mux/mp4: DTS audio (dtsc/dtsh + ddts)
Parse the DTS core header (SFREQ/AMODE/RATE/LFF/NBLKS/FSIZE) → a ddts box (sample rate, channel layout mask, core size, computed bitrate, LFE); whole access units (core + DTS-HD extension substreams) pass through, so an HD decoder finds the extension. dtsh when an extension sync follows the core, else dtsc. Fit oracle now carries DTS / DTS-HD MA / DTS-HD HR. Validated on 300 (real DTS-HD MA 7.1): freemkv's mp4 DTS track is byte-identical to ffmpeg -c copy under ffprobe (dts / 48000 / 8ch / 7.1) and decodes clean (exit 0). Channel layout correct.
This commit is contained in:
Matthew Jackson
+216
-27
@@ -1,11 +1,13 @@
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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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//! Covers the codecs that map cleanly into MP4 and play widely: **AC-3**
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//! (`ac-3` + `dac3`), **E-AC-3 / Dolby Digital Plus** (`ec-3` + `dec3`, incl.
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//! Atmos-in-DD+ JOC), and **DTS / DTS-HD** (`dtsc`/`dtsh` + `ddts`, describing
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//! the core with whole access units passed through so an HD decoder finds the
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//! extension). Config boxes are derived from the first audio frame's bitstream
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//! (ISO/IEC 14496-12 amendments; ETSI TS 102 366 / 102 114). Codecs with no
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//! clean MP4 mapping (TrueHD, LPCM, bitmap subtitles) are excluded by the fit
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//! oracle in the sink.
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use super::boxes::bx;
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use crate::disc::Codec;
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@@ -228,29 +230,188 @@ pub(super) fn audio_sample_entry(
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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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// ── DTS (dtsc/dtsh + ddts) ───────────────────────────────────────────────────
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/// DTS core `SFREQ` (4-bit) → sample rate (Hz). Reserved indices → 48 kHz.
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const DTS_SFREQ: [u32; 16] = [
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48_000, 8_000, 16_000, 32_000, 48_000, 48_000, 11_025, 22_050, 44_100, 48_000, 48_000, 12_000,
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24_000, 48_000, 96_000, 192_000,
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];
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/// DTS core base channel count per `AMODE` (0..=9); higher AMODEs are rare on disc.
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const DTS_AMODE_CH: [u8; 10] = [1, 2, 2, 2, 2, 3, 3, 4, 4, 5];
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/// Decoded DTS core parameters needed for the `ddts` box.
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struct DtsConfig {
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sample_rate: u32,
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channels: u16,
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amode: u8,
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lfe: bool,
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core_size: u32,
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/// Samples per frame ((NBLKS+1)·32).
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frame_samples: u32,
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/// Whether a DTS-HD extension substream follows the core.
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has_extension: bool,
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channel_layout: u16,
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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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/// Parse the DTS core header (ETSI TS 102 114 §5.3.1), starting at the
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/// 0x7FFE8001 big-endian core sync. Returns `None` if too short / no sync.
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fn parse_dts(frame: &[u8]) -> Option<DtsConfig> {
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let start = (0..frame.len().saturating_sub(3)).find(|&i| {
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frame[i] == 0x7F && frame[i + 1] == 0xFE && frame[i + 2] == 0x80 && frame[i + 3] == 0x01
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})?;
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let f = &frame[start..];
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if f.len() < 11 {
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return None;
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}
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// Bit fields after the 32-bit sync (MSB-first):
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// FTYPE1 SHORT5 CPF1 NBLKS7 FSIZE14 AMODE6 SFREQ4 RATE5 ...
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let nblks = (((f[4] & 0x01) as u32) << 6) | ((f[5] >> 2) as u32 & 0x3F);
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let fsize = (((f[5] & 0x03) as u32) << 12) | ((f[6] as u32) << 4) | ((f[7] >> 4) as u32 & 0x0F);
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let amode = (((f[7] & 0x0F) << 2) | ((f[8] >> 6) & 0x03)) as usize;
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let sfreq = ((f[8] >> 2) & 0x0F) as usize;
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// LFF is 2 bits at bit offset 85 → byte10 bits 2-1.
