mux/mp4: M2 — audio tracks + fit oracle (multi-track)
Generalize the sink to N tracks: one video + every MP4-mappable audio track. Audio sample entries built from the first frame's bitstream — AC-3 (ac-3/dac3) and E-AC-3 (ec-3/dec3), parsing the (E-)AC-3 BSI for fscod/bsid/bsmod/acmod/lfeon and the data rate. Per-sample audio durations from PTS deltas (no reorder → no ctts, all sync). Fit oracle (mp4_fit_report, exported): video HEVC/H264, audio AC-3/E-AC-3; TrueHD/DTS/LPCM and bitmap subs are excluded with a typed reason so the CLI can report exclusions — never a silent drop. Verified vs ffmpeg -c copy on real discs: AVC+AC3 and HEVC(HDR10)+AC3 both frame-exact on every track (video 2650/4270, audio 5567/3454), duration and colour identical, clean decode. On a pathological 4-clip title ffmpeg's OWN output emits the same DTS-monotonicity warnings (more of them) — source-inherent, not a muxer defect.
This commit is contained in:
Matthew Jackson
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//! MP4 audio sample entries and codec-config boxes for the `mp4://` muxer.
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//!
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//! Covers the Dolby family that maps cleanly into MP4 and plays widely:
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//! **AC-3** (`ac-3` + `dac3`) and **E-AC-3 / Dolby Digital Plus** (`ec-3` +
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//! `dec3`, which also carries Atmos-in-DD+ JOC). The config boxes are derived
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//! from the first audio frame's bitstream (ISO/IEC 14496-12 amendments; ETSI TS
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//! 102 366 Annexes F/G). Codecs with no clean MP4 mapping (TrueHD, DTS family,
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//! LPCM, bitmap subtitles) are handled by the fit oracle in the sink, not here.
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use super::boxes::bx;
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use crate::disc::Codec;
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/// AC-3 / E-AC-3 sample rates indexed by `fscod` (byte-4 bits 7-6).
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const FSCOD_RATES: [u32; 3] = [48_000, 44_100, 32_000];
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/// E-AC-3 reduced rates indexed by `fscod2` (byte-4 bits 5-4) when `fscod == 3`.
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const EAC3_REDUCED_RATES: [u32; 4] = [24_000, 22_050, 16_000, 48_000];
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/// Base channel count per `acmod` (A/52 Table 5.8), before the LFE.
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const ACMOD_CHANNELS: [u8; 8] = [2, 1, 2, 3, 3, 4, 4, 5];
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/// A big-endian MSB-first bit reader over a byte slice.
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struct BitReader<'a> {
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data: &'a [u8],
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bit: usize,
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}
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impl<'a> BitReader<'a> {
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fn new(data: &'a [u8]) -> Self {
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Self { data, bit: 0 }
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}
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fn skip(&mut self, n: usize) {
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self.bit += n;
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}
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/// Read `n` bits (n ≤ 32). Returns 0 past end of data (callers pre-check len).
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fn read(&mut self, n: usize) -> u32 {
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let mut v = 0u32;
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for _ in 0..n {
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let byte = self.data.get(self.bit / 8).copied().unwrap_or(0);
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let shift = 7 - (self.bit % 8);
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v = (v << 1) | ((byte >> shift) & 1) as u32;
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self.bit += 1;
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}
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v
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}
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}
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/// Decoded (E-)AC-3 stream parameters needed for the `dac3`/`dec3` config box
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/// and the audio sample entry.
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pub(super) struct DolbyConfig {
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pub fscod: u8,
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pub bsid: u8,
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pub bsmod: u8,
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pub acmod: u8,
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pub lfeon: bool,
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/// AC-3 only: `bit_rate_code` (= `frmsizecod >> 1`). Unused for E-AC-3.
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pub bit_rate_code: u8,
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/// E-AC-3 only: nominal data rate in kbps (for `dec3`). 0 for AC-3.
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pub data_rate_kbps: u16,
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pub sample_rate: u32,
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pub channels: u16,
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}
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impl DolbyConfig {
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fn channel_count(acmod: u8, lfeon: bool) -> u16 {
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ACMOD_CHANNELS[acmod as usize] as u16 + lfeon as u16
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}
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}
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/// Parse the first (E-)AC-3 frame starting at the 0x0B77 syncword. Returns
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/// `None` if the frame is too short or the syncword is absent.
