export_m4b encoded with `-c:a aac -b:a 128k` and no `-ar`, so the delivered
file came out at 24 kHz / ~96 kbps mono while the AR MP3 masters were
44.1 kHz / 192 kbps CBR mono. Measured on the shipped Deal_Hunter.m4b:
24 kHz, ~96 kbps, and peak -2.0 dB - failing the 44.1 kHz and -3 dB
requirements at Author's Republic and Spotify. AudiobookExporter had zero
loudness or peak handling of any kind.
- _encode_aac_m4b now passes -ar 44100 and -ac 1 explicitly, and takes the
spec constants from ar_export so the two paths cannot drift apart again.
- Added normalisation to the RMS target and peak enforcement on the delivered
file, plus m4b_loudness_report() so the result can be verified.
- AAC overshoot is NOT monotonic. Measured +0.8, +0.9 then +1.6 dB across
three passes, so trimming by the exact excess oscillated and exhausted the
pass budget still breaching. Trimming by excess + 1.5 dB margin converges:
verified -2.20 -> -4.5 dB in two passes.
- _concat_wavs_with_silence now targets 44.1 kHz rather than the TTS-native
24 kHz.
- Closed spec 22 gap 1: _stitch_chapter hard-coded
silence_between_chapters_ms=0 while ChapterBuilder's AudioStitcher defaults
are 500/4000, so the packaged book had different silence than the audio a
reviewer approved.
Trade-off, flagged rather than hidden: the 1.5 dB overshoot margin costs ~1.3
dB of RMS, so a trimmed m4b measures about -22.8 dB against the -20.5 target.
Still inside the -23..-18 band, but only ~0.2 dB from the floor; a chapter
needing more than ~2.5 dB of trim would fall out of band and loudness_ok will
flag it.
Verified on real chapter audio: AAC 44100 Hz mono, RMS -22.8, peak -4.5.
6 new tests. Full suite: 630 passed, 8 pre-existing failures unchanged.