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let lff = (f[10] >> 1) & 0x03;
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let lfe = lff == 1 || lff == 2;
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let sample_rate = DTS_SFREQ[sfreq];
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let base_ch = DTS_AMODE_CH.get(amode).copied().unwrap_or(6);
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let channels = base_ch as u16 + lfe as u16;
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let channel_layout = dts_channel_layout(amode, lfe);
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// DTS-HD extension substream sync (0x64582025) after the core frame.
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let ext_sync = [0x64, 0x58, 0x20, 0x25];
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let has_extension = f
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.windows(4)
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.skip((fsize as usize + 1).min(f.len()).saturating_sub(4))
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.any(|w| w == ext_sync)
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|| f.windows(4).any(|w| w == ext_sync);
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Some(DtsConfig {
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sample_rate,
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channels,
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amode: amode as u8,
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lfe,
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core_size: fsize + 1,
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frame_samples: (nblks + 1) * 32,
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has_extension,
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channel_layout,
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})
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}
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/// `ddts` ChannelLayout (16-bit speaker mask) for the common core layouts.
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/// bit0=C, bit1=L/R, bit2=Ls/Rs, bit3=LFE.
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fn dts_channel_layout(amode: usize, lfe: bool) -> u16 {
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let mut m = match amode {
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0 => 0x0001, // C (mono)
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1..=4 => 0x0002, // L/R
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5 => 0x0003, // C + L/R
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6 | 8 => 0x0006, // L/R + Ls/Rs (no centre)
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_ => 0x0007, // C + L/R + surround (amode 7, 9, …)
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};
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if lfe {
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m |= 0x0008;
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}
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m
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}
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/// The `ddts` config box (ETSI TS 102 114 Annex; DTS-in-ISO registration).
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/// Describes the DTS core; whole access units (core + any extension) are passed
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/// through as samples, so a DTS-HD-aware decoder still finds the extension.
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fn ddts_box(c: &DtsConfig) -> Vec<u8> {
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// avg/max bitrate: computed from the core frame size × frame rate (the core
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// RATE field reads "open/variable" for lossless, so it's not usable directly).
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let frames_per_sec = if c.frame_samples > 0 {
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c.sample_rate as u64 / c.frame_samples as u64
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} else {
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0
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};
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let bitrate = (c.core_size as u64 * 8 * frames_per_sec) as u32;
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let mut out = Vec::new();
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out.extend_from_slice(&c.sample_rate.to_be_bytes()); // DTSSamplingFrequency
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out.extend_from_slice(&bitrate.to_be_bytes()); // maxBitrate
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out.extend_from_slice(&bitrate.to_be_bytes()); // avgBitrate
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out.push(if c.has_extension { 24 } else { 16 }); // pcmSampleDepth
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// Bit-packed tail (56 bits):
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// FrameDuration2 StreamConstruction5 CoreLFEPresent1 CoreLayout6 CoreSize14
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// StereoDownmix1 RepresentationType3 ChannelLayout16 MultiAssetFlag1
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// LBRDurationMod1 ReservedBoxPresent1 Reserved5
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let frame_duration = match c.frame_samples {
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0..=512 => 0,
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513..=1024 => 1,
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1025..=2048 => 2,
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_ => 3,
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};
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// StreamConstruction: 1 = DTS core present. Whole-AU passthrough means an
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// HD decoder still parses the extension substreams from the stream itself.
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let stream_construction = 1u128;
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let mut v: u128 = 0;
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let mut push = |val: u128, bits: u32| v = (v << bits) | (val & ((1u128 << bits) - 1));
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push(frame_duration as u128, 2);
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push(stream_construction, 5);
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push(c.lfe as u128, 1);
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push(c.amode as u128, 6);
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push(c.core_size as u128, 14);
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push(0, 1); // StereoDownmix
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push(0, 3); // RepresentationType
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push(c.channel_layout as u128, 16);
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push(c.has_extension as u128, 1); // MultiAssetFlag
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push(0, 1); // LBRDurationMod
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push(0, 1); // ReservedBoxPresent
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push(0, 5); // Reserved
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// 56 bits → the low 7 bytes of the big-endian u128.