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pub(super) fn parse_dolby(frame: &[u8]) -> Option<DolbyConfig> {
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let start =
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(0..frame.len().saturating_sub(1)).find(|&i| frame[i] == 0x0B && frame[i + 1] == 0x77)?;
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let f = &frame[start..];
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if f.len() < 6 {
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return None;
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}
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// bsid lives in byte 5 bits 7-3 for both AC-3 and E-AC-3.
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let bsid = (f[5] >> 3) & 0x1F;
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if bsid >= 11 {
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parse_eac3(f)
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} else {
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parse_ac3(f)
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}
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}
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/// Legacy AC-3 (A/52 §5.3.2): syncword | crc(16) | fscod(2) frmsizecod(6) |
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/// bsid(5) bsmod(3) | acmod(3) …optional… lfeon.
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fn parse_ac3(f: &[u8]) -> Option<DolbyConfig> {
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if f.len() < 8 {
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return None;
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}
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let fscod = (f[4] >> 6) & 0x03;
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let frmsizecod = f[4] & 0x3F;
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let bsid = (f[5] >> 3) & 0x1F;
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let bsmod = f[5] & 0x07;
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// acmod + trailing optional 2-bit fields, then lfeon (byte 6 onward).
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let mut r = BitReader::new(f);
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r.bit = 6 * 8;
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let acmod = r.read(3) as u8;
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if (acmod & 0x1) != 0 && acmod != 0x1 {
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r.skip(2); // cmixlev
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}
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if (acmod & 0x4) != 0 {
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r.skip(2); // surmixlev
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}
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if acmod == 0x2 {
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r.skip(2); // dsurmod
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}
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let lfeon = r.read(1) == 1;
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Some(DolbyConfig {
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fscod,
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bsid,
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bsmod,
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acmod,
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lfeon,
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bit_rate_code: frmsizecod >> 1,
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data_rate_kbps: 0,
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sample_rate: FSCOD_RATES.get(fscod as usize).copied().unwrap_or(48_000),
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channels: DolbyConfig::channel_count(acmod, lfeon),
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})
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}
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/// E-AC-3 (A/52 Annex E BSI): syncword | strmtyp(2) substreamid(3) frmsiz(11) |
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/// fscod(2) numblkscod(2) acmod(3) lfeon(1) | bsid(5) …
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fn parse_eac3(f: &[u8]) -> Option<DolbyConfig> {
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if f.len() < 6 {
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return None;
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}
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let frmsiz = (((f[2] & 0x07) as u32) << 8) | f[3] as u32; // words minus one
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let fscod = (f[4] >> 6) & 0x03;
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let numblkscod = (f[4] >> 4) & 0x03;
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let acmod = (f[4] >> 1) & 0x07;
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let lfeon = (f[4] & 0x01) == 1;
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let bsid = (f[5] >> 3) & 0x1F;
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let (sample_rate, blocks) = if fscod == 0x03 {
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let fscod2 = (f[4] >> 4) & 0x03; // shares bits with numblkscod when fscod==3
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(EAC3_REDUCED_RATES[fscod2 as usize], 6u32)
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} else {
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let blocks = [1u32, 2, 3, 6][numblkscod as usize];
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(FSCOD_RATES[fscod as usize], blocks)
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};
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// Nominal data rate (kbps): frame is (frmsiz+1) 16-bit words per (blocks·256)
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// samples at sample_rate. rate = bytes·8·sr / samples / 1000.
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let frame_bytes = (frmsiz as u64 + 1) * 2;
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let samples = blocks as u64 * 256;
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let data_rate_kbps = if samples > 0 {
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((frame_bytes * 8 * sample_rate as u64) / samples / 1000) as u16
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} else {
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0
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};
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Some(DolbyConfig {
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fscod,
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bsid,
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bsmod: 0, // not in the E-AC-3 main header; dec3 default
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acmod,
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lfeon,
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bit_rate_code: 0,
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data_rate_kbps,
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sample_rate,
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channels: DolbyConfig::channel_count(acmod, lfeon),
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})
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}
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/// The `dac3` config box (ETSI TS 102 366 Annex F.4): 24 bits —
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/// fscod(2) bsid(5) bsmod(3) acmod(3) lfeon(1) bit_rate_code(5) reserved(5).
