the bit depth, the bigger the file size.
So for a Bluetooth codec to, for example, handle a CD-quality file losslessly (without losing any information), it needs to be capable of a bitrate of 1411kbps. Bear that in mind when you see the codec capabilities below...
What Bluetooth codecs are available?
There are a number of different Bluetooth codecs that you will see on the spec sheets of consumer audio devices that support Bluetooth. Typically, such devices support both the 'vanilla' SBC and AAC codecs, and then perhaps another on this list:
SBC (aka Low Complexity Subband Coding) is the most basic Bluetooth codec around. It's built into the Advanced Audio Distribution Profile (A2DP), which is a set of Bluetooth specifications for streaming audio over Bluetooth. A2DP has been around for around two decades and is free for manufacturers to use (as it's in the public domain), so every Bluetooth device you can currently buy will support it.
As well as being universally supported, it doesn't demand much power from your devices. But the trade-off is lower sound quality – it only handles up to 16-bit/48kHz audio files, with a bitrate up to 345kbps (but more often manufacturers limit their devices to 256kbps to avoid more battery drain).
AAC stands for Advanced Audio Coding. It's the successor to MP3, and the default codec used on iOS devices (though it's not owned by Apple). AAC is a more complex codec than SBC, meaning more power drain but also better sound quality. It can 'handle' 24-bit/44.1kHz audio files, with a bitrate of up to 320kbps, and is supported by Android devices (running Android 8 or later) as well as iOS.
Samsung Scalable Codec
As is so often the case, Samsung has developed its own technology just for its own devices. Samsung Scalable Codec is used with its Galaxy Buds family of earbuds and has the ability to support a wide range of bitrates depending on the stability of the Bluetooth connection.
As the Bluetooth connection gets weaker, Samsung's codec increases the compression while lowering the bitrate. That means the sound quality nosedives, but it does at least prevent the connection from dropping out altogether.
A 'next-level' codec if you like, from chipmaker Qualcomm. aptX was created in the late 1980s, mainly for use by radio and film studios (Steven Spielberg was an early fan). But now it's used primarily for Bluetooth wireless playback – you'll find it in plenty of smartphones, tablets, AV receivers and more.
You may read that aptX can transmit music at a 'CD-like' 16-bit/44.1kHz. 'CD-like' isn't quite the same as 'CD-quality', though, because aptX also uses compression and is therefore lossy. It was designed to sound better than 'standard' Bluetooth, mind. aptX has a compression ratio of 4:1 and a bitrate of 352kbps.
aptX HD was the next generation of aptX, released in 2016. Basically, it sounds better than standard aptX. Audio 'support' is bumped to 24-bit/48kHz, and the bitrate to 576kbps. It is supported by a large number of excellent headphones, such as the Sennheiser Momentum True Wireless 3 and the Focal Bathys. It's also found in Sony's older WH-1000XM3, but not the more recent XM4 and XM5 (which use Sony's own LDAC codec instead).
aptX HD was followed in 2018 by aptX Adaptive, which combines both aptX HD and aptX Low Latency (see below). By taking into account the number and intensity of radio frequencies in your surroundings, aptX Adaptive can minimise drop-outs and optimise the audio depending on whether you're on a call or listening to music.
aptX Adaptive can adjust in real-time, dynamically scaling the bitrate to adapt and adjust quality. It scales between 279kbps and 420kbps for CD and hi-res quality music. While this might look lower than aptX and aptX HD, aptX Adaptive is much more efficient than either, making for better claimed sound quality. It can wirelessly transmit 24-bit/96kHz files too – which was a first for the aptX codec at that time.
aptX Low Latency
This codec boasts audio and video syncing with less than 40 milliseconds of latency when you’re watching a video or playing a game on your connected device. It transmits music at 16-bit/44.1kHz with a bitrate of 352kbps. aptX Low Latency is less common now, as it's essentially been folded into aptX Adaptive.
Qualcomm's latest baby is aptX Lossless. This latest codec 'supports' 96kHz (with transmission bitrate scaling dynamically from 279kbps up to 860kbps), but the headline feature here is also lossless transmission at CD quality (16-bit/44.1kHz) over Classic Bluetooth and 48kHz over LE (Low Energy) Bluetooth – both unprecedented feats within the Bluetooth audio world previously. The codec's bitrate can scale between 1.1mbps and 1.2mbps (1100 and 1200kbps) to achieve this, which is just lower than a CD-quality file's 1411kbps but is apparently up to the task. Qualcomm says that "no data is lost when audio is encoded and decoded with aptX Lossless".
