ffmpeg's CLI handles Matroska priming differently by version: 9.x trims
CodecDelay, 2023-24 static builds keep the priming samples (AAC 1024,
AC-3 / E-AC-3 256, MP3 1105), so a +-40-sample best-lag search against
its decode landed on a spurious minimum (4 or 24 samples).
matroska_audio_oracle now has ffmpeg encode a reference signal we
generate (low-passed xorshift noise) and asserts our decode lines up
with that signal to the sample, the stream's own CodecDelay trimmed:
version-independent, and it fails on the old ms-rounded trim (-16).
ffmpeg's decode is compared after a best-lag search bounded by the
CodecDelay; its lag is reported, the waveform must match (< 1e-3).
E-AC-3 joins the cases. eac3_oracle's .mkv case accepts lag 0 or the
256-sample priming; MP4 / MOV / TS stay strictly at lag 0.
Verified with ffmpeg 9.0.2 and the static N-71064 (2024-06) build.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Problem: files with E-AC-3 audio (MP4 ec-3, MOV, Matroska A_EAC3, MPEG-TS)
imported silent: filmcraft-ac3 refused bsid 11-16.
Fix: clean-room E-AC-3 decoding from ATSC A/52:2012 Annex E in filmcraft-ac3:
bsi and audfrm syntax, 1/2/3/6-block syncframes, reduced sample rates,
frame-based exponent strategies (exponents reused across short syncframes),
SNR offset strategies, default band structures, spectral extension
(translation, notch filter, noise blending). Independent substream 0 is
decoded; dependent substreams and other programs are skipped (7.1 plays as
its 5.1 core). AHT and enhanced coupling are refused as unsupported. The
audio blocks share the AC-3 path; AC-3 output is unchanged. filmcraft-codecs
routes E-AC-3 in MP4/MOV, Matroska and TS to the decoder; the TS splitter
joins dependent substream frames to their access unit.
Tests: ffmpeg oracle for raw E-AC-3 (64.8-91.0 dB SNR) and in MP4, MOV,
Matroska and TS (79.5-91.3 dB, random access identical), mutation fuzz,
synthetic frames for headers, substream skipping, refusals and exponent
reuse across 1-block syncframes, spectral extension unit tests.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
