[cclements]

What this fixes for Libation users

Audiobooks downloaded with Request xHE-AAC codec enabled currently can't be seeked in Apple
Books or any iPhone/iPad/Mac app built on Apple's audio stack: resume, chapter jumps, and
scrubbing either fail outright or play audio from the wrong position, and chapter-split files
often won't even start playback without rewind-and-retry fiddling. (Sequential play works, which
is why the files seem fine at first.) ffmpeg-based players tolerate the files, which hid this
for a long time.

Root cause — confirmed by Apple engineering in response to a Feedback report — is on the
container side: USAC (xHE-AAC) requires a sync sample table (stss) enumerating its seekable
frames (ISO/IEC 23003-3 §H.1), and the remuxed files didn't have one, so Apple's demuxer seeks
to frames that aren't valid decode entry points. Mbucari/AAXClean#18 makes AAXClean write a
correct stss (derived from the USAC bitstream itself), and Mbucari/AAXClean#19 makes
chapter-split output sample-accurate via edit lists, with each split part starting on a
decodable frame. Both are merged and released in AAXClean.Codecs 3.1.0 (AAXClean 3.1.0
transitively).

Validation

• Before/after on a real Audible xHE-AAC title (Apple decoder, 31 seek probes at every chapter
  start plus known-bad offsets): 23 failures → 0, with the audio payload MD5-identical
  (the remux stays lossless).
• A 220-book library re-downloaded through a Libation build using these changes: 195 xHE-AAC
  files, every one verified correct (spot-checked bit-exact against the bitstream on titles
  from 8 minutes to 72 hours); 403/403 Apple seek probes passed.
• Libation's full test suite passes against the released 3.1.0 packages (966 tests, 0 failures,
  21 environment-dependent skips). The end-to-end runs above — including the complete macOS
  bundle build and a live liberation on macOS (arm64) — were done on pre-release builds of the
  same changes.
• AAC-LC output is unchanged.

Users' existing xHE-AAC files can be repaired without re-downloading — any lossless
re-conversion through the updated AAXClean adds the missing table in place.

Details, measurements, and Apple's response are in Mbucari/AAXClean#18 and #19.

🤖 Generated with Claude Code

[rmcrackan]

I've been casually following your conversation with @Mbucari . Thanks for helping out his project and mine.