APTX and LDAC are both coding protocols or algorithms. The original coding protocol is called SBC (Subband Coding), which is mandatory for all Bluetooth and can be said to be the most basic coding protocol.
Until ACC (Advanced Audio Coding) appeared. Relatively speaking, this kind of ACC coding is a coding algorithm with high compression ratio, which can obtain better listening and sound quality at the same bit rate.
APTX was originally the coding algorithm patent of CSR company, and was later acquired by Qualcomm. Therefore, when the smart phones on the Qualcomm platform appeared on a large scale, this coding algorithm was also widely popularized in Qualcomm, and the listening sense of this algorithm was further delayed and reduced than the first two.