What's the difference between m4a and aac? Which sound quality is better,

Ps parametric stereo, similar to sbr, uses a certain bit stream to record the limited difference between one channel and another, thus saving the bit stream and improving the sound quality at the same bit rate. These are all patented technologies of AAC. It seems that the audio files of many mobile music softwares listening to songs online are all sbr ps with aac over 20 kbps, which is a very good supplement: when both technologies need to be encoded, enable it to encode.