A brief introduction to Bluetooth audio transmission technology

With the popularity of wireless headphones, Bluetooth audio is no longer synonymous with "listening for fun". Whether it is Sony LDAC's "faith sound quality" or Apple AirPods' "spatial audio", behind them is the continuous innovation of Bluetooth audio technology. The essence of Bluetooth audio is to wirelessly transmit sound from mobile phones, computers and other devices to headphones or speakers. This process seems simple, but in fact it needs to solve three major problems: How to compress sound? (audio encoding) → How to fight wireless interference? (transmission protocol) → How to make the sound play in real time? (delay optimization).

First, let's understand its basic transmission principle. Bluetooth audio transmission relies on "A2DP (Advanced Audio Distribution Protocol)" to build a basic link, which realizes two-way audio stream transmission through asynchronous connectionless link (ACL). On top of A2DP, "AVRCP (Audio and Video Remote Control Protocol)" is responsible for the interaction of playback control commands, such as play/pause, volume adjustment and other operations. This layered design ensures the separation of audio transmission and control commands, and improves the stability of the system.

Figure 1 Protocol Block Diagram

Bluetooth audio operates in the 2.4GHz ISM band and uses frequency hopping spread spectrum technology (FHSS) to switch channels 1,600 times per second to resist interference. Bluetooth 5.0+ improves transmission efficiency through enhanced ATT (eATT) and supports transmission in open environments up to 200 meters away, while LE Audio further reduces standby power consumption based on low-power Bluetooth (BLE).

Codecs are the "lifeline" of Bluetooth sound quality, and different solutions directly determine sound quality, latency, and compatibility. The following is the technical game of mainstream codecs:

 Latency is a major flaw of Bluetooth headsets, especially in gaming and video scenarios, where the asynchrony of audio and video directly affects the experience. Where does the latency come from?

lEncoding/decoding latency: LDAC takes a long time to process due to high bitrate (about 80ms), while aptX LL compresses the latency to less than 40ms by simplifying the algorithm.

lTransmission latency: The data packet transmission time of the Bluetooth link layer is about 10-20ms, and LE Audio is further optimized through the short frame structure (7.5ms/10ms) of LC3.

lBuffer latency: To avoid stuttering, devices usually set a 50-200ms buffer, which can be reduced to 30ms in gaming mode, but the anti-interference ability is reduced.

lHardware latency: The processing time of the headphone DAC digital-to-analog conversion and amplification circuit is about 20-50ms, and high-end chips (such as Qualcomm QCC5141) reduce latency by integrating DSP.

Based on the above delay characteristics, we can choose different solutions, such as: choose a gaming headset + dedicated transmitter, such as Razer, Logitech and other brands use 2.4GHz external transmitters to achieve 15ms ultra-low latency; or we can turn off the sound enhancement function, DSP processing (such as ambient sound mode) will increase the delay by 20-30ms. In the end, users still need to choose the appropriate codec and device according to their own scenarios (music, games, multiple devices), and release more potential of hardware through environmental optimization and firmware updates.