MP4: Improve audio bitrate calculation

Author: Serial-ATA

CHANGELOG.md

@@ -48,6 +48,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  ```
  - Renamed `Popularimeter` -> `PopularimeterFrame`
  - Renamed `SynchronizedText` -> `SynchronizedTextFrame`
+- **MP4**: Bitrate calculation is now more accurate ([PR](https://github.com/Serial-ATA/lofty-rs/pull/398))
 
 ### Fixed
 - **ID3v2**: Disallow 4 character TXXX/WXXX frame descriptions from being converted to `ItemKey` ([issue](https://github.com/Serial-ATA/lofty-rs/issues/309)) ([PR](https://github.com/Serial-ATA/lofty-rs/pull/394))

lofty/src/mp4/properties.rs

@@ -210,16 +210,15 @@ impl Mp4Properties {
 }
 }
 
-pub(super) fn read_properties<R>(
-reader: &mut AtomReader<R>,
-traks: &[AtomInfo],
-file_length: u64,
-parse_mode: ParsingMode,
-) -> Result<Mp4Properties>
+struct TrakChildren {
+mdhd: AtomInfo,
+minf: Option<AtomInfo>,
+}
+
+fn get_trak_children<R>(reader: &mut AtomReader<R>, traks: &[AtomInfo]) -> Result<TrakChildren>
 where
 R: Read + Seek,
 {
-// We need the mdhd and minf atoms from the audio track
 let mut audio_track = false;
 let mut mdhd = None;
 let mut minf = None;
@@ -278,8 +277,18 @@ where
 err!(BadAtom("Expected atom \"trak.mdia.mdhd\""));
 };
 
-reader.seek(SeekFrom::Start(mdhd.start + 8))?;
+Ok(TrakChildren { mdhd, minf })
+}
 
+struct Mdhd {
+timescale: u32,
+duration: u64,
+}
+
+fn read_mdhd<R>(reader: &mut AtomReader<R>) -> Result<Mdhd>
+where
+R: Read + Seek,
+{
 let version = reader.read_u8()?;
 let _flags = reader.read_uint(3)?;
 
@@ -302,44 +311,106 @@ where
 (timescale, u64::from(duration))
 };
 
-let duration_millis = (duration * 1000).div_round(u64::from(timescale));
-let duration = Duration::from_millis(duration_millis);
-
-// We create the properties here, since it is possible the other information isn't available
-let mut properties = Mp4Properties {
+Ok(Mdhd {
+timescale,
 duration,
-..Mp4Properties::default()
-};
+})
+}
 
-let Some(minf) = minf else {
-return Ok(properties);
-};
+// TODO: Estimate duration from stts?
+// Since this has the number of samples and the duration of each sample,
+// it would be pretty simple to do, and would help in the case that we have
+// no timescale available.
+#[derive(Debug)]
+struct SttsEntry {
+_sample_count: u32,
+sample_duration: u32,
+}
 
-reader.seek(SeekFrom::Start(minf.start + 8))?;
+fn read_stts<R>(reader: &mut R) -> Result<Vec<SttsEntry>>
+where
+R: Read,
+{
+let _version_and_flags = reader.read_uint::<BigEndian>(4)?;
 
-let Some(stbl) = nested_atom(reader, minf.len, b"stbl", parse_mode)? else {
-return Ok(properties);
-};
+let entry_count = reader.read_u32::<BigEndian>()?;
+let mut entries = Vec::with_capacity(entry_count as usize);
 
-let Some(stsd) = nested_atom(reader, stbl.len, b"stsd", parse_mode)? else {
-return Ok(properties);
+for _ in 0..entry_count {
+let sample_count = reader.read_u32::<BigEndian>()?;
+let sample_duration = reader.read_u32::<BigEndian>()?;
+
+entries.push(SttsEntry {
+_sample_count: sample_count,
+sample_duration,
+});
+}
+
+Ok(entries)
+}
+
+struct Minf {
+stsd_data: Vec<u8>,
+stts: Option<Vec<SttsEntry>>,
+}
+
+fn read_minf<R>(
+reader: &mut AtomReader<R>,
+len: u64,
+parse_mode: ParsingMode,
+) -> Result<Option<Minf>>
+where
+R: Read + Seek,
+{
+let Some(stbl) = nested_atom(reader, len, b"stbl", parse_mode)? else {
+return Ok(None);
 };
 
-let mut stsd = try_vec![0; (stsd.len - 8) as usize];
-reader.read_exact(&mut stsd)?;
+let mut stsd_data = None;
+let mut stts = None;
+
+let mut read = 8;
+while read < stbl.len {
+let Some(atom) = reader.next()? else { break };
+
+read += atom.len;
+
+if let AtomIdent::Fourcc(fourcc) = atom.ident {
+match &fourcc {
+b"stsd" => {
+let mut stsd = try_vec![0; (atom.len - 8) as usize];
+reader.read_exact(&mut stsd)?;
+stsd_data = Some(stsd);
+},
+b"stts" => stts = Some(read_stts(reader)?),
+_ => {
+skip_unneeded(reader, atom.extended, atom.len)?;
+},
+}
 
-let mut cursor = Cursor::new(&*stsd);
+continue;
+}
+}
 
-let mut stsd_reader = AtomReader::new(&mut cursor, parse_mode)?;
+let Some(stsd_data) = stsd_data else {
+return Ok(None);
+};
 
