Bluetooth audio protocol introduction-APTX-HD, APTX-LL and LDAC

- Apr 19, 2020-

Bluetooth high-quality audio transmission is mainly based on the A2DP profile. A2DP simply defines the protocol and process for transmitting high-quality audio information such as mono or stereo on an asynchronous connectionless channel. This protocol is similar to the audio data transmission pipeline. The data transmitted via Bluetooth is divided into the following types according to its encoding format:

 

SBC: This is the standard encoding format for Bluetooth audio. A2DP (Advanced Audio Distribution Profile) protocol mandatory coding format. The maximum allowable rate is 320kbit / s in mono and 512kbit / s in two channels. All Bluetooth audio chips will also support this audio encoding format.

 

AAC: The technology provided by Dolby Laboratories, it’s a high compression ratio encoding algorithm. iPhone use AAC format for Bluetooth transmission. At present, Apple's Bluetooth audio devices basically use AAC encoding technology. And many receiving devices such as Bluetooth speakers / headphones on the market also support AAC decoding.

 

APTX: It is CSR's patented coding algorithm. After it was acquired by Qualcomm, it became its main coding technology. It is claimed in the publicity that it can achieve CD sound quality. Most new Android phones are equipped with APTX. This audio coding technology is more efficient than classical Bluetooth coding, and the listening feel is better than the previous two. Devices that use APTX technology need to apply for authorization from Qualcomm and pay an authorization cost, and they need to be supported by both the transmitting and receiving ends.

 

APTX-HD: aptX HD is high-definition audio, and the sound quality is almost the same as LDAC. It is based on the classic aptX, which adds channels to support 24 bit 48KHz audio format. The benefits of this are lower signal-to-noise ratio and less distortion. At the same time, the transmission rate is of course greatly increased.

 

APTX-LL: aptX LL is low-latency, the main feature is that it can achieve a latency of less than 40ms. We know that the latency limit that people can feel is 70ms, and reaching 40ms means we can't feel the delay.

 

LDAC: This is an audio coding technology developed by SONY, which can transmit high-resolution (Hi-Res) audio content. This technology can transmit about three times as much as other coding technologies through efficient coding and optimized sub-packaging The data. At present, this technology is only used in SONY's own transmitting and receiving equipment. Therefore, only the SONY set of transmitting and receiving equipment that supports LDAC audio coding technology can be purchased to support LDAC-encoded Bluetooth audio data transmission.