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pub(super) fn dac3_box(c: &DolbyConfig) -> Vec<u8> {
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let mut v: u32 = 0;
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let mut push = |val: u32, bits: u32| v = (v << bits) | (val & ((1 << bits) - 1));
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push(c.fscod as u32, 2);
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push(c.bsid as u32, 5);
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push(c.bsmod as u32, 3);
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push(c.acmod as u32, 3);
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push(c.lfeon as u32, 1);
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push(c.bit_rate_code as u32, 5);
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push(0, 5); // reserved
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// 24 bits → the top 3 bytes of the big-endian u32.
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let b = v.to_be_bytes();
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bx(b"dac3", &[b[1], b[2], b[3]])
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}
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/// The `dec3` config box (ETSI TS 102 366 Annex G.3) for a single independent
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/// substream, no dependent substreams: data_rate(13) num_ind_sub(3) then
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/// fscod(2) bsid(5) reserved(1) asvc(1) bsmod(3) acmod(3) lfeon(1) reserved(3)
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/// num_dep_sub(4) reserved(1).
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pub(super) fn dec3_box(c: &DolbyConfig) -> Vec<u8> {
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let mut v: u64 = 0;
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let mut push = |val: u64, bits: u32| v = (v << bits) | (val & ((1u64 << bits) - 1));
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push(c.data_rate_kbps as u64, 13);
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push(0, 3); // num_ind_sub - 1 = 0 (one substream)
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push(c.fscod as u64, 2);
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push(c.bsid as u64, 5);
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push(0, 1); // reserved
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push(0, 1); // asvc
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push(c.bsmod as u64, 3);
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push(c.acmod as u64, 3);
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push(c.lfeon as u64, 1);
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push(0, 3); // reserved
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push(0, 4); // num_dep_sub = 0
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push(0, 1); // reserved (chan_loc absent when num_dep_sub == 0)
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// 40 bits → the low 5 bytes of the big-endian u64.
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let b = v.to_be_bytes();
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bx(b"dec3", &[b[3], b[4], b[5], b[6], b[7]])
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}
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/// Build an audio sample entry (`ac-3` / `ec-3`) with the given config box.
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/// `AudioSampleEntry` per ISO/IEC 14496-12 §12.2.3.
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pub(super) fn audio_sample_entry(
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fourcc: &[u8; 4],
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channels: u16,
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sample_rate: u32,
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config: &[u8],
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) -> Vec<u8> {
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let mut e = Vec::new();
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e.extend_from_slice(&[0u8; 6]); // reserved
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e.extend_from_slice(&1u16.to_be_bytes()); // data_reference_index
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e.extend_from_slice(&[0u8; 8]); // reserved (version 0)
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e.extend_from_slice(&channels.to_be_bytes());
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e.extend_from_slice(&16u16.to_be_bytes()); // samplesize
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e.extend_from_slice(&0u16.to_be_bytes()); // pre_defined
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e.extend_from_slice(&0u16.to_be_bytes()); // reserved
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// samplerate is 16.16 fixed point; the integer rate in the high 16 bits.
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e.extend_from_slice(&(sample_rate << 16).to_be_bytes());
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e.extend_from_slice(config);
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bx(fourcc, &e)
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}
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/// The MP4 fourcc + config box for a Dolby audio frame, or `None` if the codec
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/// has no MP4 mapping here. Together with [`audio_fits`] this is the fit oracle
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/// for audio: only what returns `Some` is muxable.
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pub(super) fn dolby_sample_entry(codec: Codec, first_frame: &[u8]) -> Option<Vec<u8>> {
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let c = parse_dolby(first_frame)?;
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let (fourcc, config): (&[u8; 4], Vec<u8>) = match codec {
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Codec::Ac3 => (b"ac-3", dac3_box(&c)),
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Codec::Ac3Plus => (b"ec-3", dec3_box(&c)),
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_ => return None,
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};
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Some(audio_sample_entry(
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fourcc,
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c.channels,
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c.sample_rate,
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&config,
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))
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}
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/// Fit oracle for an audio codec: does `mp4://` currently carry it? M2 covers
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/// the Dolby family (AC-3 / E-AC-3). TrueHD, DTS(-HD), LPCM, AAC are not yet
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/// mapped and are skipped with a loud report (never silently dropped).
