lc3-codec 0.2.0

An implementation of the LC3 audio codec for embedded systems (no_std)
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
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181

use crate::common::{
    complex::Scaler,
    config::{FrameDuration, Lc3Config},
};
const MAX_NUM_DOWNSAMPLED: usize = 160;

// checked against spec

pub struct AttackDetector {
    // static
    config: Lc3Config,
    num_downsampled: usize,
    num_blocks: usize,
    attack_pos_limit: i32,

    // mutating state
    energy_last: Scaler,
    max_energy_last: Scaler, // last delayed long time temporal envelope
    attack_pos_last: i32,    // last attack position
    downsampled_tminus1: i32,
    downsampled_tminus2: i32,
}

impl AttackDetector {
    pub fn new(config: Lc3Config) -> Self {
        let (num_downsampled, num_blocks, attack_pos_limit) = match config.n_ms {
            FrameDuration::TenMs => (160, 4, 2),
            FrameDuration::SevenPointFiveMs => (120, 3, 1),
        };

        Self {
            config,
            max_energy_last: 0.0,
            energy_last: 0.0,
            num_downsampled,
            num_blocks,
            attack_pos_limit,
            attack_pos_last: -1,
            downsampled_tminus1: 0,
            downsampled_tminus2: 0,
        }
    }

    // NOTE that nbytes is the number of bytes of the end compressed audio (not the input audio x_s)
    pub fn run(&mut self, x_s: &[i16], nbytes: usize) -> bool {
        assert_eq!(x_s.len(), self.config.nf);
        if !self.is_active(nbytes) {
            self.energy_last = 0.0;
            self.max_energy_last = 0.0;
            self.attack_pos_last = -1;
            return false;
        }

        // downsample into smaller buffer
        let mut downsampled_buf = [0; MAX_NUM_DOWNSAMPLED];
        let downsampled = &mut downsampled_buf[..self.num_downsampled];
        self.downsample(x_s, downsampled);

        // apply highpass filter
        let mut highpass_buf = [0.0; MAX_NUM_DOWNSAMPLED];
        let highpass = &mut highpass_buf[..self.num_downsampled];
        self.filter(downsampled, highpass);

        // record the last 2 downsampled entries for the next frame
        self.downsampled_tminus1 = downsampled[downsampled.len() - 1];
        self.downsampled_tminus2 = downsampled[downsampled.len() - 2];

        // energy calculation
        let mut attack_position = -1;
        for n in 0..self.num_blocks {
            let mut energy = 0.0;
            for hp in highpass[(40 * n)..(40 * n + 40)].iter() {
                energy += *hp * *hp;
            }
            let max_energy = (0.25 * self.max_energy_last).max(self.energy_last);

            // attack detection
            if energy > 8.5 * max_energy {
                attack_position = n as i32;
            }

            self.energy_last = energy;
            self.max_energy_last = max_energy;
        }

        let attack_detected = attack_position >= 0 || self.attack_pos_last >= self.attack_pos_limit;
        self.attack_pos_last = attack_position;
        attack_detected
    }

    fn is_active(&self, nbytes: usize) -> bool {
        if self.config.fs < 32000 {
            return false;
        }

        match self.config.n_ms {
            FrameDuration::SevenPointFiveMs => {
                (self.config.fs == 32000 && nbytes >= 61 && nbytes < 150)
                    || (self.config.fs >= 44100 && nbytes >= 75 && nbytes < 150)
            }
            FrameDuration::TenMs => {
                (self.config.fs == 32000 && nbytes > 80) || (self.config.fs >= 41000 && nbytes >= 100)
            }
        }
    }

    fn downsample(&self, input: &[i16], output: &mut [i32]) {
        let block_len = self.config.nf / self.num_downsampled;
        for (n, out) in output.iter_mut().enumerate() {
            // sum all samples in range
            let from = block_len * n;
            for input in &input[from..from + block_len] {
                *out += *input as i32;
            }
        }
    }

    fn filter(&self, input: &[i32], output: &mut [Scaler]) {
        // note that the first two samples come from the previous frame
        let tminus1 = self.downsampled_tminus1 as Scaler;
        let tminus2 = self.downsampled_tminus2 as Scaler;
        output[0] = 0.375 * input[0] as Scaler - 0.5 * tminus1 + 0.125 * tminus2;
        output[1] = 0.375 * input[1] as Scaler - 0.5 * input[0] as Scaler + 0.125 * tminus1;

        for (n, output) in output.iter_mut().enumerate().skip(2) {
            *output = 0.375 * input[n] as Scaler - 0.5 * input[n - 1] as Scaler + 0.125 * input[n - 2] as Scaler;
        }
    }
}

#[cfg(test)]
mod tests {
    extern crate std;
    use super::*;
    use crate::common::config::{FrameDuration, SamplingFrequency};

