FFmpeg 9.1's new AAC encoder

Original Article ↗ Hacker News Discussion ↗

Link and thread context

  • Original HN submission had a bad URL; it was later corrected after moderator intervention.
  • Some meta-discussion about how to contact HN mods and occasional email black holes.

Technical changes in FFmpeg’s AAC encoder

  • New encoder works around a long‑standing stereo PNS decoding bug in FFmpeg and possibly other decoders.
  • Prior behavior combined PNS with temporal and stereo tools in ways that broke noise shaping and stereo imaging; the new approach only uses PNS when both channels are sufficiently noise‑like.
  • Encoder tuning is focused on 48 kHz; 44.1 kHz and 96 kHz work but are not primary optimization targets.

Audio quality, testing, and comparisons

  • Users report FFmpeg’s old AAC encoder produced audible artifacts and was clearly worse than Apple’s Core Audio and FDK AAC, sometimes even worse than high‑bitrate MP3.
  • Benchmarks in the linked forum show the new encoder outperforming Apple’s AAC in some metrics, at least under the tested conditions.
  • Some emphasize that such metrics are only proxies; real validation still requires ABX/MUSHRA tests with listeners.
  • One commenter notes the encoder is CBR‑centric; others clarify that a quality‑based VBR mode exists but doesn’t hit the absolute metric peak.

Sample rate debates (48 kHz vs 44.1 kHz and higher)

  • Strong view: 48 kHz is the present‑day “standard” (video, streaming, OS defaults, Opus design), and resampling is cheap.
  • Counterview: most legacy music (CDs) is 44.1 kHz; lack of explicit 44.1‑kHz tuning is a practical concern for serious pipelines.
  • Extended subthread on whether higher sample rates (>48 kHz) matter beyond specialized production and scientific use; consensus leans toward little benefit for end‑user listening.

AAC vs Opus and other codecs

  • Many see the thread as a showcase for Opus, which is said to beat AAC at low bitrates and remain highly competitive at higher ones, with lower latency.
  • Others stress AAC’s entrenched role: RTMP live streaming (YouTube, Twitch), HLS, car stereos, older devices, and widespread hardware support.
  • Discussion of Opus downsides: weaker support in some consumer tools, higher CPU than Vorbis in game audio, and concerns about spec text complicating public‑domain implementations.

Adoption, compatibility, and tooling

  • Several people currently rely on Apple’s AAC or FDK AAC encoders and hope the new FFmpeg encoder will be “good enough” to drop external dependencies.
  • Some highlight that for archival or editing they still prefer lossless (FLAC/ALAC) or Opus, but AAC remains the pragmatic choice where compatibility rules.

Reactions and reservations

  • General enthusiasm for a high‑quality native AAC encoder in FFmpeg and appreciation for the detailed write‑up.
  • Skepticism centers on: CBR‑first design, 48‑kHz‑centric tuning, and uncertainty about performance at 44.1 kHz and in VBR/TVBR workflows, especially for CD‑derived content.