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#809: own MPEG audio Layer I / II decoder plays MP2 at ffmpeg's default bit rates

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Problem: symphonia rejects every Layer II frame whose bit rate / mode
combination ISO/IEC 11172-3 does not allow (mono above 192 kbit/s, stereo
below 64 kbit/s). ffmpeg writes mono MP2 at 384 kbit/s by default, so such
tracks played as silence in MPEG-TS / PS, Matroska and raw .mp2 files.

Fix: new L0 crate filmcraft-mpa, a clean-room Layer I / II decoder written
from ISO/IEC 11172-3 and 13818-3 (all modes incl. joint stereo, MPEG-1/2/2.5
rates, the disallowed combinations decoded with the allocation table their
bit rate per channel selects). PacketDecoder::mpeg_audio uses it for layers
I and II (packets of several frames too); raw .mp2 files decode with it,
resyncing after damage. Oracle tests vs ffmpeg: 73-84 dB SNR on ffmpeg /
twolame streams and synthetic Layer I / joint stereo frames; fuzzed damaged
frames never panic.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
vangel76
2026-10-11 00:48:45 +02:00
co-authored by Claude Opus 5.5
parent 120883d41c
commit 39afd2f6c3
15 changed files with 1619 additions and 8 deletions
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docs/architecture.md
+2 -1
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@@ -29,7 +29,7 @@ Design principles:
L2 edit · codecs · interchange · captions · speech · tts · tts-text
L1 frame · media · project · audio-dsp · text
L0 foundation: time · geom · color · bitstream · testkit (dev-dependency only)
codecs/containers: isobmff · matroska · mxf · cfb · mpegts · ogg · h264 · h264enc · hevc · vp9 · av1 · mpeg2v · prores · dnx · apv · aac · ac3 · opus
codecs/containers: isobmff · matroska · mxf · cfb · mpegts · ogg · h264 · h264enc · hevc · vp9 · av1 · mpeg2v · prores · dnx · apv · aac · ac3 · mpa · opus
```
Crates are named `filmcraft-<dir>` (`crates/time` is `filmcraft-time`). The apps are `filmcraft`
@@ -56,6 +56,7 @@ and `filmcraft-cli`.
| `mpeg2v` | L0 | MPEG-2 video (Main / 4:2:2 profile; frame and field pictures, dual prime) and MPEG-1 video decoder ([README](../crates/mpeg2v/README.md)) |
| `aac` | L0 | AAC-LC decoder and encoder |
| `ac3` | L0 | AC-3 (ATSC A/52) decoder ([README](../crates/ac3/README.md)) |
| `mpa` | L0 | MPEG-1/2 Audio Layer I and II (MP1 / MP2) decoder ([README](../crates/mpa/README.md)) |
| `testkit` | L0 | test-only helpers, used only as a dev-dependency: ffmpeg/ffprobe discovery, fixture dirs, golden images ([testing.md](testing.md)) |
| `frame` | L1 | `VideoFrame` (planar YUV / RGBA8 / linear RGBA f32, colour metadata), `AudioBuffer` |
| `media` | L1 | `MediaSource` trait, probing/openers, frame cache, generators, stills, image sequences, WAV / Broadcast WAV |