In principle, audio can be transmitted via Bluetooth in various ways. One of the oldest protocols that is still in use is the Headset Profile (HSP), which allows 64 kbit mono sound. The extensions in the Hands-Free Profile (HFP) allow for more control and slightly improved mono sound for better voice quality during phone calls. Both protocols were actually used for music playback at the beginning, but of course they are not suitable for demanding audio transmission.
This is where the Advanced Audio Distribution Profile (A2DP) comes into play. This enables the transmission of audio signals via Bluetooth with a significantly higher bandwidth. Any A2DP-enabled device (both transmitter and receiver) can handle the Subband Codec (SBC), which is capable of transmitting stereo audio at up to 345 kbit/s. SBC was developed with the goal of low resource consumption, so the codec is not highly complex and cannot compress music very efficiently. In addition, it is designed in such a way that the bandwidth is dynamically adapted to the circumstances, i.e. if reception is poor, the bandwidth is cut and the sound quality suffers.
The bad reputation of Bluetooth audio stems from the fact that poor transmission devices, i.e. playback devices that provide little power for the Bluetooth module (both sides send and receive in a Bluetooth connection) in connection with SBC usually lead to a very low audio signal bandwidth. A remedy is already provided in A2DP, because the protocol also supports the direct transmission of other audio codecs – such as mp3, which is now license-free. In principle, the transmission of lossless codecs with A2DP is also possible.
Unfortunately, the industry didn’t get involved with these formats – and so two other Bluetooth formats have prevailed: AAC and aptX.
Both (in contrast to SBC) incur license fees, so that a manufacturer may think twice about which formats he supports. The crux: For a successful transmission in one of these two formats, both the transmitter and the receiver must support the codecs. In connection with smartphones from the manufacturer Apple, for example, this can lead to aptX Bluetooth headphones designed for high quality falling back to SBC with an iPhone – these do not support aptX! Apple’s technical design papers speak of a bandwidth of just under 265 kbit/s AAC via A2DP, so that in principle the data available on the iPhone can be transmitted bit by bit at up to 256 kbit/s and only be converted in the playback device.
Anyhow, 265kbit AAC is already very high quality and probably difficult for many people to distinguish from the original. aptX provides even more bandwidth: the full 345 kbit/s can be used for the stereo signal. In contrast to SBC, the full bandwidth is always used, ie you either have a high-quality signal or none at all. In addition, the aptX codec is significantly more effective and can push more audio information into the bandwidth. With aptX-HD it has been further enhanced to enable up to 48 kHz/24 bit.
However, none of these options are true lossless transmissions and on the sending side, a digital/digital conversion from one codec (MP3, AAC, FLAC etc.) to another lossy codec such as AAC or aptX must first take place as well as in the Bluetooth receiver then finally its decoding into the analogue audio signal. Therefore, both Bluetooth devices must also support the standard used.