+Ok(Some(Minf { stsd_data, stts }))
+}
+
+fn read_stsd<R>(reader: &mut AtomReader<R>, properties: &mut Mp4Properties) -> Result<()>
+where
+R: Read + Seek,
+{
 // Skipping 4 bytes
 // Version (1)
 // Flags (3)
-stsd_reader.seek(SeekFrom::Current(4))?;
-let num_sample_entries = stsd_reader.read_u32()?;
+reader.seek(SeekFrom::Current(4))?;
+let num_sample_entries = reader.read_u32()?;
 
 for _ in 0..num_sample_entries {
-let Some(atom) = stsd_reader.next()? else {
+let Some(atom) = reader.next()? else {
 err!(BadAtom("Expected sample entry atom in `stsd` atom"))
 };
 
@@ -348,9 +419,9 @@ where
 };
 
 match fourcc {
-b"mp4a" => mp4a_properties(&mut stsd_reader, &mut properties)?,
-b"alac" => alac_properties(&mut stsd_reader, &mut properties)?,
-b"fLaC" => flac_properties(&mut stsd_reader, &mut properties)?,
+b"mp4a" => mp4a_properties(reader, properties)?,
+b"alac" => alac_properties(reader, properties)?,
+b"fLaC" => flac_properties(reader, properties)?,
 // Maybe do these?
 // TODO: dops (opus)
 // TODO: wave (https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html#//apple_ref/doc/uid/TP40000939-CH205-134202)
@@ -371,23 +442,85 @@ where
 },
 }
 
-// We do the mdat check up here, so we have access to the entire file
-let duration_millis = properties.duration.as_millis();
-if duration_millis > 0 {
-let overall_bitrate = u128::from(file_length * 8) / duration_millis;
-properties.overall_bitrate = overall_bitrate as u32;
+// We only want to read the properties of the first stream
+// that we can actually recognize
+break;
+}
 
-if properties.audio_bitrate == 0 {
-log::warn!("Estimating audio bitrate from 'mdat' size");
+Ok(())
+}
+
+pub(super) fn read_properties<R>(
+reader: &mut AtomReader<R>,
+traks: &[AtomInfo],
+file_length: u64,
+parse_mode: ParsingMode,
+) -> Result<Mp4Properties>
+where
+R: Read + Seek,
+{
+// We need the mdhd and minf atoms from the audio track
+let TrakChildren { mdhd, minf } = get_trak_children(reader, traks)?;
 
-properties.audio_bitrate =
-(u128::from(mdat_length(reader)? * 8) / duration_millis) as u32;
+reader.seek(SeekFrom::Start(mdhd.start + 8))?;
+let Mdhd {
+timescale,
+duration,
+} = read_mdhd(reader)?;
+
+// We create the properties here, since it is possible the other information isn't available
+let mut properties = Mp4Properties::default();
+
+if timescale > 0 {
+let duration_millis = (duration * 1000).div_round(u64::from(timescale));
+properties.duration = Duration::from_millis(duration_millis);
+}
+
+// We need an `mdhd` atom at the bare minimum, everything else can be optional.
+let Some(minf_info) = minf else {
+return Ok(properties);
+};
+
+reader.seek(SeekFrom::Start(minf_info.start + 8))?;
+let Some(Minf { stsd_data, stts }) = read_minf(reader, minf_info.len, parse_mode)? else {
+return Ok(properties);
+};
+
+// `stsd` contains the majority of the audio properties
+let mut cursor = Cursor::new(&*stsd_data);
+let mut stsd_reader = AtomReader::new(&mut cursor, parse_mode)?;
+read_stsd(&mut stsd_reader, &mut properties)?;
+
+// We do the mdat check up here, so we have access to the entire file
+if duration > 0 {
+// TODO: We should keep track of the `mdat` length when first reading the file.
+// This extra read is unnecessary.
+let mdat_len = mdat_length(reader)?;
+
+if let Some(stts) = stts {
+let stts_specifies_duration = !(stts.len() == 1 && stts[0].sample_duration == 1);
+if stts_specifies_duration {
+// We do a basic audio bitrate calculation below for each stream type.
+// Up here, we can do a more accurate calculation if the duration is available.
+let audio_bitrate_bps = (((u128::from(mdat_len) * 8) * u128::from(timescale))
+/ u128::from(duration)) as u32;
+
+// kb/s
+properties.audio_bitrate = audio_bitrate_bps / 1000;
 }
 }
 
-// We only want to read the properties of the first stream
-// that we can actually recognize
-break;
+let duration_millis = properties.duration.as_millis();
+
+let overall_bitrate = u128::from(file_length * 8) / duration_millis;
+properties.overall_bitrate = overall_bitrate as u32;
+
+if properties.audio_bitrate == 0 {
+log::warn!("Estimating audio bitrate from 'mdat' size");
+
+properties.audio_bitrate =
+(u128::from(mdat_length(reader)? * 8) / duration_millis) as u32;
+}
 }
 
 Ok(properties)
@@ -576,7 +709,7 @@ where
 return Ok(());
 }
 
-// Unlike the mp4a atom, we cannot read the data that immediately follows it
+// Unlike the "mp4a" atom, we cannot read the data that immediately follows it
 // For ALAC, we have to skip the first "alac" atom entirely, and read the one that
 // immediately follows it.
 //
@@ -694,7 +827,7 @@ where
 
 while let Ok(Some(atom)) = reader.next() {
 if atom.ident == AtomIdent::Fourcc(*b"mdat") {
-return Ok(atom.len);
+return Ok(atom.len - 8);
 }
 
 skip_unneeded(reader, atom.extended, atom.len)?;

lofty/src/properties/tests.rs

@@ -129,7 +129,7 @@ const MP4_ALAC_PROPERTIES: Mp4Properties = Mp4Properties {
 extended_audio_object_type: None,
 duration: Duration::from_millis(1428),
 overall_bitrate: 331,
-audio_bitrate: 1536,
+audio_bitrate: 326,
 sample_rate: 48000,
 bit_depth: Some(16),
 channels: 2,