    #[test]
    fn attack_detector_run() {
        let x_s = [
            836, 739, 638, 510, 352, 200, 72, -56, -177, -297, -416, -520, -623, -709, -791, -911, -1062, -1199, -1298,
            -1375, -1433, -1484, -1548, -1603, -1648, -1687, -1724, -1744, -1730, -1699, -1650, -1613, -1594, -1563,
            -1532, -1501, -1473, -1441, -1409, -1393, -1355, -1280, -1201, -1118, -1032, -953, -860, -741, -613, -477,
            -355, -261, -168, -80, -8, 57, 127, 217, 296, 347, 385, 413, 456, 517, 575, 640, 718, 806, 888, 963, 1041,
            1080, 1081, 1083, 1072, 1062, 1067, 1056, 1035, 1019, 999, 964, 934, 909, 876, 854, 835, 813, 795, 781,
            783, 772, 750, 747, 728, 713, 726, 716, 680, 638, 580, 516, 451, 393, 351, 307, 244, 161, 79, 18, -45,
            -123, -215, -301, -389, -512, -644, -764, -888, -1006, -1126, -1253, -1378, -1500, -1614, -1716, -1813,
            -1926, -2051, -2176, -2301, -2416, -2514, -2595, -2680, -2783, -2883, -2977, -3068, -3163, -3262, -3341,
            -3381, -3392, -3392, -3379, -3368, -3353, -3318, -3292, -3244, -3169, -3109, -3049, -2989, -2922, -2844,
            -2790, -2743, -2672, -2588, -2490, -2371, -2222, -2046, -1861, -1695, -1546, -1384, -1214, -1058, -913,
            -761, -602, -441, -280, -124, 24, 169, 302, 421, 546, 661, 738, 796, 851, 924, 1055, 1227, 1412, 1588,
            1707, 1787, 1853, 1905, 1963, 2015, 2048, 2072, 2082, 2093, 2099, 2095, 2097, 2086, 2063, 2060, 2069, 2052,
            2012, 1977, 1956, 1948, 1918, 1843, 1748, 1641, 1533, 1435, 1342, 1252, 1163, 1081, 1024, 989, 962, 937,
            911, 879, 841, 769, 657, 541, 445, 365, 289, 202, 104, -4, -119, -245, -381, -523, -655, -770, -874, -957,
            -1017, -1069, -1118, -1173, -1256, -1370, -1497, -1629, -1745, -1827, -1882, -1934, -2021, -2115, -2165,
            -2196, -2230, -2258, -2282, -2302, -2320, -2332, -2340, -2344, -2338, -2313, -2269, -2215, -2152, -2072,
            -1978, -1885, -1793, -1704, -1621, -1528, -1419, -1310, -1213, -1116, -1014, -914, -820, -736, -656, -578,
            -514, -445, -358, -276, -206, -136, -62, 0, 56, 124, 190, 253, 316, 379, 458, 552, 630, 686, 725, 735, 709,
            661, 612, 572, 538, 507, 476, 453, 448, 453, 444, 415, 370, 316, 257, 203, 159, 125, 107, 114, 137, 162,
            181, 189, 186, 166, 145, 145, 154, 154, 161, 184, 200, 217, 254, 294, 325, 332, 320, 302, 286, 273, 260,
            266, 294, 297, 274, 251, 221, 170, 100, 29, -31, -82, -134, -187, -232, -278, -347, -426, -490, -548, -613,
            -677, -727, -755, -769, -770, -757, -741, -729, -713, -684, -659, -647, -631, -606, -588, -585, -577, -555,
            -534, -527, -528, -513, -480, -456, -440, -415, -382, -333, -244, -132, -32, 47, 130, 225, 308, 383, 460,
            533, 607, 687, 757, 817, 889, 977, 1038, 1064, 1100, 1165, 1250, 1349, 1456, 1563, 1665, 1755, 1829, 1890,
            1935, 1973, 2008, 2033, 2044, 2054, 2076, 2106, 2125, 2115, 2097, 2092, 2093, 2082, 2067, 2068, 2095, 2135,
            2169, 2193, 2213, 2219, 2202, 2163, 2101, 2033, 1992, 1985, 1990, 1986, 1978, 1977, 1976, 1969, 1959, 1956,
            1960, 1955, 1930, 1907, 1884, 1844, 1790, 1733, 1687, 1649, 1611, 1586,
        ];
        let config = Lc3Config::new(SamplingFrequency::Hz48000, FrameDuration::TenMs);
        let mut detector = AttackDetector::new(config);

        let attack_detected = detector.run(&x_s, 150);

        assert_eq!(detector.max_energy_last, 905588.875);
        assert_eq!(detector.energy_last, 549861.5);
        assert_eq!(detector.num_downsampled, 160);
        assert_eq!(detector.attack_pos_last, 0);
        assert_eq!(detector.downsampled_tminus1, 4846);
        assert_eq!(detector.downsampled_tminus2, 5210);
        assert_eq!(attack_detected, true);
    }
}