Bluetooth audio transmission is now systematically integrated in smartphones and tablets as well as most laptops. This is due to the fact that it is still the easiest way to stream music wirelessly, without needing to be connected to a local network. The stability of transmissions has improved greatly over time, and recent Bluetooth controllers are both economical and capable of high data flow (an important factor for sound quality).
How does Bluetooth transmission work?
Let’s take the example of a smartphone and a Bluetooth speaker. When both devices are paired (by detection or touch with an NFC chip), they connect and list their respective capabilities. As far as audio transmission is concerned, up to five compression modes are possible. The choice of transmission mode is (almost) never left up to the user and it is the Bluetooth chips that determine which to use. If the radio frequency conditions allow it (low interference, the proximity of the devices, possible Bluetooth keyboards or mouse devices connected to the source) the most high-quality mode is selected by default.
When transmission conditions are not optimal (in public transport for example, due to the proximity of other smartphones and Bluetooth headphones), the emitter and receiver adapt and change their mode of communication seamlessly. The aptX HD codec is dropped in favor of the next best codec in terms of quality.
Note: when you listen to Spotify, Deezer, Qobuz, Google Music or Apple music, the stream is automatically recompressed according to the codec selected by the emitting and receiving devices.
Which Bluetooth codec should you choose: LDAC, aptX, SBC or AAC?
Many Bluetooth codecs exist, the main ones being LDAC, aptX HD, aptX, AAC and SBC. These codecs make it easy to identify the maximum transfer rate possible for Bluetooth signals, meaning the best sound quality possible. Here are the characteristics of each of these Bluetooth codecs.
Sony’s LDAC is currently the highest quality Bluetooth codec available. It operates using the Bluetooth 4.0 standard to ensure that CD quality audio files (16-bit/44.1kHz) can be streamed losslessly. Transmission up to 24-bit/96kHz is possible, but still requires some compression. For a long time, the technology remained exclusive to the Japanese brand and could only be used to connect Sony products. Fortunately, other brands have had access to this codec for a few years now. It is featured on many Bluetooth devices such as the Technics F70, Technics F50 and Sony WH-1000XM3 headphones, as well as certain DAPs, like the Shanling M0, the FiiO M6, the Shanling M2X and the Sony NW-ZX300.
Note: as with any other Bluetooth codec, in order to benefit from LDAC audio transmission the receiver (speaker, amp, etc.) and the emitter (computer, smartphone, tablet) must both be compatible with LDAC.
aptX et aptX HD
Now rather widespread, aptX codecs provide high quality transmission, with a bitrate of 576 kb/s for aptX HD and 350 kb/s for aptX. The method used to compress sound is lossy (information is erased), but the restitution is still very good. Qualcomm, the company that developed the aptX HD codec and manufactures Bluetooth aptX HD chips, claims the quality is close to that of a CD. These codecs ensure a wide frequency response and a spacious soundstage.
If you own an iPhone, an iPod touch or an iPad, you won’t be able to use the aptX codec as Apple has chosen a different Bluetooth controller supplier than Qualcomm (the only manufacturer to commercialize aptX chips) for its devices. As a result, the aptX codec has been disregarded for the time being. Apple has opted for AAC audio compression, which is also the format used on iTunes and Apple Music.
The difference in quality between aptX and AAC is minor, and if you listen to music via iTunes in AAC format, no recompression is applied.
The Sub Band Codec (SBC) uses a compression algorithm not unlike Microsoft’s WMA. When radio conditions are favorable, the bitrate is 350 kb/s with decent sound quality. However, when the bitrate drops to 128 kb/s, sonic artifacts are audible in high frequencies. If you have ever had the impression that your Bluetooth headphones were hissing, you weren’t just imagining it. The SBC codec was to blame.
Note: it is obligatory for manufacturers to integrate the SBC codec. Therefore, when communication in LDAC, aptX or AAC isn’t possible, the emitter and receiver “fall back on” the SBC codec.
High-quality Bluetooth listening experience
If you are looking for a lossless (without loss of information) Bluetooth transmission, you must choose devices that are LDAC compatible, such as a Sony DAP or smartphone, and your Bluetooth headphones, Bluetooth speakers or other receiving devices must also be compatible with LDAC.
If you wish to have a wider choice of compatible equipment, it is better to opt for a source and transmitter compatible with aptX or aptX HD. You can find aptX HD Bluetooth emitters for computers in the form of micro USB flash drives. Many smartphones (Samsung, Motorola, etc.) and most DAPs are compatible.
If you own an iPhone, we recommend AAC compatible Bluetooth audio devices. This is the case for many DENON amplifiers, for example.
Lastly, if after checking your smartphone’s manual you discover that it only has a SBC Bluetooth transmitter, know that some headphones and speakers feature settings that force the smartphone to stream music with a 350 kb/s bitrate.