Wireless signal transmission via Bluetooth is convenient. Music can be transmitted from a smartphone or television to wireless headphones, for example. The bandwidth of Bluetooth was originally very limited. Since the Bluetooth standard is backwards compatible, this must be taken into account despite the new possibilities. The audio signal is always compressed for transmission and reassembled at the receiver. It quickly becomes clear that the sound quality does not improve. The basis for Bluetooth audio transmissions is the A2DP profile (Advanced Audio Distribution Profile) and the SBC codec (Low Complexity Subband Codec). This is the common denominator of all Bluetooth audio devices. This method has low hardware requirements, but the music quality is okay. However, the conversion causes delays between the audio source and receiver. If, for example, TV sound is played back on the TV and Bluetooth speakers, a clear echo is the result. This so-called latency cannot be completely eliminated, but it can be reduced depending on the codec.
aptX
aptX is a special codec that allows a higher data rate of up to 384 kbps and better sound quality. The process was invented for digital telephony and later developed further for professional audio transmissions. If the signal source and the receiver are connected via Bluetooth and both can use aptX, the result is a significant increase in quality. Many smartphones and tablets, but also the operating systems Windows 10 and Mac OS X, support aptX. iPhone and iPad, however, do not.
aptX Low Latency
aptX Low Latency is a further development and is used when the sender and receiver support this codec. Audio quality is about the same as aptX. The data rate is 352 kbps. The additional advantage here is the low latency. Latency is the delay caused by the transmission. It takes a few milliseconds to hear the sound from the transmitter. If there are loudspeakers on the transmitter and receiver, this is perceived as reverberation or echo. aptX Low Latency limits latency to under 40 ms, which is also the value recommended for audio-video applications. With conventional transmission with SBC codec, the latency is 150 ms (+/- 50 ms). When the audio is streamed over Bluetooth with SBC, it is not lip-synching with the TV picture. It is also irritating with computer games when the game noises are transmitted with a delay. Therefore, aptX Low Latency is ideal for time-critical applications such as video, gaming and (to a certain extent) making music. If you want to use Bluetooth for this, you should check whether both devices support the aptX LL standard.
aptX is a technology from Qualcomm (formerly called CSR).
aptX HD
This standard delivers even better audio quality with a data rate of up to 576 kbps. The latency is relatively high at 200 ms. The typical application is therefore music transmission, where time delay is not an issue.