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pub(super) fn audio_fits(codec: Codec) -> bool {
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matches!(codec, Codec::Ac3 | Codec::Ac3Plus)
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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/// A synthetic legacy AC-3 header: syncword, crc, fscod=0 (48k),
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/// frmsizecod, bsid=8, bsmod=0, acmod=7 (3/2), lfeon=1 → 5.1.
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fn ac3_frame_5_1() -> Vec<u8> {
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let mut f = vec![0x0B, 0x77, 0x00, 0x00];
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// byte4: fscod(2)=0 | frmsizecod(6)=0b010110 (22)
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f.push(0b00_010110);
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// byte5: bsid(5)=8 (0b01000) | bsmod(3)=0
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f.push(0b01000_000);
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// byte6: acmod(3)=7 (0b111) | cmixlev(2) | surmixlev(2) | lfeon(1)...
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// acmod=7 has centre (needs cmixlev) and surround (needs surmixlev):
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// 111 | 00 | 00 | 1(lfeon) = 0b111_00_00_1
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f.push(0b111_00_00_1);
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f.push(0x00);
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f
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}
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#[test]
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fn ac3_bsi_and_dac3() {
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let c = parse_dolby(&ac3_frame_5_1()).expect("parsed");
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assert!(c.bsid < 11, "legacy AC-3");
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assert_eq!(c.fscod, 0);
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assert_eq!(c.sample_rate, 48_000);
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assert_eq!(c.bsid, 8);
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assert_eq!(c.acmod, 7);
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assert!(c.lfeon);
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assert_eq!(c.channels, 6, "3/2 + LFE = 5.1");
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assert_eq!(c.bit_rate_code, 22 >> 1);
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let dac3 = dac3_box(&c);
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// [size:4]["dac3"][3-byte payload] = 11 bytes.
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assert_eq!(dac3.len(), 11);
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assert_eq!(&dac3[4..8], b"dac3");
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}
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#[test]
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fn eac3_bsi_and_dec3() {
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// E-AC-3: syncword | strmtyp/substreamid/frmsiz | fscod/numblks/acmod/lfeon | bsid
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let mut f = vec![0x0B, 0x77];
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f.push(0x00); // strmtyp=0, substreamid=0, frmsiz high=0
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f.push(0x3F); // frmsiz low = 63 → frame 128 bytes
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// byte4: fscod(2)=0 | numblkscod(2)=3 (6 blocks) | acmod(3)=7 | lfeon(1)=1
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f.push(0b00_11_111_1);
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// byte5: bsid(5)=16 (E-AC-3) | dialnorm high(3)
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f.push(0b10000_000);
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f.push(0x00);
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let c = parse_dolby(&f).expect("parsed");
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assert!(c.bsid >= 11, "E-AC-3");
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assert_eq!(c.bsid, 16);
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assert_eq!(c.fscod, 0);
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assert_eq!(c.sample_rate, 48_000);
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assert_eq!(c.acmod, 7);
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assert!(c.lfeon);
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assert_eq!(c.channels, 6);
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let dec3 = dec3_box(&c);
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// [size:4]["dec3"][5-byte payload] = 13 bytes.
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assert_eq!(dec3.len(), 13);
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assert_eq!(&dec3[4..8], b"dec3");
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}
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#[test]
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fn sample_entry_shape() {
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let c = parse_dolby(&ac3_frame_5_1()).unwrap();
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let e = audio_sample_entry(b"ac-3", c.channels, c.sample_rate, &dac3_box(&c));
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assert_eq!(&e[4..8], b"ac-3");
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// channelcount at entry-body offset 16 (after 6 reserved + 2 dri + 8 reserved).
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let ch = u16::from_be_bytes([e[8 + 16], e[8 + 17]]);
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assert_eq!(ch, 6);
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}
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#[test]
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fn fit_oracle_dolby_only() {
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assert!(audio_fits(Codec::Ac3));
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assert!(audio_fits(Codec::Ac3Plus));
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assert!(!audio_fits(Codec::TrueHd));
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assert!(!audio_fits(Codec::DtsHdMa));
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assert!(!audio_fits(Codec::Lpcm));
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}
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}
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+537
-324
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