Explainers
Bluetooth codecs and latency: SBC, AAC, aptX, LDAC, LC3
Bluetooth does not have the bandwidth to carry uncompressed sound. It therefore needs a codec, a process that lightens the signal before it is sent. Manufacturers make a selling point of it; the reality is more nuanced, and the codec is not always where quality is won or lost.
Why compression is needed
The music on a CD amounts to 1,411 kilobits per second. A classic Bluetooth audio link offers, in good conditions, a few hundred kilobits, and far less in a Tube carriage saturated with radio signals. A codec (coder-decoder) reduces the bitrate by discarding the information the ear perceives least, following the principles of psychoacoustics: a quiet sound masked by a loud one nearby, for example, is not transmitted.
The phone encodes, the headphones decode. The two devices must therefore speak the same codec. On connecting, they choose the best one they have in common; failing that, they fall back on SBC, which every Bluetooth audio device is required to support.
The codecs in one table
| Codec | Maximum bitrate | Compatibility | Worth knowing |
|---|---|---|---|
| SBC | ≈ 328 kbit/s | Universal | Decent at full bitrate, often throttled |
| AAC | ≈ 256 kbit/s | Apple, most Android phones | Excellent on iPhone, uneven on Android |
| aptX | 352 kbit/s | Android, Windows | In practice, equivalent to a good SBC |
| aptX HD | 576 kbit/s | Android | Up to 24-bit / 48 kHz |
| aptX Adaptive | 279 – 420 kbit/s, variable | Recent Android phones (Qualcomm chips) | Adapts to radio conditions, low latency |
| aptX Lossless | ≈ 1 Mbit/s | A few recent Android phones | Lossless CD quality if the signal allows |
| LDAC | 330 / 660 / 990 kbit/s | Android | Up to 24-bit / 96 kHz, sensitive to interference |
| LC3 | Variable | LE Audio devices | Better than SBC at the same bitrate, energy-efficient |
Can you hear it?
Less than the spec sheets suggest. Blind listening tests have shown as much for a long time: well-encoded AAC at 256 kbit/s is indistinguishable from the original for the vast majority of listeners, on the vast majority of music. SBC at full bitrate is not far behind.
Audible differences arise mainly in three situations:
- A collapsing bitrate. When the radio signal is poor, the codec reduces its bitrate. LDAC that has dropped to 330 kbit/s does no better than AAC.
- A mediocre encoder. The quality of AAC depends heavily on its implementation: remarkable at Apple, variable from brand to brand on Android.
- Double compression. Streamed music is already compressed; the Bluetooth codec compresses it again. Two successive compressions add up, which argues for a good-quality source.
The real hierarchy is clear: the headphones (their drivers, their tuning, their fit) weigh infinitely more than the codec. Better excellent headphones over AAC than average headphones over LDAC.
Which codec with which phone
- iPhone, iPad: AAC, and nothing better. aptX and LDAC are of no use in this ecosystem.
- Android with a Qualcomm chip: aptX and its variants, plus LDAC on most models.
- Other Android phones: SBC, AAC and generally LDAC, built into the system since Android 8.
- Windows: SBC, AAC and aptX, depending on the version and the adapter.
On Android, the developer options show the codec actually in use: it is the only way to check that the promised “high resolution” really is active.
Latency, where the codec really matters
Encoding, transmitting, buffering and decoding all take time. This delay goes unnoticed when you are listening to music. For video, apps compensate by delaying the picture. When you are gaming or playing an instrument, nothing can compensate for it.
SBC and AAC commonly run at 150 to 250 ms. aptX Adaptive and LC3 come down to around 50 to 90 ms in their low-latency mode. Only proprietary 2.4 GHz links and the cable dip below 30 ms. The consequences for gaming are set out in sound in gaming.
LE Audio, LC3 and Auracast: the new generation
The “classic” Bluetooth used for audio dates from the early 2000s. Its successor, LE Audio, is built on the low-energy variant of Bluetooth and on a new mandatory codec, LC3. What it brings:
- Better quality at the same bitrate, and so more battery life for the same quality, or the reverse.
- A good-quality mic while you listen. Classic Bluetooth switches to a telephone mode, mono and muffled, as soon as the mic comes on; LE Audio maintains decent sound in both directions.
- Auracast: broadcasting a single stream to an unlimited number of receivers. Sharing your music with a friend, or listening to a gym's television or an airport's announcements through your own headphones.
- Support for hearing aids on the same footing as ordinary earbuds.
LE Audio is becoming widespread on recent phones and headphones, but requires both devices to support it. In the meantime, classic Bluetooth remains the common denominator.
What matters more than the codec
- Multipoint: staying connected to your computer and your phone at the same time. Invaluable when working from home.
- Link stability: a modest codec that never drops out is better than an ambitious one that makes the sound stutter.
- The headphones' internal processing: equalisation, noise cancelling and amplification weigh more on the result than the way the signal is carried.
- A wired input: via 3.5 mm jack or USB-C, it sidesteps the whole question, and saves a journey when the battery is flat.
Sources and references
Frequently Asked Questions
What is the best Bluetooth codec?
On paper, LDAC at 990 kbit/s and aptX Lossless offer the highest bitrates. In practice, AAC on iPhone and aptX Adaptive or LDAC on Android all give excellent results. The best codec is the one that both your phone and your headphones support and that stays stable where you listen.
Does the iPhone support aptX or LDAC?
No. iPhones and iPads use only the SBC and AAC codecs. Headphones that support aptX or LDAC will work with an iPhone, but over AAC. Apple's AAC is very well implemented and is enough for high-quality listening.
Does Bluetooth degrade sound quality?
It compresses the sound, with a theoretical loss. With a modern codec at a decent bitrate, that loss is inaudible to most listeners under normal conditions. The quality of the headphones, their tuning and their fit influence the result far more than Bluetooth compression.