tl;dr: FFmpeg developer Lynne rewrote the native AAC encoder from scratch, overhauling rate control, RDO, and all coding tools (PNS, TNS, I/S, M/S), with benchmarks (Zimtohrli, ViSQOL) showing it outperforms qaac and fdk-aac at most bitrates, though still trailing Opus. The encoder is CBR-only, optimized for 48kHz, and works around a stereo PNS bug present in FFmpeg's decoder. Early user tests confirm strong quality at 128kbps+, though fdk-aac still edges it out at low bitrates (~64kbps), and some testers reported TNS-related ticking artifacts on specific samples.