FFmpeg 9.1 introduces a groundbreaking rewrite of its AAC encoder, reengineering rate control, RDO, and coding tools like PNS, TNS, I/S, and M/S. The update, merging today, replaces outdated implementations with a CBR-focused approach optimized for 48kHz audio. Benchmarks against qaac and fdk-aac show dramatic improvements in perceptual quality, measured via Zimtohrli and Vi SQOL metrics.

Users are advised to avoid `-q:a` for VBR and prioritize 48kHz resampling for peak performance, though 44.1kHz and 96kHz remain compatible. The encoder dynamically disables coding tools when they offer no benefit, ensuring efficiency without sacrificing quality. Lynne, FFmpeg contributor and USAC decoder author, highlighted the encoder’s focus on audible frequency bands, leaving non-essential ones unprocessed to enhance clarity.