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let b = v.to_be_bytes();
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out.extend_from_slice(&b[9..16]);
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bx(b"ddts", &out)
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}
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/// The MP4 fourcc + config box for an audio frame, or `None` if the codec has no
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/// MP4 mapping here. Together with [`audio_fits`] this is the fit oracle for
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/// 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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match codec {
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Codec::Ac3 => {
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let c = parse_dolby(first_frame)?;
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Some(audio_sample_entry(
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b"ac-3",
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c.channels,
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c.sample_rate,
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&dac3_box(&c),
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))
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}
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Codec::Ac3Plus => {
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let c = parse_dolby(first_frame)?;
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Some(audio_sample_entry(
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b"ec-3",
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c.channels,
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c.sample_rate,
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&dec3_box(&c),
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))
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}
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Codec::Dts | Codec::DtsHdMa | Codec::DtsHdHr => {
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let c = parse_dts(first_frame)?;
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// `dtsc` = DTS core; `dtsh` = DTS-HD (core + extension substreams).
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let fourcc: &[u8; 4] = if c.has_extension { b"dtsh" } else { b"dtsc" };
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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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&ddts_box(&c),
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))
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}
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_ => None,
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}
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}
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/// Fit oracle for an audio codec: does `mp4://` currently carry it? Covers the
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/// Dolby family (AC-3 / E-AC-3) and DTS (core / DTS-HD HRA / DTS-HD MA — the core
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/// is described, whole access units pass through). TrueHD, 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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matches!(
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codec,
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Codec::Ac3 | Codec::Ac3Plus | Codec::Dts | Codec::DtsHdMa | Codec::DtsHdHr
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)
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}
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#[cfg(test)]
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@@ -327,11 +488,39 @@ mod tests {
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}
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#[test]
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fn fit_oracle_dolby_only() {
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fn fit_oracle_covers_dolby_and_dts() {
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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::Dts));
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assert!(audio_fits(Codec::DtsHdMa));
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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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#[test]
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fn dts_core_5_1_and_ddts() {
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// Synthetic DTS core: SFREQ=13 (48k), AMODE=9 (5ch), LFF=1 (LFE) → 5.1.
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let f = vec![
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0x7F, 0xFE, 0x80, 0x01, 0x00, 0x3C, 0x05, 0xF2, 0x77, 0x00, 0x02, 0x00,
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];
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let c = parse_dts(&f).expect("dts core parsed");
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assert_eq!(c.sample_rate, 48_000);
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assert_eq!(c.amode, 9);
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assert!(c.lfe);
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assert_eq!(c.channels, 6, "5 core + LFE = 5.1");
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assert_eq!(c.channel_layout, 0x000F, "C + L/R + Ls/Rs + LFE");
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assert_eq!(c.core_size, 96);
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assert_eq!(c.frame_samples, 512);
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let ddts = ddts_box(&c);
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assert_eq!(&ddts[4..8], b"ddts");
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// DTSSamplingFrequency (first field) = 48000.
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assert_eq!(
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u32::from_be_bytes([ddts[8], ddts[9], ddts[10], ddts[11]]),
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48_000
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);
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// Sample entry uses dtsc (no extension in this synthetic frame).
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let e = dolby_sample_entry(Codec::DtsHdMa, &f).unwrap();
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assert_eq!(&e[4..8], b"dtsc");
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}
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}
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+4
-3
@@ -12,9 +12,10 @@
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//! ## Track model
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//!
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//! One video track (HEVC / H.264) plus every audio track whose codec has a clean
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//! MP4 mapping (AC-3 → `ac-3`/`dac3`, E-AC-3 → `ec-3`/`dec3`). This is the fit
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//! oracle: a codec MP4 can't carry (TrueHD, DTS, LPCM) or that has no sample
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//! entry here is **excluded, never silently dropped** — [`fit_report`] lets the
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//! MP4 mapping (AC-3 → `ac-3`/`dac3`, E-AC-3 → `ec-3`/`dec3`, DTS/DTS-HD →
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//! `dtsc`/`dtsh`/`ddts`). This is the fit oracle: a codec MP4 can't carry
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//! (TrueHD, LPCM) or that has no sample entry here is **excluded, never silently
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//! dropped** — [`fit_report`] lets the
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//! CLI enumerate exactly what was left out and why. Video NALs pass through
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//! unchanged (the demux hands us length-prefixed hvcC/avcC framing — already
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//! MP4's form). Decode timestamps are derived (the pipeline carries presentation
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