Like Samsung, Sony has decided to plough its own furrow to create its own Bluetooth codec. LDAC handles hi-res audio up to 32-bit/96kHz over Bluetooth at up to 990kbps – pretty good on paper compared with all others but aptX Lossless. According to Sony, LDAC allows approximately three times more data to be streamed over Bluetooth because of the use of more efficient coding and "optimised packetization" of the data.
The tech isn't limited to Sony devices, though they are more likely to support it. Compatible devices include the Award-winning Sony WH-1000XM4 and XM5, the similarly Award-winning Sony WH-1000XM5, Fiio M11 Pro portable music player, Technics EAH-A800 over-ear headphones, iFi Zen One Signature DAC, and Sony HT-ST5000 soundbar.
LHDC stands for Low Latency and High Definition Audio Codec (we know, the initialism doesn't really work, but LLHDAC doesn't exactly roll off the tongue). LHDC was created by Savitech as an alternative to LDAC, and delivers 24-bit/192KHz audio quality over Bluetooth at up to 1000kbps. That's similar to LDAC, but LHDC actually performs better in terms of lowlatency audio. It's being marketed as "the next generation standard for high quality Bluetooth", and has been around for a while – the first smartphone to support it was the Huawei mate 10 from 2017.
LC3 (Low Complexity Communications Codec) was introduced in 2020 as part of Bluetooth Special Interest Group's LE (Low Energy) Audio standard, and is set to represent the next generation of Bluetooth audio technology. It's mostly about improving transmission efficiency and so promises low bitrates for easier transmission while retaining 'decent' sound quality: 32-bit/48kHz audio is supported, at a 345kbps bitrate. Because it is more power efficient, LC3-supporting headphones should be able to boast better battery lives.
The first generation of devices that support LC3 has launched. But because both the source device and what you listen through will need to support LC3, it's a way off being a mainstream proposition.
|Codec||Max bitrate||Max bit depth||Max sample rate|
What's the future of wireless audio?
Bluetooth only has limited capabilities. As you will hopefully understand, only now has Bluetooth been able to come close to transmitting CD-quality audio losslessly. Lossless Hi-res audio? No chance. That tube simply doesn't go wide enough. But where Bluetooth falls short, other wireless technologies are waiting to take up the baton.
UWB stands for Ultra Wide Band. It uses less power and can deliver a higher bitrate than Bluetooth. Most excitingly of all, it's already widely available – every iPhone since the iPhone 11, the Google Pixel 7 Pro and 8 Pro, and a host of Samsung Galaxys feature UWB. But they use UWB for accurate location finding rather than wireless audio. That's because UWB can time how long signals take to get places, giving actual distances between devices, something Bluetooth can't work out.
The main hurdle preventing UWB from being used for wireless audio is body blocking, where the wearer's body blocks the wireless signal, causing a drop-out. AntennaWare's BodyWave technology claims to solve this to allow it to be used in personal devices, one of which will be the newly announced PSB headphones due in 2024. Watch this space...
The other emerging technology is SCL6. This codec is made by MQA and works across Bluetooth, UWB and wi-fi. What's exciting about SCL6 is that it's adaptable – able to react to the quality of the link between the sending device and the receiver. It can vary in data rate from 200kbps (lossy) all the way up to 20mbps (lossless) depending on the bandwidth of the connection.
Other codecs (such as aptX Adaptive and Samsung Scalable Codec) can adapt in real-time, but SCL6 is the only one to switch between lossless and lossy operations. At the top end, it is able to losslessly carry 24-bit/384kHz PCM files. Those higher mbps transfers will be over wi-fi and UWB, mind you – not Bluetooth, unless it comes a very long way from where it is now.
SCL6 also claims to optimise the time-domain aspects of the signal, rather than prioritising the frequency domain, like most compression codecs. The claim is of a better, more accurate sound than the alternatives, with fewer audible artefacts. It's not power-hungry either and so shouldn't put too much of a battery strain on wireless devices using it.
Most excitingly of all, SCL6 doesn't have specific hardware requirements. As long as a device has the processing power, SCL6 could be activated with a firmware upgrade. MQA is in talks with a number of potential partners, including phone companies. Definitely one to watch...
Read our SCL6 hands-on review
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