moq-mux 0.5.6

Media muxers and demuxers for MoQ
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134

//! Dolby Digital Plus (E-AC-3 / Enhanced AC-3).
//!
//! Broadcast audio carried verbatim: each sync frame is published whole. The
//! header is parsed only to find frame boundaries and the catalog config
//! (sample rate, channels); the audio is never decoded.
//!
//! Scope: a single independent substream (strmtyp 0 or 2, substreamid 0), the
//! shape broadcast encoders emit for up to 5.1. Dependent substreams (7.1 and
//! beyond) and additional substream ids are rejected explicitly.

use super::legacy;

pub(crate) static DESCRIPTOR: legacy::Descriptor = legacy::Descriptor {
	track_suffix: ".eac3",
	codec: hang::catalog::AudioCodec::Ec3,
	min_header_len: 6,
	parse: parse_header,
};

const SAMPLE_RATE: [u32; 3] = [48000, 44100, 32000];
// Reduced rates selected by fscod2 when fscod is 0b11.
const SAMPLE_RATE_REDUCED: [u32; 3] = [24000, 22050, 16000];
// Audio blocks per frame, indexed by numblkscod; each block is 256 samples.
const BLOCKS: [u64; 4] = [1, 2, 3, 6];
// Full-bandwidth channels per acmod: 1+1, 1/0, 2/0, 3/0, 2/1, 3/1, 2/2, 3/2.
const CHANNELS: [u32; 8] = [2, 1, 2, 3, 3, 4, 4, 5];

/// Parse an E-AC-3 sync frame header from the start of `data` (needs >= 6 bytes).
pub(crate) fn parse_header(data: &[u8]) -> anyhow::Result<legacy::Header> {
	anyhow::ensure!(data.len() >= 6, "E-AC-3 header needs 6 bytes");
	anyhow::ensure!(data[0] == 0x0B && data[1] == 0x77, "missing E-AC-3 sync word");

	// bsid 11..=16 is E-AC-3; plain AC-3 (bsid <= 8) is routed by stream_type and
	// never reaches this parser.
	let bsid = data[5] >> 3;
	anyhow::ensure!((11..=16).contains(&bsid), "not an E-AC-3 bitstream (bsid {bsid})");

	// A dependent substream (strmtyp 1) extends a prior program to 7.1+, and a
	// nonzero substreamid carries additional programs in the same PID. Either
	// would make this track's channel count a lie, so they are rejected rather
	// than mis-described.
	let strmtyp = data[2] >> 6;
	anyhow::ensure!(strmtyp != 3, "reserved E-AC-3 stream type");
	anyhow::ensure!(
		strmtyp != 1,
		"E-AC-3 dependent substream (7.1+ layout) is not supported; only a single independent substream"
	);
	let substreamid = (data[2] >> 3) & 0x07;
	anyhow::ensure!(
		substreamid == 0,
		"E-AC-3 additional substream {substreamid} is not supported; only a single independent substream"
	);

	let frmsiz = (((data[2] & 0x07) as usize) << 8) | data[3] as usize;
	let len = (frmsiz + 1) * 2;
	// frmsiz is a raw field; corrupt input can declare a "frame" shorter than the
	// header just parsed, which would surface later as a confusing sync error.
	anyhow::ensure!(len >= 6, "E-AC-3 frame length {len} shorter than its header");

	let fscod = data[4] >> 6;
	let (sample_rate, blocks) = if fscod == 0b11 {
		let fscod2 = (data[4] >> 4) & 0x03;
		anyhow::ensure!(fscod2 != 3, "reserved E-AC-3 sample-rate code");
		// Reduced rates always run 6 blocks.
		(SAMPLE_RATE_REDUCED[fscod2 as usize], 6)
	} else {
		(SAMPLE_RATE[fscod as usize], BLOCKS[((data[4] >> 4) & 0x03) as usize])
	};

	let acmod = (data[4] >> 1) & 0x07;
	let lfeon = data[4] & 0x01;

	Ok(legacy::Header {
		len,
		sample_rate,
		channel_count: CHANNELS[acmod as usize] + lfeon as u32,
		samples: 256 * blocks,
	})
}

#[cfg(test)]
mod test {
	use super::parse_header;

	#[test]
	fn parses_independent_substream() {
		// strmtyp 0, substreamid 0, frmsiz 255 (512 bytes), 48 kHz, 6 blocks,
		// acmod 3/2 + lfeon = 6 channels, 1536 samples.
		let h = parse_header(&[0x0B, 0x77, 0x00, 0xFF, 0x3F, 0x80]).unwrap();
		assert_eq!(
			(h.len, h.sample_rate, h.channel_count, h.samples),
			(512, 48_000, 6, 1536)
		);

		// Reduced rate: fscod 0b11 + fscod2 0 selects 24 kHz, always 6 blocks.
		let h = parse_header(&[0x0B, 0x77, 0x00, 0xFF, 0xCF, 0x80]).unwrap();
		assert_eq!(
			(h.len, h.sample_rate, h.channel_count, h.samples),
			(512, 24_000, 6, 1536)
		);
	}

	#[test]
	fn rejects_reserved_codes() {
		// strmtyp 3 is reserved.
		let err = parse_header(&[0x0B, 0x77, 0xC0, 0xFF, 0x3F, 0x80]).unwrap_err();
		assert!(err.to_string().contains("reserved E-AC-3 stream type"), "{err}");

		// fscod 0b11 + fscod2 0b11 is a reserved sample-rate code.
		let err = parse_header(&[0x0B, 0x77, 0x00, 0xFF, 0xFF, 0x80]).unwrap_err();
		assert!(err.to_string().contains("reserved E-AC-3 sample-rate code"), "{err}");

		// frmsiz 1 declares a 4-byte "frame", shorter than the 6-byte header.
		let err = parse_header(&[0x0B, 0x77, 0x00, 0x01, 0x3F, 0x80]).unwrap_err();
		assert!(err.to_string().contains("shorter than its header"), "{err}");
	}

	#[test]
	fn rejects_out_of_scope_substreams() {
		// Dependent substream (strmtyp 1): a 7.1+ extension of a prior program,
		// not a standalone track. The error must say so; this is the first thing
		// a real 7.1 feed will hit.
		let err = parse_header(&[0x0B, 0x77, 0x40, 0xFF, 0x3F, 0x80]).unwrap_err();
		assert!(err.to_string().contains("dependent substream"), "{err}");

		// A second program multiplexed in the same PID (substreamid 1).
		let err = parse_header(&[0x0B, 0x77, 0x08, 0xFF, 0x3F, 0x80]).unwrap_err();
		assert!(err.to_string().contains("additional substream"), "{err}");

		// Plain AC-3 bsid (8): wrong bitstream for this parser.
		let err = parse_header(&[0x0B, 0x77, 0x00, 0xFF, 0x3F, 0x40]).unwrap_err();
		assert!(err.to_string().contains("bsid"), "{err}");
	}
}