How Wireless Earbuds Send Music: SBC, AAC and LC3
Wireless earbuds compress music, send it over Bluetooth radio and decode it again. The codec in the middle, such as SBC, AAC or LC3, shapes quality and battery life.
In this article
Wireless earbuds receive music as compressed digital data over a 2.4 GHz Bluetooth radio link, then decode it into sound. The method of compression is called a codec. Every device that streams music over the common Bluetooth profile must support one codec, called SBC, and many support better ones. This article explains the chain and the main codecs.
The radio link
The Bluetooth Special Interest Group describes two radio types, Classic and Low Energy (LE). Both work in the 2.4 GHz band, between 2.402 and 2.480 GHz, and both hop between frequencies to reduce interference. Transmit power is capped at 100 milliwatts, subject to local rules.
Classic uses 79 channels one megahertz apart and is described as mainly used for point-to-point links such as audio to speakers and headphones. LE uses 40 channels two megahertz apart and is designed for very low power.
The profile and the mandatory codec
A profile is an agreed set of rules for one job. Wikipedia explains that the Advanced Audio Distribution Profile (A2DP) streams audio from a phone or laptop to a headset, in up to two-channel stereo, one way at a time. It requires support for the SBC codec. SBC is described as low-complexity with modest bit rates.
Because every A2DP device supports SBC, two devices that share no other codec can still connect. That is the usual fallback.
What a codec does
Raw digital music is too large to send comfortably over a small radio link, so the phone compresses it. The earbud then decompresses it and a converter turns the numbers into the electrical signal that drives its speaker. This compression is lossy, meaning some detail is thrown away in ways designed to be hard to hear.
Wikipedia says SBC was designed for reasonably good audio quality at medium bit rates with low computational complexity. Decoders must handle up to 512 kilobits per second for stereo, and the High Quality setting that most operating systems use by default runs at about 328 to 345 kilobits per second.
Beyond SBC
A2DP also allows optional codecs. The Wikipedia list includes MP3, AAC and HE-AAC, and manufacturer-defined codecs such as aptX and LDAC. AAC, short for Advanced Audio Coding, was developed by Dolby, AT&T, Fraunhofer and Sony as a successor to MP3. Wikipedia says it generally gives higher sound quality than MP3 at the same bit rate and that tests found it transparent at 128 kilobits per second for stereo.
Note that this is a general statement about AAC, not a measurement of any earbud. Whether an earbud sounds better with one codec than another also depends on its drivers, its fit and the music. Both the phone and the earbuds must support the same optional codec for it to be used.
- SBC: required for A2DP, works everywhere
- AAC: optional, widely used for stored and streamed music
- aptX and LDAC: optional, defined by manufacturers
- LC3: the codec of LE Audio
LE Audio and LC3
LE Audio is the newer system, running on the Low Energy radio instead of the Classic one. It uses LC3, the Low Complexity Communications Codec, which Wikipedia says was developed by Fraunhofer IIS and Ericsson and is royalty-free. The Bluetooth SIG's page says listening tests show quality gains over SBC, with LC3 delivering better quality at a 50 percent lower bit rate. The company says developers can use the saving for longer battery life or a smaller battery. Wikipedia mentions that some results against the Opus codec are disputed.
LE Audio also adds multi-stream audio, which sends separate synchronised streams from a source to one or more devices, for example one stream to each earbud. It adds support for hearing aids too. A feature called Auracast lets one source broadcast audio to an unlimited number of listening devices, for uses such as TVs in public places.
What this means in practice
The path is therefore the same every time: the phone's player decodes a music file, the Bluetooth stack compresses it with the codec both devices agreed on, the radio sends it in small packets, and the earbud decodes it. Where two devices share only SBC, that is what they use. Where both support something newer, they can choose it when they connect.
This is general information. Actual sound, delay and battery life vary with the product, the phone and the surroundings, and the pages checked here do not give delay figures for the codecs.
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