glint v0.8.0 — the AAC release

glint v0.8.0 — the AAC release

23 commits since v0.7.0, all in three days. glint is no longer an MP3
encoder: it is a dual-codec (MP3 + AAC-LC), MIT-licensed, clean-room
encoder whose AAC side places behind only Apple and Fraunhofer FDK
in quality at 128 kbps — while needing 47.4 KB of RAM and encoding
with no per-coefficient floating point on FPU-less microcontrollers.
Every claim below is measured; the full experiment log including every
dead end lives in PLAN.md (§A0–A6).

To our knowledge this makes glint the only competitive-quality AAC-LC
encoder under a plain OSI license: ffmpeg's native encoder is LGPL (and
behind on quality), FDK's license is not OSI-approved and disclaims
patent grants, and vo-aacenc (Apache-2.0) is unmaintained and places
last on every clip in our league.

The AAC-LC encoder

Written from the ISO 13818-7 / 14496-3 specs in three days of measured,
gated passes:

• Wire format: all four window sequences (short blocks with
  attack-split grouping), per-band M/S stereo, selective TNS,
  optimal-sectioning Huffman (per-band codebook DP with exact bit
  accounting — the count==emission identity is unit-tested), CBR with a
  bit-debt controller, constant-quality VBR (V0–V9), ADTS output, all
  12 standard sample rates, mono/stereo.
• Normative tables extracted from two independent implementations
  (vo-aacenc and ffmpeg), cross-checked bit-for-bit and verified
  prefix-free by the generator (tools/gen_aac_tables.py).
• Psychoacoustics: metric-aligned Bark masks with tonality-aware
  offsets at low rates, and a distortion-controlled allocator —
  per-band scalefactors in closed form from noise targets
  mask^0.6 · k, with the loudness knob bisected to the bit budget.
  Short frames get per-group masks and their own allocation tilt.
• Validated by two independent decoders: every configuration
  decodes with zero errors in both ffmpeg and Apple CoreAudio, which
  produce metrically identical output. A decode-based ctest gate
  covers 8 configurations including a transient torture case and VBR.

Quality (measured league, 2026-07-06)

128 kbps stereo, mean noise-to-mask ratio in dB (lower is better;
rank ①–⑥ among the six encoders; -q normal):

clip	Apple	FDK	glint	LAME-MP3	ffmpeg	vo-aacenc
speech	−6.9	−5.4	−3.6 ⑶	−2.2	−0.7	+1.4
electronic	−9.9	−11.4	−4.3 ⑶	−3.1	−1.9	+2.3
quartet	−2.9	−2.6	−5.8 ①	−2.0	+0.2	+1.2
industrial	−1.1	−0.8	−1.4 ①	−0.0	+1.6	+1.3
piano	−9.4	−8.5	−10.2 ①	−7.5	−4.1	−1.8
castanets	−7.4	−9.0	−8.5 ⑵	+2.6	+7.1	+18.6

On castanets glint's PEAQ ODG is 0.00 — the best of all six encoders
(Apple and FDK: −0.08): the short blocks, TNS and per-group shaping land
the transients transparently. At 256 kbps everything from
Apple/FDK/glint is ODG-transparent; glint has the best NMR of all
encoders on quartet (−18.7 vs FDK's −13.3) and the highest SNR of the
AAC field on most clips. The full 6-clip × 2-rate log with
SNR/NMR/ODG/PESQ/STOI columns ships in tests/aac_league_v0.8.txt.
vo-aacenc — the obvious "just port it" candidate when this work started
— finishes last on essentially every cell.

Constant-quality VBR: V0 = 311 kbps / 0.0 % audible band-frames on
the speech clip (better than CBR-256), V4 ≈ 130–160 kbps
content-adaptive, V9 ≈ 42 kbps. No Xing-style header machinery needed —
ADTS frames are self-describing.

Speed

After a profile-driven, byte-identity-gated perf pass (−52…54 % encode
time): ~272× realtime at -q speed and ~168× at -q best on an
Apple M1 (integer build: 241× / 140×) — faster at -q speed than every
encoder in the league, including Apple's.

Embedded: 47.4 KB and no FPU

• GLINT_MODE=fixed builds the AAC encoder in 47.4 KB (24.3 KB
  context + 23.1 KB tables) — under vo-aacenc's measured 48.0 KB — with
  storage-only type changes that are metrics-identical to the desktop
  build, then adds GLINT_AAC_INT: integer MDCT (131 dB transform
  SNR), a Q16 log-domain integer quantizer (0.04 coefficient
  mismatches per frame vs the double formula), and int64 energies so
  that at -q speed no per-coefficient float instruction remains.
• MP3 got the same treatment (GLINT_MP3_INT): the old "fixed"
  path was Q31 only through the filterbank; now the -q speed CBR
  chain is integer end-to-end (new integer 36-pt MDCT + the shared
  log-domain quantizer), metrics-identical to the double rate loop
  within 0.02 dB. MP3 quality is otherwise bit-for-bit unchanged from
  v0.7.0 (regression gate deltas: +0.00 across the board).
• Provable: tools/check_nofpu.sh disassembles a Cortex-M0+ build
  and fails if any per-coefficient function contains a soft-float
  call. It passes.
• Runnable without hardware: embedded/qemu/ runs the benchmark
  on an emulated Cortex-M3; the semihosted output streams decode
  cleanly in ffmpeg and CoreAudio, and the MP3 stream is bit-exact
  with a host run (the integer path is deterministic across
  architectures). Ready-to-flash benchmark projects for the Raspberry
  Pi Pico (embedded/pico/) and ESP32 (esp-idf/example/) are
  included — QEMU is not cycle-accurate, so real-silicon throughput
  numbers are the one thing still open.

API, bindings, tooling

• New C API: glint_aac_create/encode/encode_float/flush/destroy,
  glint_version(). The AAC config struct carries a reserved tail and
  a zero-init contract so future options never break the ABI.
  Encoder delay is 2048 samples; glint_aac_flush returns the two
  tail frames.
• AAC in all three bindings (Python AacEncoder, Rust
  AacEncoder/encode_pcm_aac, Dart GlintAacEncoder) — and a fix
  for a latent bug in all of them: the mirrored MP3 config structs
  were missing fields the C struct has had since the quality/VBR
  features, so glint_create read uninitialized memory.
• The encoder league harness gained --codec aac
  (tests/compare_encoders.py): glint vs Apple afconvert, fdkaac,
  ffmpeg native and vo-aacenc, with PEAQ ODG / PESQ / STOI columns.
• A listening pack generator convention and a double-blind ABX tool
  (tests/abx.py) for ear-verification of any pair.

Compatibility notes

• struct glint_aac_config must be zero-initialized (= {0} or
  memset) before filling — reserved fields select defaults.
• The fixed-point (GLINT_MODE=fixed) AAC build trades ~0.7–1.5 dB
  NMR on stereo versus the float builds (the irreducible half-LSB of
  integer M/S, proven by an L==R experiment; mono is identical,
  castanets is better). Desktop builds are unaffected.
• MP3 encoding is bit-for-bit identical to v0.7.0 in the double path
  and at -q normal/best in the fixed path; only fixed -q speed
  changed (integer quantizer, metrics-identical).